RFM DNT2400 Series Quick setup guide

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DNT2400 Series
2.4 GHz Spread Spectrum
Wireless Transceivers
Integration Guide

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Important Regulatory Information
RFM Product FCC ID: HSW-DNT2400
IC 4492A-DNT2400
Note: This unit has been tested and found to comply with the limits for a Class B digital device, pursuant
to Part 15 of the FCC Rules. These limits are designed to provide reasonable protection against harmful
interference when the equipment is operated in a commercial environment. This equipment generates,
uses, and can radiate radio frequency energy and, if not installed and used in accordance with the in-
struction manual, may cause harmful interference to radio communications. Operation of this equipment
in a residential area is likely to cause harmful interference in which case the user will be required to
correct the interference at their expense.
FCC Antenna Gain Restriction:
The DNT2400 has been designed to operate with any dipole antenna of up to 9 dBi of gain, or any patch
of up to 6 dBi gain.
The antenna(s) used for this transmitter must be installed to provide a separation distance of at least
20 cm from all persons and must not be co-located or operating in conjunction with any other antenna or
transmitter.
IC RSS-210 Detachable Antenna Gain Restriction:
This device has been designed to operate with the antennas listed below, and having a maximum gain of
9 dB. Antennas not included in this list or having a gain greater than 9 dB are strictly prohibited for use
with this device. The required antenna impedance is 50 ohms:
RFM OMNI249 Omnidirectional Dipole Antenna, 9 dB
RFM PA2400 Patch Antenna, 6 dB
To reduce potential radio interference to other users, the antenna type and its gain should be so chosen
that the equivalent isotropically radiated power (e.i.r.p.) is not more than that permitted for successful
communication.
See Section 3.10 of this manual for regulatory notices and labeling requirements. Changes or modifica-
tions to a DNT2400 not expressly approved by RFM may void the user’s authority to operate the module.

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Table of Contents
1.0 Introduction ......................................................................................................................................... 5
1.1 Why Spread Spectrum?................................................................................................................... 5
1.2 Frequency Hopping versus Direct Sequence..................................................................................6
2.0 DNT2400 Radio Operation ................................................................................................................. 7
2.1 Network Synchronization and Registration...................................................................................... 7
2.2 Authentication.................................................................................................................................. 8
2.3 Transparent and Protocol Serial Port Modes .................................................................................. 9
2.4 RF Data Communications................................................................................................................ 9
2.5 RF Transmission Error Control........................................................................................................ 9
2.6 Transmitter Power Management ................................................................................................... 10
2.7 Network Configurations ................................................................................................................. 10
2.7.1 Point-to-Point Network Operation............................................................................................... 10
2.7.2 Point-to-Multipoint Network Operation ....................................................................................... 11
2.7.3 Peer-to-Peer Network Operation................................................................................................ 11
2.8 Full-Duplex Serial Data Communications...................................................................................... 11
2.9 Channel Access............................................................................................................................. 12
2.9.1 Polling Mode............................................................................................................................... 12
2.9.2 CSMA Mode ............................................................................................................................... 13
2.9.3 TDMA Modes.............................................................................................................................. 14
2.10 Transmission Configuration Planning............................................................................................ 14
2.10.1 TDMA Throughput...................................................................................................................... 15
2.10.2 Polling Throughput ..................................................................................................................... 15
2.10.3 CSMA Throughput...................................................................................................................... 16
2.10.4 Latency....................................................................................................................................... 16
2.10.5 Configuration Validation ............................................................................................................. 17
2.11 Serial Port Operation.................................................................................................................. 19
2.12 Sleep Modes.................................................................................................................................. 20
2.13 Encryption...................................................................................................................................... 22
2.14 Synchronizing Co-located Bases................................................................................................... 22
3.0 DNT2400 Hardware.......................................................................................................................... 24
3.1 Specifications................................................................................................................................. 25
3.2 Module Interface............................................................................................................................ 26
3.3 DNT2400C RFIO Stripline............................................................................................................. 27
3.4 DNT2400 Antenna Connector ....................................................................................................... 28
3.5 Input Voltages................................................................................................................................ 28
3.6 ESD and Transient Protection....................................................................................................... 28
3.7 Interfacing to 5 V Logic Systems................................................................................................... 29
3.8 Power-On Reset Requirements..................................................................................................... 29
3.9 Mounting and Enclosures .............................................................................................................. 29
3.10 Labeling and Notices ..................................................................................................................... 29
4.0 Protocol Messages............................................................................................................................ 31
4.1 Protocol Message Formats............................................................................................................ 31
4.1.1 Message Types .......................................................................................................................... 31
4.1.2 Message Format Details............................................................................................................. 32
4.1.3 /CFG Select Pin.......................................................................................................................... 33
4.1.4 Flow Control ............................................................................................................................... 33
4.1.5 Protocol Mode Data Message Example..................................................................................... 34

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4.2 Configuration Registers ................................................................................................................. 34
4.2.1 Bank 0 - Transceiver Setup........................................................................................................ 34
4.2.2 Bank 1 - System Settings........................................................................................................... 36
4.2.3 Bank 2 - Status Registers........................................................................................................... 39
4.2.4 Bank 3 - Serial............................................................................................................................ 41
4.2.5 Bank 4 - Host Protocol Settings ................................................................................................. 42
4.2.6 Bank 5 - I/O Peripheral Registers............................................................................................... 43
4.2.7 Bank 6 - I/O Setup...................................................................................................................... 43
4.2.8 Bank 7 - Authentication List........................................................................................................ 45
4.2.9 Bank FF - Special Functions ...................................................................................................... 46
4.2.10 Protocol Mode Configuration Message Example....................................................................... 46
4.2.11 Protocol Mode Sensor Message Example ................................................................................. 47
4.2.12 Protocol Mode Event Message Example ................................................................................... 47
5.0 DNT2400DK Developer’s Kit ............................................................................................................ 48
5.1 DNT2400DK Kit Contents.............................................................................................................. 48
5.2 Additional Items Needed................................................................................................................ 48
5.3 Developer’s Kit Operational Notes................................................................................................. 48
5.4 Developer’s Kit Default Operating Configuration........................................................................... 49
5.5 Developer’s Kit Hardware Assembly ............................................................................................. 49
5.6 DNT2400 Demo Utility Program.................................................................................................... 51
5.6.1 Initial Kit Operation ..................................................................................................................... 52
5.6.2 Serial Communication and Radio Configuration ........................................................................ 55
5.7 DNT2400 Wizard Utility Program................................................................................................... 61
5.8 DNT2400 Interface Board Features .............................................................................................. 69
6.0 Demonstration Procedure................................................................................................................. 72
7.0 Troubleshooting ................................................................................................................................ 73
7.1 Diagnostic Port Commands........................................................................................................... 73
8.0 Appendices ....................................................................................................................................... 76
8.1 Ordering Information...................................................................................................................... 76
8.2 Technical Support.......................................................................................................................... 76
8.3 DNT2400 Mechanical Specifications............................................................................................. 77
8.4 DNT2400 Development Board Schematic..................................................................................... 80
9.0 Warranty............................................................................................................................................ 82

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1.0 Introduction
The DNT2400 series transceivers provide highly reliable wireless connectivity for point-to-point, point-to-
multipoint and peer-to-peer applications. Frequency hopping spread spectrum (FHSS) technology en-
sures maximum resistance to multipath fading and robustness in the presence of interfering signals, while
operation in the 2.4 GHz ISM band allows license-free use world wide. The DNT2400 supports all stan-
dard serial data rates for host communications from 1.2 to 460.8 kb/s. On-board data buffering and an
error correcting radio protocol provide smooth data flow and simplify the task of integration with existing
applications. Key DNT2400 features include:
•Multipath fading resistant frequency hop-
ping technology with up to 37 frequency
channels per subband
•Dynamic TDMA slot assignment that maxi-
mizes throughput and CSMA modes that
maximizes network size
•Support for point-to-point or point-to-
multipoint applications
•AES encryption provides protection from
eavesdropping
•FCC 15.247, IC and ETSI certified for
license-free operation
•Nonvolatile memory stores DNT2400 con-
figuration when powered off
•10 mile plus range with omnidirectional
antennas (antenna height dependent)
•Selectable 1, 10 and 63 mW transmit power
levels
•Transparent ARQ protocol with data
buffering ensures data integrity
•Selectable RF data rates of 38.4, 115.2, 200
and 500 kb/s
•Analog and digital I/O simplifies wireless
sensing
•Auto-reporting I/O mode simplifies applica-
tion development
1.1 Why Spread Spectrum?
A radio channel can be very hostile, corrupted by noise, path loss and interfering transmissions from
other radios. Even in an interference-free environment, radio performance faces serious degradation from
a phenomenon known as multipath fading. Multipath fading results when two or more reflected rays of the
transmitted signal arrive at the receiving antenna with opposing phases, thereby partially or completely
canceling the signal. This problem is particularly prevalent in indoor installations. In the frequency do-
main, a multipath fade can be described as a frequency-selective notch that shifts in location and intensity
over time as reflections change due to motion of the radio or objects within its range. At any given time,
multipath fades will typically occupy 1% - 2% of the band. From a probabilistic viewpoint, a conventional
radio system faces a 1% - 2% chance of signal impairment at any given time due to multipath fading.
Spread spectrum reduces the vulnerability of a radio system to both multipath fading and jammers by
distributing the transmitted signal over a larger frequency band than would otherwise be necessary to
send the information. This allows a signal to be reconstructed even though part of it may be lost or cor-
rupted in transmission.

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Narrow-band versus spread-spectrum transmission
Figure 1.1.1
1.2 Frequency Hopping versus Direct Sequence
The two primary approaches to spread spectrum are direct sequence spread spectrum (DSSS) and
frequency hopping spread spectrum (FHSS), either of which can generally be adapted to a given applica-
tion. Direct sequence spread spectrum is produced by multiplying the transmitted data stream by a much
faster, noise-like repeating pattern. The ratio by which this modulating pattern exceeds the bit rate of the
base-band data is called the processing gain, and is equal to the amount of rejection the system affords
against narrow-band interference from multipath and jammers. Transmitting the data signal as usual, but
varying the carrier frequency rapidly according to a pseudo-random pattern over a broad range of chan-
nels produces a frequency hopping spread spectrum system.
Forms of spread spectrum - direct sequence and frequency hopping
Figure 1.1.2

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One disadvantage of direct sequence systems is that due to design issues related to broadband transmit-
ters and receivers, they generally employ only a minimal amount of spreading, often no more than the
minimum required by the regulating agencies. For this reason, the ability of DSSS systems to overcome
fading and in-band jammers is relatively weak. By contrast, FHSS systems are capable of hopping
throughout the entire band, statistically reducing the chances that a transmission will be affected by
fading or interference. This means that a FHSS system will degrade gracefully as the band gets noisier,
while a DSSS system may exhibit uneven coverage or work well until a certain point and then give out
completely.
Because it offers greater immunity to interfering signals, FHSS is often the preferred choice for co-located
systems. Since direct sequence signals are very wide, they can offer only a few non-overlapping chan-
nels, whereas multiple hoppers can interleave, minimizing interference. Frequency hopping does carry
some disadvantage, in that it requires an initial acquisition period during which the receiver must lock on
to the moving carrier of the transmitter before any data can be sent, which typically takes several sec-
onds. In summary, frequency hopping systems generally feature greater coverage and channel utilization
than comparable direct sequence systems. Of course, other implementation factors such as size, cost,
power consumption and ease of implementation must also be considered before a final radio design
choice can be made.
2.0 DNT2400 Radio Operation
2.1 Network Synchronization and Registration
As discussed above, frequency hopping spread spectrum radios such as the DNT2400 periodically
change the frequency at which they transmit. In order for the other radios in the network to receive the
transmission, they must be listening to the frequency on which the current transmission is being sent. To
do this, all the radios in the network must be synchronized to the same hopping pattern.
In point-to-point or point-to-multipoint networks, one radio module is designated as the base. All other
radios are designated as remotes. One of the responsibilities of the base is to transmit a synchronization
signal (beacon) to the remotes to allow them to synchronize with the base. Since the remotes know the
hopping pattern, once they are synchronized with the base, they know which frequency to hop to and
when. Every time the base hops to a different frequency, it immediately transmits a synchronizing signal.
When a remote is powered on, it rapidly scans the frequency band for the synchronizing signal. Since the
base is transmitting on up to 37 frequencies and the remote is scanning up to 37 frequencies, it can take
several seconds for a remote to synchronize with the base.
Once a remote has synchronized with the base, it will request registration information to allow it to join the
network. Registration is handled automatically by the base. When a remote is registered, it receives
several network parameters from the base, including HopDuration, InitialNwkID, FrequencyBand and
Nwk_Key (see Section 4.2 for parameter details). Note that if a registration parameter is changed at the
base, it will update the parameter in the remotes over the air.
Among other things, registration allows the tracking of remotes entering and leaving a network, up to a
limit of 254 remotes. The base builds a table of serial numbers of registered remotes using their three-
byte serial numbers (MAC addresses). To detect if a remote has gone offline or out of range, the registra-
tion is “leased”and must be “renewed”once every 250 hops. Any transmission from a remote running on
a leased registration renews the lease with the base. If a remote does not transmit within 250 hops, the

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radio automatically renews the lease with the base. There is nothing the remote host needs to do to keep
the lease renewed. Note that more remotes can join the network but their entering and leaving the net-
work cannot be tracked by the base radio. The DNT2400 base can be configured to send join announce-
ments to a host application for an unlimited number of remotes. The application can then verify the con-
tinued presence of remotes in the network through periodic polling of each remote.
In addition, the DNT2400 supports a RemoteLeave command that allows a host application to cause a
remote to leave the network. This is useful to remove any rogue remotes that may have joined when
authentication is not being used. It is also useful to remove remotes from the network once they have
been serviced by the application. In this manner, the base can use the lease times to keep track of re-
motes that have not yet been serviced thereby allowing networks of more than 254 remotes to be tracked.
The RemoteLeave command includes the amount of time the remote must leave the network which can
be set from 2 seconds to more than 36 hours. In addition, a remote can be told to leave and not rejoin
until it has been power-cycled or reset.
2.2 Authentication
In many applications it is desirable to control which remote devices may join the network. This provides
security from rogue nodes joining the network and simplifies network segregation for co-located networks.
Network registration of remotes is controlled by the AuthMode parameter in the base. The AuthMode
parameter can be set to one of four values, 0..3. The default value is 0, which allows any remote to regis-
ter with a base.
When the AuthMode parameter is set to 1, a remote’s address must be listed in Parameter Bank 7 before
it will be allowed to register with the base. This is referred to as base authentication. Bank 7 must be
preloaded with the addresses of the authorized remotes before using base authentication. If a remote
whose MAC address is not in Bank 7 attempts to join the network the base radio will deny the registration
request. A maximum of 16 remotes can be entered into Bank 7. To support larger networks, mode 2 must
be used.
When the AuthMode parameter is set to 2, the address of a remote attempting to register with the base is
sent to the host for authentication in a JoinRequest message. The host application determines if the
remote should be allowed to register and returns a JoinReply message to the base containing a Permit-
Status parameter that allows or blocks the remote from registering. The host application has 30 seconds
in which to respond, after which time the base denies registration to the remote. Up to 16 join requests
can be pending at any one time. If more than 16 remotes are asking to join, the first 16 will be serviced
and additional remotes will be serviced after the earlier requests are handled. The RegDenialDelay pa-
rameter controls how often a remote will request registration after it has been denied. If it is anticipated
that more than 16 remotes will request registration before the application can service the first 16, this
parameter should be set to the time it will take the application to service four requests as this will speed
the authentication process by freeing the base from issuing multiple denials to the same remotes.
When the AuthMode parameter is set to 3, authentication is locked to the addresses of the remotes
currently registered with the base. Mode 3 is typically used in conjunction with Mode 0 during a commis-
sioning process. AuthMode is set to 0, remotes are turned on and allowed to register with the base, and
AuthMode is then switched to 3 to lock the network membership.

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2.3 Transparent and Protocol Serial Port Modes
DNT2400 radios can work in two serial port data modes: transparent and packet protocol. Transparent
mode formatting is simply the raw user data. Protocol mode formatting includes a start-of-packet framing
character, length byte, addressing, command bytes, etc. Transparent mode operation is especially useful
in point-to-point systems that act as simple cable replacements. In point-to-multipoint systems where the
base needs to send data specifically to each remote, protocol formatting must be used unless the data
being sent includes addressing information that the devices connected to the remote radios can use to
determine the intended destination of the broadcast data. Protocol formatting is also required for configu-
ration commands and responses, and sensor I/O commands and responses. Protocol mode can be used
at the base radio while transparent mode is used at the remotes. The one caution about protocol mode is
that the length of a protocol mode message cannot exceed the BaseSlotSize or RemoteSlotSize or the
packet will be discarded. Protocol formatting details are covered in Section 4.
The DNT2400 provides two ways to switch between transparent and protocol modes. If /CFG input Pin 18
on the DNT2400 is switched from logic high to low, protocol mode is invoked. Or if the EnterProtocolMode
command is sent, the DNT2400 will switch to the protocol mode. When input Pin 18 is switched from logic
low to high, or an ExitProtocolMode command is sent to the primary serial input, the DNT2400 will switch
to transparent operation. Note that it is possible that part of the EnterProtocolMode command will be sent
over the air as transparent data.
When operating in transparent mode, two configuration parameters control when a DNT2400 radio will
send the data in its transmit buffer. The MinPacketLength parameter sets the minimum number of bytes
that must be present in the transmit buffer to trigger a transmission. The TxTimeout parameter sets the
maximum time data in the transmit buffer will be held before transmitting it, even if the number of data
bytes is less than MinPacketLength. The default value for MinPacketLength parameter is one and the
TxTimeout parameter is zero, so that any bytes that arrive in the DNT2400 transmit buffer will be sent on
the next hop. As discussed in Section 2.7.2, it is useful to set these parameters to values greater than
their defaults in point-to-multipoint systems where some or all the remotes are in transparent mode.
2.4 RF Data Communications
At the beginning of each hop the base transmits a beacon, which always includes a synchronizing signal.
After synchronization is sent, the base will transmit any user data in its transmit buffer, unless in transpar-
ent mode the MinPacketLength and/or TxTimeout parameters have been set above their default values.
The maximum amount of user data bytes that the base can transmit per hop is limited by the BaseSlot-
Size parameter, as discussed in Section 2.7.1. If there is no user data or reception acknowledgements
(ACKs) to be sent on a hop, the base will only transmit the synchronization signal in the beacon.
The operation for remotes is similar to the base, but without a synchronizing signal. The RemoteSlotSize
parameter indicates the maximum number of user data bytes a remote can transmit on one hop and is a
read-only value. The RemoteSlotSize is determined by the HopDuration and BaseSlotSize parameters
and the number of registered remotes. The MinPacketLength and TxTimeout parameters operate in a
remote in the same manner as in the base.
2.5 RF Transmission Error Control
The DNT2400 supports two error control modes: automatic transmission repeats (ARQ), and redundant
transmissions for broadcast packets. In both modes, the radio will detect and discard any duplicates of

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messages it receives so that the host application will only receive one copy of a given message. In the
redundant transmission mode, broadcast packets are repeated a fixed number of times based on the
value of the ARQ_AttemptLimit parameter. In ARQ mode, a packet is sent and an acknowledgement is
expected on the next hop. If an acknowledgement is not received, the packet is transmitted again on the
next available hop until either an ACK is received or the maximum number of attempts is exhausted. If the
ARQ_AttemptLimit parameter is set to its maximum value, a packet transmission will be retried without
limit until the packet is acknowledged. This is useful in some point-to-point cable replacement applications
where it is important that data truly be 100% error-free, even if the destination remote goes out of range
temporarily.
2.6 Transmitter Power Management
The DNT2400 includes provisions for setting the base transmit power level and the remote maximum
transmit power level with the TxPower parameter. DNT2400 networks covering a small area can be
adjusted to run at lower transmitter power levels, reducing potential interference to other nearby systems
such as 802.11 networks. Remotes that are located close to their base can be adjusted to run at lower
maximum power, further reducing potential interference. Base units transmit at the fixed power level set
by the TxPower parameter. Remotes automatically adjust their transmitter power to deliver packets to the
base at an adequate but not excessive signal level, while not transmitting more power than set by their
TxPower parameter. Remotes make transmitter power adjustments using the strength of the signals
received from the base and the base transmitter power setting, which is periodically transmitted by the
base. The remote automatic transmit power adjustment is enabled by default but can be disabled if so
desired. Refer to Section 4.2.1 for details.
2.7 Network Configurations
The DNT2400 supports three network configurations: point-to-point, point-to-multipoint, and peer-to-peer.
In a point-to-point network, one radio is set up as the base and the other radio is set up as a remote. In a
point-to-multipoint network, a star topology is used with the radio set up as a base acting as the central
communications point and all other radios in the network set up as remotes. In this configuration, each
communication takes place between the base and one of the remotes. Peer-to-peer communications
between remotes using the base as a relay is also supported, as discussed in Section 2.7.3.
2.7.1 Point-to-Point Network Operation
Most point-to-point networks act as serial cable replacements and both the base and the remote use
transparent mode. Unless the MinPacketLength and TxTimeout parameters have been set above their
default values, the base will send the data in its transmit buffer on each hop, up to a limit controlled by the
BaseSlotSize parameter. In transparent mode, if the base is buffering more data than can be sent on one
hop, the remaining data will be sent on subsequent hops. The base adds the address of the remote, a
packet sequence number and error checking bytes to the data when it is transmitted. These additional
bytes are not output at the remote in transparent mode. The sequence number is used in acknowledging
successful transmissions and in retransmitting corrupted transmissions. A two-byte CRC and a one-byte
checksum allow a received transmission to be checked for errors. When a transmission is received by the
remote, it will be acknowledged if it checks error free. If no acknowledgment is received, the base will
retransmit the same data on the next hop. Note that acknowledgements from remotes are suppressed on
broadcast packets from the base.

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In point-to-point operation, by default a remote will send the data in its transmit buffer on each hop, up to
the limit controlled by its RemoteSlotSize parameter. If desired, the MinPacketLength and TxTimeout
parameters can be set above their default values, which configures the remote to wait until the specified
amount of data is available or the specified delay has expired before transmitting. In transparent mode, if
the remote is buffering more data than can be sent on one hop, it will send the remaining data in subse-
quent hops. The remote adds its own address, a packet sequence number and error checking bytes to
the data when it is transmitted. These additional bytes are not output at the base if the base is in trans-
parent mode. When a transmission is received by the base, it will be acknowledged if it checks error free.
If no acknowledgment is received, the remote will retransmit the same data on the next hop.
2.7.2 Point-to-Multipoint Network Operation
In a point-to-multipoint network, the base is usually configured for protocol formatting, unless the applica-
tions running on each remote can determine the data’s destination from the data itself. Protocol formatting
adds addressing and other overhead bytes to the user data. If the addressed remote is using transparent
formatting, the source (originator) address and the other overhead bytes are removed. If the remote is
using protocol formatting, the source address and the other overhead bytes are output with the user data.
A remote can operate in a point-to-multipoint network using either transparent or protocol formatting, as
the base is the destination by default. In transparent operation, a remote DNT2400 automatically adds
addressing, a packet sequence number and error checking bytes as in a point-to-point network. When the
base receives the transmission, it will format the data to its host according to its formatting configuration.
A remote running in transparent mode in a point-to-multipoint network can have the MinPacketLength and
TxTimeout parameters set to their default values to reduce latency, or above their default values to re-
duce the volume of small packet transmissions.
2.7.3 Peer-to-Peer Network Operation
After a remote has joined the network, it can communicate with another remote through peer-to-peer
messaging, where the base acts as an automatic message relay. In protocol mode, if a remote specifies a
destination address other than the base address, peer-to-peer messaging is enabled. In transparent
mode, the RmtTransDestAddr parameter sets the destination address. Changing RmtTransDestAddr from
the default base address to the address of another remote enables peer-to-peer messaging. The broad-
cast address can also be used as a peer-to-peer destination address. In this case, the message will be
unicast from the remote to the base (using ARQ) and then broadcast by the base (no ARQ). For peer-to-
peer broadcasts, no acknowledgement is sent and no TxDataReply packet is reported to the host.
2.8 Full-Duplex Serial Data Communications
From a host application’s perspective, DNT2400 serial communications appear full duplex. Both the base
host application and each remote host application can send and receive serial data at the same time. At
the radio level, the base and remotes do not actually transmit at the same time. If they did, the transmis-
sions would collide. As discussed earlier, the base transmits a beacon with a synchronization signal at the
beginning of each hop, followed by its user data. After the base transmission, the remotes can transmit.
Each base and remote transmission may contain all or part of a complete message from its host applica-
tion. From an application’s perspective, the radios are communicating in full duplex since the base can
receive data from a remote before it completes the transmission of a message to the remote and vice-
versa.

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2.9 Channel Access
The DNT2400 provides five methods of channel access: one Polling, two CSMA, and three TDMA, as
shown in the table and figure below. The channel access method is selected by the AccessMode parame-
ter in Bank 1. See section 4.2.2. The channel access setting is distributed to all remotes by the base, so
changing it at the base sets the entire network. Polling refers to an application sending a command from
the base to one or more remote devices where only one remote device will respond at a time. Polling is
suitable for both large and small networks where unsolicited or event reporting by remotes is not required.
Carrier Sense Multiple Access (CSMA) is very effective at handling packets with varying amounts of data
and/or packets sent at random times from a large number of remotes. Time Division Multiple Access
(TDMA) provides a scheduled time slot for each remote to transmit on each hop. The default DNT2400
access mode is TDMA dynamic mode.
Access Mode Description Max Number of Remotes
0 Polling unlimited
1 CSMA unlimited
2 TDMA dynamic slots up to 16
3 TDMA fixed slots up to 16
4 TDMA with PTT up to 16
Table 2.9.1
B a s e
R e m o t e # 1
R e m o t e # 2
R e m o t e # 3
T D M A
B a s e
R e m o t e # 1
R e m o t e # 2
R e m o t e # 3
C S M A
XX
B a c k o f f
L i s t e n
L i s t e n
B a s e
R e m o t e # 1
R e m o t e # 2
R e m o t e # 3
P O L L
Figure 2.9.1
2.9.1 Polling Mode
Polling channel access is used for point-to-point and point-to-multipoint systems where only one remote
will attempt to transmit data at a time, usually in response to a command from the base.
Polling (mode 0) is a special case of CSMA mode 1. The user can set the BaseSlotSize and CSMA_
RemtSlotSize parameters when using this mode. Since only one remote will attempt to transmit at a time,

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to minimize latency, the CSMA_Predelay and CSMA_Backoff parameters are not used. Lease renewals
are also not used, again to minimize latency. Thus, when the base is operated in protocol mode with
Announce messages enabled, only join messages are generated. This mode provides high throughput
since there is no contention between remotes and the entire portion of the hop frame following the base
transmission is available for a remote to transmit. Applications where more than one remote may attempt
to transmit at a time, or where periodic reporting and/or event reporting are enabled should not use this
mode.
2.9.2 CSMA Mode
When using CSMA channel access, each remote with data to send listens to hear if the channel is clear
and then transmits. If the channel is not clear, a remote will wait a random period of time and listen again.
CSMA works best when a large or variable number of remotes transmit infrequent bursts of data. There is
no absolute upper limit on the number of remote radios that can be supported in this mode - it depends
on message density. A maximum of 254 remotes can be supported if base join-leave tracking is required,
or an unlimited number of remotes if base join-leave tracking is not required or will be handled by the host
application.
There are two important parameters related to CSMA operation. The CSMA_Backoff parameter defines
the initial time that a remote will wait when it determines the channel is busy before again checking to see
if the channel is clear (back-off interval). If, after finding the channel busy and backing off, the radio finds
the channel busy a second time, the amount of time the remote will wait before checking the channel will
increase. It will continue to increase each subsequent time the channel is busy until the channel is finally
found idle. This is the classic CSMA technique that handles the situation where a number of remotes hold
data to send at the same time. The CSMA_ Predelay parameter controls the maximum time that a remote
will wait before first listening to see if the channel is clear for a transmission. This parameter is used to
make sure that all the remotes do not transmit immediately after the base finishes transmitting.
CSMA (mode 1) provides classical CSMA channel access, and gives the user control over both the
CSMA_Predelay and CSMA_Backoff parameters. This mode is well suited for large numbers of uncoordi-
nated remotes, and/or where periodic/event reporting is used. In addition to CSMA_ Predelay and
CSMA_ MaxBackoff, the user can set the BaseSlotSize and CSMA_RemtSlotSize parameters when
using this mode. The following guidelines are suggested for setting CSMA_Predelay:
•For lightly loaded CSMA contention networks, decrease CSMA_Predelay
to 0x20 or less to reduce latency.
•For heavily loaded CSMA contention networks, increase CSMA_Predelay
to 0x80 or more for better throughput.
As an option, CSMA mode allows the base to directly track remotes entering and leaving the network, for
up to 254 remotes. The base is operated in protocol mode and is configured to send Announce messages
to its host when a remote joins and when the remote’s registration lease expires.
While a base in a CSMA network can track a maximum of 254 remotes entering and leaving the network,
it can generate join Announce messages for an unlimited number of remotes. This allows the host appli-
cation to track remotes entering and leaving a CSMA network with more than 254 remotes by creating its
own table of MAC addresses and periodically sending a GetRemoteRegister command to each remote in
the table. Failure to answer a GetRemoteRegister command indicates the remote is no longer active in
the network.

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CSMA modes work well in many applications, but CSMA has some limitations, as summarized below:
•Bandwidth is not guaranteed to any remote.
•Marginal RF links to some remotes can create a relatively high chance of
collisions in heavily loaded networks.
2.9.3 TDMA Modes
TDMA modes provide guaranteed bandwidth to some or all of the remotes in a network. Remotes that
register with the base receive several special parameters, including ranging information and a specific
channel access time slot assignment. TDMA registrations are always leased and must be renewed every
250 hops. The DNT2400 provides three TDMA access modes, as discussed below.
TDMA Dynamic Slots (mode 2) is used for general-purpose TDMA applications where scaling the capac-
ity per slot to the number of active remotes is automatic. Each remote that registers with the base
receives an equal time slice. As new remotes join, the size of the TDMA remote slots shrinks accordingly.
The number of slots, individual slot start times, and the RemoteSlotSize are computed automatically by
the DNT2400 network in this mode. The user should note that the bandwidth to each remote will change
immediately as remotes join or leave the network. When running in protocol mode on a remote, care must
be taken not to generate messages too long to be sent in a single hop due to automatic RemoteSlotSize
reduction.
TDMA Fixed Slots (mode 3) is used for applications that have fixed data throughput requirements, such
as isochronous voice or streaming telemetry. The slot start time and the RemoteSlotSize are computed
automatically by the DNT2400 network in this mode. The user must set the number of slots using the
MaxSlots parameter. The base radio will allocate remote slot sizes as if MaxSlots number of remotes are
linked with the base, even when fewer remotes are actually linked. In this mode, the remote slot sizes are
constant.
TDMA with PTT (mode 4) supports remotes with a "push-to-talk" feature, also referred to as "listen-
mostly" remotes. This mode uses fixed slot allocations. Remotes can be registered for all but the last slot.
The last slot is reserved for the group of remotes that are usually listening, but occasionally need to
transmit. In essence, the last slot is a shared channel for this group of remotes. When one of them has
data to send it keys its transmitter much like a walkie-talkie, hence the name push-to-talk (PTT). There is
no limit to the number of remotes that can listen to the last slot.
The slot start time and the RemoteSlotSize are computed automatically by the DNT2400 network in mode
4. The user must specify the number of slots using the MaxSlots parameter. The last slot is reserved for
the PTT remotes. The user must configure PTT remotes individually to select mode 4 operation. The
user's application must ensure that only one PTT remote at a time is using the slot.
2.10 Transmission Configuration Planning
Because frequency hopping radios change frequency periodically, a single message may be sent in one
or more RF transmissions. The length of time the radio stays on a frequency, the hop duration, impacts
both latency and throughput. The longer the radio stays on a single frequency, the higher the throughput
since the radio is transmitting for a higher percentage of the time, but latency is also higher since radios
may have to wait longer to transmit. So latency and throughput trade off against one another. The

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DNT2400 has several configuration parameters that allow latency and throughput to be optimally bal-
anced to the needs of an application.
2.10.1 TDMA Throughput
For TDMA channel access, throughput and latency are controlled by the RF data rate, the serial port
baud rate, the BaseSlotSize, the HopDuration, and the number of remotes. A wide range of throughput
and latency combinations can be obtained by adjusting these parameters. The throughput of a radio in a
TDMA network is simply:
Number of bytes per hop/Hop Duration
For the base, the number of byes per hop is controlled by the BaseSlotSize parameter so the throughput
of the base radio is:
BaseSlotSize/HopDuration
Note that if fewer bytes than the BaseSlotSize limit are sent to the base radio by its host during the hop
duration time in transparent mode, the observed throughput of the base radio will be reduced. If the base
is in protocol mode, it will wait until a protocol formatted message is completely received from its host
before transmitting it. If the message is not completely received by the time the base transmits, the base
will wait until the next hop to transmit the message. The throughput for each remote is:
RemoteSlotSize/HopDuration
In a TDMA mode, the RemoteSlotSize is set automatically based on the number of remotes and the
BaseSlotSize. Note that the base radio always reserves BaseSlotSize amount of time in each hop
whether or not the base has user data to send.
To help select appropriate parameter values, RFM provides the DNT Throughput Calculator utility pro-
gram (DNTCalc.exe). This program is on the development kit CD. Enabling encryption (security) adds
additional bytes to the data to be sent but the Calculator has a mode to take this into account.
2.10.2 Polling Throughput
In polling mode, the application sends data from the base to a specific remote, which causes the remote
and/or its host to send data back to the application. The network operates like a point-to-point network in
this case. In polling, the HopDuration should be set just long enough to accommodate a base transmis-
sion up to the limit allowed by the BaseSlotSize parameter, plus one remote transmission up to the limit
allowed by the CSMA_RemtSlotSize parameter. These slot sizes and the hop duration set the polling
throughput as in TDMA channel access.
The throughput in Polling mode is also determined by the amount of time it takes for the remote host
device to respond to the poll. For example, consider the situation where a remote host device communi-
cates with the DNT2400 at 38.4 kb/s, receives a 16-byte poll command, and takes 1 ms to generate a 32-
byte response which it then sends to the DNT2400. Sixteen bytes over a UART port is 160 bits using
8,N,1 serial parameters. Sending 160 bits at 38.4 kb/s takes 4.2 ms. Add 1 ms for the host device to
process the command and begin sending the 32-byte response. The 32-byte response takes 8.4 ms to
send at 38.4 kb/s, for a total turnaround time of 13.6 ms. This amount of time could be added to the base
and remote slot times to allow the entire transaction to take place in a single hop. However, except at the
38.4 kb/s over-the-air data rate, this is likely to be much longer than the base and remote slot times.
Thus, in practice, lengthening the hop duration to complete the transaction in a single hop doesn’t really

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affect the throughput. Nevertheless, it is important to note that the throughput for the remote in the exam-
ple above is substantially less than the remote slot size in bytes divided by the hop duration.
It is not a DNT2400 requirement that the complete application message be sent in a single hop, nor that
the remote response is returned in a single hop, when in transparent mode. If either transmission occurs
over more than one hop, then depending on the length of the data, the RF data rate and the serial port
data rate at the receiving end, there may be a gap in the serial data. Some protocols, such as Modbus
RTU, use gaps in data to determine packet boundaries.
2.10.3 CSMA Throughput
In CSMA mode, remote radios do not have a fixed throughput, which is why applications requiring guar-
anteed throughput should use polling or a TDMA mode. The reason that the throughput of a CSMA
remote is not fixed is because its ability to transmit at any given time depends on whether another radio is
already transmitting. The throughput of a remote is further affected by how many other remotes are
waiting for the channel to become clear so they can transmit. This is not a problem when remotes, even a
large numbers of remotes, only send data infrequently. The DNT2400 includes several configuration
parameters that can be used to optimize the performance of a CSMA network.
It is often desirable to limit the amount of data a CSMA remote can send in one transmission. This pre-
vents one remote from hogging network throughput. To accommodate this, the DNT2400 provides a
CSMA_RemtSlotSize parameter that is user configurable. When a remote has transmitted CSMA_
RemtSlotSize bytes on a given hop, it will stop transmitting until the next hop. Note that this remote will
have to contend for the channel on the next hop, so it is not guaranteed that it will be the first remote to
transmit on the next hop or that it will be able to transmit on the next hop at all. To allow multiple remotes
a chance to transmit on the same hop, the HopDuration parameter must be set long enough to support
the BaseSlotSize, plus the number of remotes to transmit per hop multiplied by the CSMA_ RemtSlotSize,
plus the number of remotes to transmit per hop multiplied by the CSMA_Backoff. Because of the way
CSMA channel access works, this does not guarantee that the desired number of remotes to transmit on
a hop will always be able to transmit on a single hop. This is due to the fact that when a remote with data
to send finds the channel busy a second time, it waits for a longer period to time before testing the chan-
nel again. This time will continue to increase until the remote finds the channel clear. In practice this is
unlikely to present a problem, as CSMA networks are used with devices that infrequently have data to
send.
The DNT Throughput Calculator can be used to determine the HopDuration, but it will be necessary to
increase the number of remotes in DNTCalc to a value greater than the number of remotes to transmit on
a single hop to account for the backoffs. It is indeterminate how many backoffs may occur during a single
hop, which is why the number of remotes that transmit on a given hop cannot be guaranteed. Note that
the CSMA_ Backoff parameter sets the length of time a remote will wait to recheck the channel when it
has detected that the channel is busy. The second time a remote detects that the channel is busy, it will
increase the amount of time it waits until it checks again. Every subsequent time it detects a busy channel
it will increase the amount of time it will wait in a geometric fashion. This continues until it detects an idle
channel. So while a short CSMA_Backoff can decrease the time between when one remote transmits and
the next remote transmits, it can actually lead to a longer time between remote transmissions than a
longer backoff. This can occur when the remote checks the channel multiple times during the transmitting
remote’s transmission causing the back-off time to be increased.

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2.10.4 Latency
The worst case latency for TDMA access, excluding retries, occurs when the radio receives data just after
its turn to transmit. In this instance, it will have to wait the length of time set by the HopDuration to begin
transmitting the data. If the radio is receiving data over its serial port at a rate higher than its throughput,
this will only occur at the beginning of a transmission that spans several hops.
In a polling application, latency is affected by how long the remote and/or its host takes to respond, and
when in the hop data is ready to be transmitted. Since a remote can begin transmitting at practically any
time during the hop after the base has transmitted, the latency can be less than HopDuration. However,
the remote transmission may extend over two hops if it starts late in the first hop.
Latency for any given remote in a CSMA network is particularly difficult to characterize. If many remotes
have data to send, the latency for the last remote to send will be the length of time it takes all the other
remotes to send. The CSMA scheme used in the DNT2400 is designed to allow each remote an equal
opportunity to transmit, so the concern is not that one remote is locked out, but just how long it will take a
number of remotes with data to sent to each gain access to the channel and send their data. The more
data that needs to be sent, the more time will be consumed checking the channel and backing off when
the channel is busy. Again, this is why CSMA networks are best used when there are a large number of
nodes that send data infrequently.
The other factor impacting latency is retries. This impact is not unique to frequency hopping radios but is
common among all wireless technologies. A radio only transmits data once per hop. It needs to wait until
the next hop to see if the transmission was received at the destination. If not, the radio will transmit the
data again and wait for the acknowledgement. This can happen up to ARQAttemptLimit number of times
which is equal to ARQAttemptLimit times HopDuration amount of time.
2.10.5 Configuration Validation
Although slot durations are automatically calculated by the DNT2400, the RF data rate, hop duration, etc.,
must be coordinated by the user to assure a valid operating configuration based on the following criteria:
1. Regardless of the RF data rate, the maximum DNT2400 hop duration is limited to 200 ms. A
DNT2400 network must be configured accordingly.
2. In protocol mode, the BaseSlotSize and RemoteSlotSize parameters must be large enough to
hold all the data bytes in the largest protocol formatted message being used. Protocol formatted
messages must be sent in a single transmission. Any protocol formatted messages too large for
the slot size setting will be discarded
3. In TDMA mode 2, the RemoteSlotSize will be reduced automatically when a new remote joins the
network. This can cause a network to suddenly malfunction if the hop duration is not set to pro-
vide an adequately large remote slot allocation when fully loaded with remotes
4. When operating in polling mode 0, the CSMA_RemtSlotSize and HopDuration parameters are
usually set to accommodate the number of data bytes in a maximum size transmission. This con-
figuration provides low latency for polled messages.

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5. When operating in CSMA mode 1 with multiple remotes, the CSMA_RemtSlotSize and HopDura-
tion parameters are usually set to accommodate three times the number of data bytes in one
maximum size transmission, to allow time for more than one remote to attempt to transmit during
a single hop.
Figure 2.10.5.1
The DNT Throughput Calculator utility program is shown in Figure 2.10.5.1. Decimal data is entered by
default. Hexadecimal data can also be entered using a 0x prefix, as shown in the Hop Duration Counts
text box. When using the DNT Throughput Calculator, parameter coordination depends on the operating
mode of a DNT2400 network, as outlined below:
Polling (mode 0) - the user can set and must coordinate the RF data rate, hop duration, base slot size
and remote slot size. First, set the BaseSlotSize to accommodate the maximum number of data bytes in a
base transmission. Next, set the CSMA_RemtSlotSize to accommodate the maximum number data bytes
in a remote transmission. Use these slot sizes, the RF data rate and the maximum operating range (20
miles is the default) as inputs to the Calculator program to determine minimum valid HopDuration.
CSMA contention (mode 1) - the same procedure as for polling is used, except that the CSMA_
RemtSlotSize typically should be set at three times the maximum number of data bytes for point-to-
multipoint networks. The default values for CSMA pre-delay and back-off are assumed.
TDMA dynamic (mode 2) - this is the DNT2400’s default operating mode and the default settings are
optimized for point-to-point transparent operation. For other configurations the user must coordinate the
RF data rate, hop duration, base slot size and maximum number of remotes. Although the remote slot
size and remote slot time allocation are automatically set in mode 2, the user must predetermine these
values to assure a valid operating configuration. First, set the BaseSlotSize to accommodate the maxi-
mum number of data bytes in a base transmission. Next, determine the RemoteSlotSize required to
accommodate the maximum number of data bytes in a remote transmission. Use these slot sizes, the
maximum number of remotes that will be used in the network, the RF data rate and the maximum operat-
ing range as inputs to the Calculator to determine the minimum valid HopDuration time. Note that when
there are fewer remotes on the network than the maximum specified, the remotes will automatically be
configured with a bigger RemoteSlotSize parameter.

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TDMA fixed (mode 3) - First, set the BaseSlotSize to accommodate the maximum number of data bytes in
a transmission. Next, determine the RemoteSlotSize required to accommodate the maximum number of
data bytes in a remote transmission. Then set the number of remote slots. Use the slot sizes, the number
of remotes, the RF data rate and maximum operating range as inputs to the Calculator to determine the
minimum valid hop duration.
TDMA PTT (mode 4) - use the same procedure as for TDMA fixed mode 3.
The DNT2400 base firmware can detect a significant number of invalid configurations and override the
HopDuration parameter to establish a valid configuration. To take advantage of this feature, configure a
DNT2400 network in the following order:
1. In all system radios, set the RF_DataRate parameter and save it. Then reset all radios to estab-
lish the new RF data rate.
2. Set the BaseSlotSize and TDMA_MaxSlots or CSMA_RemtSlotSize as needed. Use the default
maximum operating range unless links of more than 20 miles are planned.
3. Set the HopDuration parameter and then read it back. If the HopDuration parameter readout is
different than the value set, the firmware detected an invalid configuration and is overriding it.
2.11 Serial Port Operation
DNT2400 networks are often used for wireless communication of serial data. The DNT2400 supports
serial baud rates from 1.2 to 460.8 kb/s. Listed in Table 2.11.1 below are the supported data rates and
their related byte data rates and byte transmission times for an 8N1 serial port configuration:
Baud Rate kb/s Byte Data Rate kB/s Byte Transmission Time ms
1.2 0.12 8.3333
2.4 0.24 4.1667
4.8 0.48 2.0833
9.6 0.96 1.0417
19.2 1.92 0.5208
28.8 2.88 0.3472
38.4 3.84 0.2604
57.6 5.76 0.1736
76.8 7.68 0.1302
115.2 11.52 0.0868
230.4 23.04 0.0434
460.8 46.08 0.0217
Table 2.11.1
To support continuous full-duplex serial port data flow, an RF data rate higher than the serial port baud
rate is required for FHSS. Radios transmissions are half duplex, and there are overheads related to
hopping frequencies, assembling packets from the serial port data stream, transmitting them, sending
ACK’s to confirm error-free reception, and occasional transmission retries when errors occur.

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For example, consider a TDMA mode 2 system with one remote operating up to 20 miles at 500 kb/s with
the BaseSlotSize parameter set to 64 bytes and the RemoteSlotSize parameter at 64 bytes. As shown in
Figure 2.11.1, the hop duration from the DNT Throughput Calculator program for this configuration is
4.70 ms
Figure 2.11.1
The average full-duplex serial port byte rate that can be supported under error free conditions is:
64 Bytes/4.70 ms = 13.62 kB/s, or 136.2 kb/s for 8N1
Continuous full-duplex serial port data streams at a baud rate of 115.2 k/bs can be supported by this
configuration, provided only occasional RF transmission errors occur. Plan on an average serial port data
flow of 90% of the calculated error-free capacity for general-purpose applications.
The DNT2400 transmit and receive buffers hold at least 1024 bytes and will accept brief bursts of data at
high baud rates, provided the average serial port data flow such as shown in the example above is not
exceeded. It is strongly recommended that the DNT2400 host use hardware flow control in applications
where the transmit buffer can become full. The host must send no more than 32 additional bytes to the
DNT2400 when the DNT2400 de-asserts the host’s CTS line. In turn, the DNT2400 will send no more
than one byte following the host de-asserting its RTS line. Three-wire serial port operation is allowed
through parameter configuration, as discussed in Section 4.2.4. However, data loss is possible under
adverse RF channel conditions when using three-wire serial operation due to buffer overruns.
2.12 Sleep Modes
To save power in applications where a remote transmits infrequently, the DNT2400 supports hardware
and firmware sleep modes. Hardware sleep mode is entered by switching SLEEP/DTR Pin 11 on the
DNT2400 from logic low to high. While in hardware sleep mode, the DNT2400 consumes less than 50 µA
at room temperature. This mode allows a DNT2400 to be powered off while its host device remains
powered. After leaving hardware sleep mode, the radio must re-synchronize with the base and re-
register.
In addition to the sleep mode controlled by Pin 11, in CSMA mode the DNT2400 remotes support an
additional sleep mode to support battery-powered applications. When this mode is enabled, the DNT2400