Murata WIT2420 Quick setup guide

WIT2420
2.4GHz Spread Spectrum
Wireless Industrial Transceiver
Integration Guide
June 2nd 2021
Note: This unit has been tested and found to
comply with the limits for a class Adigital 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
instruction 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 his
own expense.

The modular transmitter is only FCC authorized for the specific rule parts listed on the grant;
when it is installed in a host device, the host product manufacturer is responsible for
compliance to any other FCC rules that apply to the host not covered by the modular
transmitter grant of certification. The final host product still requires Part 15 Subpart B
compliance testing with the modular transmitter installed.
If the WIT2420 is installed within another device the outside of the device into which the
WIT2420 is installed must also display a label referring to the enclosed module. The label
required for the WIT2420 module is as follows: Contains FCC ID: HSW-2420 and Contains
IC: 4492A-2420.
This device complies with Part 15 of the FCC Rules. Operation is subject to the following two
(1) This device may not cause harmful interference, and (2) this device must accept any
interference received, including inerference that may cause undesired operation.
WARNING: changes not expressly approved by the party responsible for compliance could
void the user’s authority to operate this device.
WARNING: a 20cm separation distance must be maintained between this device and the user.
This device complies with Industry Canada license-exempt RSS standard(s). Operation is
subject to the following two conditions: (1) this device may not cause interference, and (2) this
device must accept any interference, including interference that may cause undesired operation
of the device. Le présent appareil est conforme aux CNR d'Industrie Canada applicables aux
appareils radio exempts de licence. L'exploitation est autorisée aux deux conditions suivantes :
(1) l'appareil ne doit pas produire de brouillage, et (2) l'utilisateur de l'appareil doit accepter
tout brouillage radioélectrique subi, même si le brouillage est susceptible d'en compromettre le
fonctionnement.
About This Manual
This manual is designed to allow integration of the Murata WIT2420 OEM module into
complete products. Care has been taken to try and make sure all of the information in this
manual is accurate. However, specifications can change over time and Murata cannot
guarantee the accuracy of this information. If you have any questions on any information in
this manual, please contact Murata Technical Support at [phone].

TABLE OF CONTENTS
1. INTRODUCTION ...................................................................................................................................................1
1.1 Why Spread Spectrum?...................................................................................................................................1
1.2 Frequency Hopping vs. Direct Sequence ........................................................................................................2
2. RADIO OPERATION.............................................................................................................................................4
2.1. Synchronization and Registration ..................................................................................................................4
2.2. Data Transmission .........................................................................................................................................5
2.2.1. Point-to-Point.......................................................................................................................................5
2.2.2. Point-to-Multipoint................................................................................................................................6
2.2.3. TDMA Mode.........................................................................................................................................6
2.2.4. Full Duplex Communication.................................................................................................................8
2.2.5. Error-free Packet Transmission Using ARQ........................................................................................8
2.3. Modes of Operation........................................................................................................................................9
2.3.1. Control and Data Modes......................................................................................................................9
2.3.2. Sleep Mode..........................................................................................................................................9
2.3.3. Low Power Mode and Duty Cycling.................................................................................................. 10
2.3.4. RF Flow Control Mode....................................................................................................................... 10
3. PROTOCOL MODES ......................................................................................................................................... 11
3.1. Packet Formats........................................................................................................................................... 13
3.1.1. Data Packet ...................................................................................................................................... 13
3.1.3. Connect Packet................................................................................................................................. 14
3.1.4. Disconnect Packet (base only, receive only)................................................................................... 14
4. MODEM INTERFACE ........................................................................................................................................ 15
4.1. Interfacing to 5 Volt Systems ...................................................................................................................... 16
4.2 Evaluation Unit and OEM Module Differences............................................................................................ 16
4.3 Three Wire Operation.................................................................................................................................. 16
4.4 Power-On Reset Requirements.................................................................................................................. 17
5. MODEM COMMANDS ....................................................................................................................................... 18
5.1. Serial Commands........................................................................................................................................ 18
5.2. Network Commands.................................................................................................................................... 19
5.3. Protocol Commands.................................................................................................................................... 21
5.4. Status Commands....................................................................................................................................... 24
5.5. Memory Commands.................................................................................................................................... 25
5.6. Modem Command Summary...................................................................................................................... 26
6. WIT2420 DEVELOPER’S KIT ............................................................................................................................ 27
6.1. COM24/WinCOM24 .................................................................................................................................... 27
6.2. Demonstration Procedure........................................................................................................................... 28
6.3. Troubleshooting .......................................................................................................................................... 29
7. APPENDICES..................................................................................................................................................... 31
7.1. Technical Specifications.............................................................................................................................. 31
7.1.1 Ordering Information.......................................................................................................................... 31
7.1.2. Power Specifications ........................................................................................................................ 31
7.1.3. RF Specifications.............................................................................................................................. 31
7.1.4. Mechanical Specifications ................................................................................................................ 31
7.2. Serial Connector Pinouts ............................................................................................................................ 32
7.3. Approved Antennas..................................................................................................................................... 32
7.4. Technical Support ....................................................................................................................................... 32
7.5. Reference Design ....................................................................................................................................... 33
7.6. Mechanical Drawing......................................................................................Error! Bookmark not defined.
7.7. Warranty........................................................................................................Error! Bookmark not defined.

© 1999, 2000 MurataCorporation 1 01/11/00
1. INTRODUCTION
The WIT2420 radio transceiver provides reliable wireless connectivity for either
point-to-point or multipoint applications. Frequency hopping spread spectrum technology
ensures maximum resistance to noise and multipath fading and robustness in the presence of
interfering signals, while operation in the 2.4GHz ISM band allows license-free use and
worldwide compliance. A simple serial interface supports asynchronous data up to 230400
bps. An on-board 3 KB buffer and an error-correcting over-the-air protocol provide smooth
data flow and simplify the task of integration with existing applications.
-Multipath fading impervious
frequency hopping technology
with 75 frequency channels
(2401-2475 MHz).
- Supports point-to-point or
multipoint applications.
- Meets FCC rules 15.247 and ETS
300.328 for worldwide license-
free operation.
- Superior range to 802.11 wireless
LAN devices.
- Transparent ARQ protocol
w/3KB buffer ensures data
integrity.
- Digital addressing supports up to
64 networks, with 62 remotes per
network.
- Low power 3.3v CMOS signals
- Simple serial interface handles both
data and control at up to 230400
bps.
- Fast acquisition typically locks to
hopping pattern in 2 seconds or less.
- Selectable 10 mW or 100 mW
transmit power.
- Support for diversity antenna.
- Built-in data scrambling reduces
possibility of eavesdropping.
- Nonvolatile memory stores
configuration when powered off.
- Smart power management features
for low current consumption.
- Dynamic TDMA slot assignment
that maximizes throughput.
1.1 Why Spread Spectrum?
The radio transmission channel is very hostile, corrupted by noise, path loss and
interfering transmissions from other radios. Even in a pure interference-free
environment, radio performance faces serious degradation through 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 phase, thereby
partially or completely canceling the desired signal. This is a problem particularly
prevalent in indoor installations. In the frequency domain, 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 2.4 GHz band. This means
that from a probabilistic viewpoint, a conventional radio system faces a 1% - 2% chance
of signal impairment at any given time due to multipath.

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Spread spectrum reduces the vulnerability of a radio system to both interference from
jammers and multipath fading by distributing the transmitted signal over a larger region
of the frequency band than would otherwise be necessary to send the information. This
allows the signal to be reconstructed even though part of it may be lost or corrupted in
transit.
Figure 1
Narrowband vs. spread spectrum in the presence of interference
1.2 Frequency Hopping vs. Direct Sequence
The two primary approaches to spread spectrum are direct sequence (DS) and frequency
hopping (FH), either of which can generally be adapted to a given application. 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 baseband data is called the processing gain, and is equal to the
amount of rejection the system affords against narrowband 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 channels produces a
frequency hopping spectrum system.

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Figure 2
Forms of spread spectrum
One disadvantage of direct sequence systems is that due to spectrum constraints and the
design difficulties of broadband receivers, they generally employ only a minimal amount
of spreading (typically no more than the minimum required by the regulating agencies).
For this reason, the ability of DS systems to overcome fading and in-band jammers is
relatively weak. By contrast, FH systems are capable of probing the entire band if
necessary to find a channel free of interference. Essentially, this means that a FH
system will degrade gracefully as the channel gets noisier while a DS 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, FH is often the preferred
choice for co-located systems. Since direct sequence signals are very wide, they tend to
offer few non-overlapping channels, whereas multiple hoppers may interleave with less
interference. Frequency hopping does carry some disadvantage in that as the transmitter
cycles through the hopping pattern it is nearly certain to visit a few blocked channels
where no data can be sent. If these channels are the same from trip to trip, they can be
memorized and avoided; unfortunately, this is generally not the case, as it may take
several seconds to completely cover the hop sequence during which time the multipath
delay profile may have changed substantially. To ensure seamless operation throughout
these outages, a hopping radio must be capable of buffering its data until a clear channel
can be found. A second consideration of frequency hopping systems is that they require
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 seconds. 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.
As an additional benefit, RF spectrum has been set aside at 2.4 GHz in most countries
(including the U.S.) for the purpose of allowing compliant spread spectrum systems to
operate freely without the requirement of a site license. This regulatory convenience
alone has been a large motivation for the industry-wide move toward spread spectrum.

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2. RADIO OPERATION
2.1. Synchronization and Registration
As discussed above, frequency hopping radios 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 over which the current transmission is being sent. To do
this, all the radios in the net must be synchronized and must be set to the same hopping
pattern. All radios in a net must be set to the same hopping pattern before attempting to
communicate.
In point-to-point or point-to-multipoint arrangements, one radio module is designated as the
base station. All other radios are designated remotes. One of the responsibilities of the base
station is to transmit a synchronization signal to the remotes to allow them to synchronize
with the base station. Since the remotes know the hopping pattern, once they are
synchronized with the base station, they know which frequency to hop to and when. Every
time the base station 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 station is transmitting over 75 frequencies and the remote is scanning
75 frequencies, it can take several seconds for a remote to synch up with the base station.
Once a remote has synchronized with the base station, it must request registration from the
base station. The registration process identifies to the base station the remotes from which
transmissions will be received and not discarded. Registration also allows tracking of
remotes entering and leaving the network. The base station builds a table of serial numbers
of registered remotes. To improve efficiency, the 24-bit remote serial number is assigned a
6-bit “handle” number. Two of these are reserved for system use, thus each base station can
register 62 separate remotes. This handle is how user applications will know the remotes. If
necessary, the automatic handle assignment can be overridden to explicitly tie certain handles
to certain remotes. See the section on Network Commands for details on the Set Default
Handle command.
To detect if a remote has gone offline or out of range, the registration must be “renewed”
once every 256 hops. Registration is completely automatic and requires no user application
intervention. When the remote is registered, it will receive several network parameters from
the base. This allows the base to automatically update these network parameters in the
remotes over the air. Once a parameter has been changed in the base, it is automatically
changed in the remotes. The parameters automatically changed are hop duration and the
duty cycle.
At the beginning of each hop, the base station transmits a synchronizing signal. After the
synchronizing signal has been sent, the base will transmit any data in its buffer unless data
transmit delay has been set. The data transmit delay parameter allows for the transmission
of groups of continuous data in transparent mode (protocol mode 0). The amount of data that

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the base station can transmit per hop is determined by the base slot size parameter. The
maximum amount of data sent by a base station per hop is 192 bytes. If there is no data to be
sent, the base station will not transmit until the next frequency.
The operation for remotes is similar to the base station without the synchronizing signal. The
amount of data a remote can send on one hop is dependent upon the hop duration, the base
slot size and the number of registered remotes. 212 bytes per hop is the maximum data length
a remote can transmit per hop, subject to limitations imposed by the hop duration, the base
slot size and the number of registered remotes. A detailed explanation of this relationship is
provided in Section 2.2.3. Minimum data length and data transmit delay operate the same as
with the base station.
Except for the registration process which occurs only when a remote logs onto the network,
the whole procedure is repeated on every frequency hop. Refer to the section on Modem
Commands for complete details on parameters affecting the transmission of data.
2.2. Data Transmission
The WIT2420 supports two network configurations: point-to-point and point-to-multipoint.
In a point-to-point network, one radio is set up as the base station 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 station acting as the central communications point and all other radios in the network
set up as remotes. In this configuration, all communications take place between the base
station and any one of the remotes. Remotes cannot communicate directly with each other.
It should be noted that point-to-point mode is a subset of point-to-multipoint mode and
therefore there is no need to specify one mode or the other.
2.2.1. Point-to-Point
In point-to-point mode, unless data transmit delay or minimum data length have been set, the
base station will transmit whatever data is in its buffer limited to 192 bytes or as limited by
the base slot size. If the base station has more data than can be sent on one hop, the
remaining data will be sent on subsequent hops. In addition to the data, the base station adds
some information to the transmission over the RF link. It adds the address of the remote to
which it is transmitting, even though in a point-to-point mode there is only one remote. It
also adds a sequence number to identify the transmission to the remote. This is needed in the
case of acknowledging successful transmissions and retransmitting unsuccessful
transmissions. Also added is a 24-bit CRC to allow the base to check the received
transmission for errors. When the remote receives the transmission, it will acknowledge the
transmission if it was received without errors. If no acknowledgment is received, the base
station will retransmit the same data on the next frequency hop.
In point-to-point mode, a remote will transmit whatever data is in its buffer up to the limit of
its maximum data length. If desired, minimum data length and data transmit delay can also
be set, which force the remote to wait until a certain amount of data is available or the

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specified delay is exceeded before transmitting. If the remote has more data than can be sent
on one hop, it will send as much data as possible as a packet, adding its own address, a
packet sequence number and 24-bit CRC. These additional bytes are transparent to the user
application if the protocol mode is 00 (which is the default). In the event a remote has more
data to send, the data will be sent on subsequent hops. If the transmission is received by the
base station without errors, the base station will acknowledge the transmission. If the remote
does not receive an acknowledgment, it will retransmit the data on the next frequency hop.
To the user application, acknowledgments and retransmissions all take place behind the
scenes without the need for user intervention.
2.2.2. Point-to-Multipoint
In point-to-multipoint mode, data sent from the user application to the base station must be
packetized by the user application unless the remote device can distinguish between
transmissions intended for it and transmissions intended for other remote devices. This is
necessary to identify the remote to which the base station should send data. When the user
packet is received by the remote, if the remote is in transparent mode (protocol mode 0), the
packetization bytes are stripped by the remote. In this instance the remote host receives just
data. If the remote is not in transparent mode, the remote host will receive the appropriate
packet header as specified by the remote’s protocol mode. Refer to the section Protocol
Modes for details on the various packet formats.
When a remote sends data to a base station in point-to-multipoint mode, the remote host does
not need to perform any packetization of the data. Remotes can operate in transparent mode
even though the base is operating in a packet mode. The remote will add address, sequence
and CRC bytes as in the point-to-point mode. When the base station receives the data, the
base station will add packetization header bytes according to its protocol mode setting.
2.2.3. TDMA Mode
For applications needing guaranteed bandwidth availability, the TDMA mode of the
WIT2420 can meet this requirement. In TDMA mode, each remote has an assigned time slot
during which it can transmit. The base station time slot is set independently of the remote
time slots through the Set Base Slot Size command. The base station assigns each remote a
time slot and informs the remotes of the size of the time slot. All remote time slots are the
same size that is determined by the number of remotes registered with the base station. The
slot size is a dynamic variable that changes as the number of registered remotes changes.
The remotes are continually updated with the time slot size. This approach continually
maximizes the data throughput. The base station divides the amount of time available per
hop by the number of registered remotes up to a maximum of 16 times slots per hop. If the
number of registered remotes is greater than 16, the time slots will be spread across the
required number of hops. For networks with more than 16 possible remotes, the Set Duty
Cycle command must be used to specify a duty cycle -- the number of hops over which the
time slots must be spread. For 1 to 16 remotes, no duty cycle is required; for 17 to 32
remotes a duty cycle of at least ½ is required; and for 33 to 62 remotes a duty cycle of ¼ or

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more is necessary. An added benefit of using the power save mode to set a duty cycle is
improved average current consumption efficiency. Refer to the Status Commands section for
details of this command.
When setting up a TDMA network, keep in mind that time slot length, maximum packet size
and hop duration are all interrelated. The hop duration parameter will determine the time
slot size and the maximum amount of data that can be transmitted per hop by the remotes.
There is a hard limit of the absolute maximum amount of data that can be sent on any given
hop of 212 bytes regardless of any parameters. The base station requires 1.7 ms overhead for
tuning, the synchronization signal and parameter updating, as well as a guard time of 500s
between each remote slot. Thus the amount of time allocated per remote slot is roughly:
hop duration –base slot –1.7ms - ( # of registered remotes-1)∙500s
( # of registered remotes)
Take for example a network comprised of a base station and 10 remotes. A hop duration of
10 ms is chosen. We decide that the base station needs to be able to send up to 32 bytes each
hop (equivalent to a capacity for the base of ~ 32 kbps). Counting the 1.7 ms overhead for
the base packet and making use of the fact that our RF rate is 460.8 kbps, we determine that
the base slot requires approximately:
Each remote time slot will be:
10 ms –2.3 ms –(9)∙0.5 ms
10
From our RF data rate of 460.8kbps we see that it takes 17.36 s to send a byte of data, so
each remote will be able to send up to
= 18 bytes of data per hop.
Note that the 18 bytes is the actual number of data bytes that can be sent. If the WIT2420 is
using a protocol mode, the packet overhead does not need to be considered. So in this
example, the total capacity per remote would be:
If we figure a minimum margin of safety for lost packets and retransmissions of about 20%,
we see that this would be more than sufficient to support 14.4 kbps of continuous data per
remote. It is also useful to remember that the asynchronous data input to the WIT2420 is
stripped of its start and stop bits during transmission by the radio, yielding a "bonus" of 10/8
or 25% in additional capacity.
= 0.32 ms
0.32 ms
17.36s
us
32∙8
460.8kbps
+ 1.7 ms = 2.3 ms
18 bytes
10 ms
= 18 kbps

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The above calculations are provided as a means of estimating the capacity of a multipoint
WIT2420 network. To determine the precise amount of capacity, you can actually set up the
radio system and then query the maximum data length from one of the remotes in control
mode to discover its exact setting. Divide this number by the hop duration as above to get
the remote's exact capacity.
2.2.4. Full Duplex Communication
From an application perspective, the WIT2420 communicates in full duplex. That is, both
the user application and the remote terminal can be transmitting data without waiting for the
other to finish. At the radio level, the base station and remotes do not actually transmit at the
same time. If they did, the transmissions would collide. As discussed earlier, the base
station transmits a synchronization signal at the beginning of each hop followed by up to
three packets of data. After the base station transmission, the remotes will transmit. Each
base station and remote transmission may be just part of a complete transmission from the
user application or the remote terminal. Thus, from an application perspective, the radios are
communicating in full duplex mode since the base station will receive data from a remote
before completing a transmission to the remote.
2.2.5. Error-free Packet Transmission Using ARQ
The radio medium is a hostile environment for data transmission. In a typical office or
factory environment, 1% - 2% of the 2.4GHz frequency band may be unusable at any given
time at any given station due to noise, interference or multipath fading. For narrowband
radio systems (and also many spread spectrum radio systems which use direct sequence
spreading), this would imply a loss of contact on average of over 30 seconds per hour per
station. The WIT2420 overcomes this problem by hopping rapidly throughout the band in a
pseudo-random pattern. If a message fails to get through on a particular channel, the
WIT2420 simply tries again on the next channel. Even if two thirds of the band are
unusable, the WIT2420 can still communicate reliably.
Data input to the WIT2420 is broken up by the radio into packets. A 24-bit checksum is
attached to each packet to verify that it was correctly received. If the packet is received
correctly, the receiving station sends an acknowledgment, or ACK, back to the transmitting
station. If the transmitter doesn't receive an ACK, at the next frequency hop it will attempt to
send the packet again. When ARQ is enabled, the transmitting radio will attempt to send a
packet packet attempts limit times before discarding the packet. A value of 00H disables
ARQ. When it is disabled, any transmission received with errors is discarded. It is the
responsibility of the user application to track missing packets. A second parameter, ARQ
Mode, allows the choice between using ARQ to resend unsuccessful transmissions or always
sending a transmission packet attempts limit times regardless of the success or failure of any
given transmission.

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All of this error detection and correction is transparent to the user application. All the user
application sees is error-free data from the modem. However, if the ARQ mode is disabled,
transmissions with errors are discarded, and missing data detection will be the responsibility
of the user application. Refer to the Protocol Commands section for complete details.
2.3. Modes of Operation
2.3.1. Control and Data Modes
The WIT2420 has two modes of operation: Control mode and Data mode. When in Control
Mode, the various radio and modem parameters can be modified. When in Data Mode, only
data can be transmitted. The default mode is Data Mode. There are two ways to enter
Control Mode. The first way is to assert the Configure (CFG) pin on the modem. Upon
entering Control Mode, the modem will respond with a >prompt. After each command is
entered, the modem will again respond with a >prompt. As long as the CFG pin is asserted,
data sent to the modem will be interpreted as command data. Once the CFG pin is
deasserted, the modem will return to Data Mode.
The second method for entering Control Mode is to send the escape sequence :WIT2420 (all
lower case) followed by a carriage return. In the default mode, the escape sequence is only
valid immediately after power up or after deassertion of the Sleep pin on the modem. The
modem will respond in the same way with a >prompt. To return to Data Mode, enter the
Exit Modem Control Mode command, z>, or assert and deassert the Sleep pin. There are
three modes for the escape sequence, controlled by the Set Escape Sequence Mode command,
zc:
zc = 0 Escape sequence disabled
zc = 1 Escape sequence available once at startup (default setting)
zc = 2 Escape sequence available at any time
The zc2 mode setting is useful if the user application has a need to change the modem
settings "on the fly". In this mode the escape sequence is always enabled and may be sent at
any time after a pause of at least 20ms. The modem will respond in the same way as when in
the default mode. It is necessary to issue the Exit Modem Control Mode command, z>, before
resuming data transmission. The escape sequence must be interpreted as data until the last
character is received and as such may be transmitted by the modem to any listening modems.
2.3.2. Sleep Mode
To save power consumption for intermittent transmit applications, the WIT2420 supports a
Sleep Mode. Sleep Mode is entered by asserting the Sleep pin on the modem interface.
While in Sleep Mode, the modem consumes less than 50µA. This mode allows the radio to
be powered off while the terminal device remains powered. After leaving Sleep Mode, the
radio must re-synchronize with the base station and re-register.

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2.3.3. Low Power Mode and Duty Cycling
To conserve power, WIT2420 remotes power down the receiver and transmitter between
hops when not in use. Base stations must remain active all the time to handle any
transmission from any remote. Remotes can save even more power by enabling the duty
cycle feature. This feature causes a remote to power down for 2Nfrequency hops where 1/2N
is the duty cycle. Rather than attempting to transmit on every frequency hop when data is in
the transmit buffer, a remote will attempt to transmit only every 2Nhops. Roughly speaking,
this will proportionately reduce the average power consumption while increasing average
latency. When there are more than 16 remotes being operated in TDMA mode, duty cycling
must be enabled since a maximum of 16 time slots is available per hop.
2.3.4. RF Flow Control Mode
Because of slight differences in baud rates between transmitting and receiving hosts, when
sending large amounts of data (100’s of KB) in one direction in a point-to-point application,
it is possible to overrun the receive buffer of the receiving radio. For example a nominal
115.2Kbaud at the transmitting radio’s host might really be 115,201 and at the receiving
radio’s host it might be 115,199. This is similar to a situation where the transmitting radio is
sent data at a higher baud rate than the baud rate at which data is received by the receiving
host. To compensate for the variations in nominal baud rates, the WIT2420 supports an RF
flow control mode for point-to-point operation. In this mode, when the receive buffer of the
receiving WIT2420 is close to full, the receiving WIT2420 stops acknowledging
transmissions. The transmitting radio is set to infinite retries which invokes the RF flow
control mode (See Set Packet Attempts Limit in Section 5.3). The receiving radio will not
begin acknowledging transmissions from the transmitting radio until more room in the
receive buffer has become available. This will cause data in the transmit buffer of the
transmitting radio to back up. If it backs up to the point where the transmit buffer fills up, the
transmitting radio will deassert CTS stopping data from the transmitting radio’s host device.
Once room is available in the receiving radio’s buffer, the receiving radio will begin
acknowledging transmissions from the transmitting radio allowing the transmitting radio’s
buffer to begin to empty which will cause the transmitting radio to reassert CTS. Either one
or both of the radios in a point-to-point installation can be configured for the RF flow
control. If this mode is invoked in a point-to-multipoint installation, communications with all
radios will be stopped when any one radio’s receive buffer becomes full.

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3. PROTOCOL MODES
In point-to-point applications, it is generally desired that the radios operate in a transparent
mode. That is, raw unformatted data is sent from the host to the radio and is received as raw
data from the receiving end. The addressing and error detection and correction are still
performed by the radios, but it is transparent to the user application. To set up a point-to-
point network, one radio has to be set up as a base station. When the radios are powered on,
the base station will send out the synchronization signal at the beginning of each hop. The
remote will synchronize with the base and automatically request registration. Once the
remote is registered, the radios can transmit data. Protocol mode operation is available in
point-to-point mode if desired.
If the base station is to be responsible for directing data to a specific remote in point-to-
multipoint mode, the data sent to the base station by the user application must adhere to a
packet format. This allows transmissions from the base station to be directed to a specific
remote. Data received by a base station from a remote is similarly formatted to identify to
the user application the remote that sent the transmission. The remotes may still use
transparent mode without formatting to send data to the base, if desired. The WIT2420
supports 10 protocol formats that are described in detail below. The protocol format is
selected through the Set Protocol Mode command.
Base and remote radios can use protocol modes to insure that a packet is transmitted to the
base without being broken up over multiple hops. The data length value in the data packet
becomes the effective minimum packet length and maximum packet length for that packet.
Note that if the remote data length is set to a number of bytes that is longer than the number
of bytes that can be transmitted by a remote on a single hop, the packet will be discarded. For
the base, this value is set by the Set Base Slot Size command. For remotes this value is
dynamically available through the Get Maximum Data Length command or may be
calculated based on the maximum number of remotes that can ever be registered at one time.
See Sections 5.3 and 2.2.3 respectively. Also note that using protocol modes effectively
disables Data Transmit Delay. This means that a packet will not be transmitted until the
entire packet has been sent to the radio, regardless of the amount of time it takes.
If the remote hosts can determine what data is directed to them in point-to-multipoint mode,
the data can be sent to the base station without using a packet format. In this situation,
broadcast mode is selected at the base station by using the Set Default Handle and selecting
3FH as the default handle. In this mode, the automatic retransmission of unsuccessful
transmissions is disabled. This is required since all of the remote modems will attempt to
acknowledge each base transmission when ARQ is enabled. Transmissions that are received
with errors are discarded by the radio. The remote devices must be able to detect a missing
packet and request a retransmission by the base device.

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Protocol Modes Definitions
mode 00 Transparent mode used for point-to-point networks or
multipoint remotes; does not support any packet types.
mode 01 This is the simplest protocol mode supporting Data
packets only. No CONNECT or DISCONNECT packets
are supported and no sequence numbers are provided.
packet types supported: Data
mode 02 This mode includes notification when remotes are
registered or dropped through CONNECT and
DISCONNECT packets that are sent to the user
application at the base station and at the remote. No
sequence numbers are provided.
packet types supported: Data
CONNECT
DISCONNECT
mode 04 This is the packet format used by the WIT2400. This
allows legacy software to operate the WIT2420. Note
however, that since different air data rates are used,
WIT2420s and WIT2400s cannot be mixed in a
network.
packet types supported: 2400 data format
(addresses must be limited to 1..62)
modes 05 –08 reserved for future use.
mode 09 This mode sends the protocol mode 01 packets during
transmit but receives data transparently.
mode 0A This mode sends the protocol mode 02 packets during
transmit but receives data transparently.
mode 0C This mode sends the protocol mode 04 packet during
transmit but receives data transparently.
modes 0D –0F reserved for future use.
mode 11 This mode sends data transparently but supports
protocol mode 1 during reception.
mode 12 This mode sends data transparently but supports
protocol mode 2 during reception.
mode 14 This mode sends data transparently but supports
protocol mode 4 during reception.

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3.1. Packet Formats
The byte formats for each packet type are shown in the table below. Packet fields are
organized to fall on byte boundaries. In the case of bit-level fields, most-significant bits are
on the left.
WIT2400 packet type (mode 04):
Base DATA 0000 0010 00HH HHHH LLLL LLLL <0-212 bytes data> 0000 0011
Remote DATA 0000 0010 0000 0000 LLLL LLLL <0-212 bytes data> 0000 0011
MRTP (WIT2420) packet types (modes 01-03):
Transmit and Receive:
Base DATA 1110 1001 00HH HHHH LLLL LLLL <0-212 bytes data>
Remote DATA 1110 1001 0000 0000 LLLL LLLL <0-212 bytes data>
Receive only:
CONNECT 1110 1001 10HH HHHH RRRR TTTT 00NN NNNN <3 byte remote ID>
DISCONNECT 1110 1001 11HH HHHH 0111 1111
H: handle number (0-63)
L: data length (0-212)
N: remote's previous network number (if roamed)
R: receive sequence number (from previous cell)
T: transmit sequence number (from previous cell)
Note that while the packet length can be set to 212, the maximum number of bytes transmitted
per hop is limited to the lesser of 212 or the length specified by maximum data length. Packets
with a data length longer than that will be discarded and not sent. See Get Maximum Data
Length for more details.
3.1.1. Data Packet
Modes 01 & 02:
Base 1110 1001 00HH HHHH LLLL LLLL <0-212 bytes data>
Remote 1110 1001 0000 0000 LLLL LLLL <0-212 bytes data>
Mode 04 (WIT2400):
Base 0000 0010 00HH HHHH LLLL LLLL <0-212 bytes data> 0000 0011
Remote 0000 0010 0000 0000 LLLL LLLL <0-212 bytes data> 0000 0011
H: handle number (0-63)
L: data length (0-212)
This packet carries user data. The handle number is the handle of the receiving remote.
When data is being sent from a remote to the base, no handle number is required. Up to 212
bytes of user data may be carried per data packet but no more than is specified by the
maximum data length parameter. The radio will not break up a packet over multiple hops.
Packets with a data length greater than maximum data length will not be sent and will be
discarded. This parameter is variable and depends on the number of remotes currently
registered.

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Handle 63 is reserved for broadcast packets from the base to all remotes. Acknowledgment
requests are not supported for broadcasts. For this reason, it is a good idea to send broadcast
messages several times to increase the odds of reaching all remotes.
3.1.3. Connect Packet
1110 1001 10HH HHHH RRRR TTTT 00NN NNNN <3-byte remote ID> (base, receive only)
H: handle number (0-62)
R: receive sequence number (from previous cell)
T: transmit sequence number (from previous cell)
N: network number of the previous base (if roamed)
1110 1001 10HH HHHH RRRR TTTT 00NN NNNN <3-byte base ID> (remote, receive only)
H: handle number (0-62)
R: receive sequence number
T: transmit sequence number
N: network number of base
Remotes must go through an automatic registration process when roaming from one base to
another, after loss of contact, or when acquiring a base signal for the first time after power
up. The base then assigns the remote a handle value, may or may not assign it a dedicated
time slice depending on the user settings, and notifies the user application of the new remote
with a connect packet.
The network number of the last base the remote was connected to is given to aid user
software in resending orphan packets that may have been sent to the remote's previous cell.
If the remote has been powered up for the first time and this is the first base contacted, the
last base ID will be reported as 80H.
3.1.4. Disconnect Packet (base only, receive only)
1110 1001 11HH HHHH 0111 1111
H: handle number (1-62)
When a remote goes out of range or roams to another cell, the base issues a disconnect packet
to indicate that the remote is no longer available.

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4. MODEM INTERFACE
Electrical connection to the WIT2420 is made through a 16-pin male header on the modem
module. The signals are 3.3 volt signals and form an RS-232 style asynchronous serial
interface. The table below provides the connector pinout.
Pin
Signal
Type
Description
1
GND
-
Signal and chassis ground
2
TXD
Input
Transmit data. Input for serial data to be transmitted. In Control
Mode also used to transmit modem commands to the modem.
3
RXD
Output
Receive data. Output for received serial data. In Control Mode,
also carries receive modem status from the modem.
4
Input
Configuration selector. Used to switch between Control and Data
Modes. Normally, CFG will be set for Data Mode. An internal 10K
pull-up enables Data Mode if this signal is left unconnected.
Control Mode is also accessible by transmitting an escape
sequence immediately after wake up or power up.
(0v) 1 = Control Mode
(3.3v) 0 = Data Mode
5
Input
Request to send. Gates the flow of receive data from the radio to
the user on or off. In normal operation this signal should be
asserted. When negated, the WIT2420 buffers receive data until
RTS is asserted.
(0v) 1 = Receive data (RxD) enabled
(3.3v) 0 = Receive data (RxD) disabled.
6
SLEEP
Input
Sleeps/wakes radio transceiver. In sleep mode all radio functions
are disabled consuming less than 50µA. At wake up, any user
programmed configuration settings are refreshed from non-volatile
memory, clearing any temporary settings that may have been set.
(3.3v) 1 = Sleep Radio
(0v) 0 = Wake Radio
7
Output
Data carrier detect. For remotes, indicates the remote has
successfully acquired the hopping pattern of the base station.
(0v) 1 = Carrier detected (synchronized)
(3.3v) 0 = No carrier detected (not synchronized)
8
Output
Clear to send. Used to control transmit flow from the user to the
radio.
(0v) 1 = Transmit buffer not full, continue transmitting
(3.3v) 0 = Transmit buffer full, stop transmitting
9
-
-
Reserved for future use. Do not connect.
10
Input
Resets the radio.
11-15
-
-
Reserved for future use. Do not connect.
16
VCC
-
Positive supply. Min 3.3 v, 5.0 v nominal, 10.0 v max.
RTS
DCD
CTS
CFG
Reset

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4.1. Interfacing to 5 Volt Systems
The modem interface signals on the WIT2420 are 3.3 volt signals. To interface to 5 volt
signals, the resistor divider network shown below must be placed between the 5 volt signal
outputs and the WIT2420 signal inputs. The output voltage swing of the WIT2420 3.3 volt
signals is sufficient to drive 5 volt logic inputs.
4.2 Evaluation Unit and OEM Module Differences
The evaluation unit has an RS-232 transceiver that translates RS-232 level signals to 3.3 volt
signals for input into the OEM module inside the evaluation unit. A typical schematic is
shown in Appendix 7.5. The OEM module does not have any type of RS-232 transceiver and
cannot handle the RS-232 voltages. This allows the OEM module to be easily integrated into
any 3.3 volt system without any logic signal translation. In order for the OEM module to
function properly several pins need to be driven low or tied to ground. Pin 5 (RTS) and pin 6
(SLEEP) need to be pulled to ground on the 16-pin male header. If you have the OEM
module interfaced to an RS-232 transceiver, RTS and DTR need to be pulled high on the
transceiver side. In the evaluation unit, RTS and DTR are pulled high on the transceiver side
so the evaluation unit will work with these signals not connected.
4.3 Three Wire Operation
The WIT2420 can be operated in a three wire configuration using just TxD, RxD and
Ground. To operate the WIT2420 in this configuration, the Sleep and RTS signals must be
tied to ground. These signals are pulled up on the WIT2420 module and if left disconnected
will put the radio into sleep mode and RTS will be deasserted.
The WIT2420 does not support software flow control (XON/XOFF). Thus when using a
three wire configuration, there is no flow control. The radio configuration and/or the
application must insure the transmit and receive buffers do not overflow. The WIT2420 has a
2048-byte transmit buffer and a 1024-byte receive buffer. For example, the default settings
for the base slot size and hop duration are 08H and 90H respectively. The 08H base slot size
allows the base to send 32 bytes of data per hop. The 90H hop duration provides a 10ms hop
dwell time. These default settings provide a base throughput of 40kbps (Since the over the air
transmission is synchronous, the 32kbps synchronous over the air rate is equivalent to 40kbps
10 k
20 k
From 5v
Output
To 3.3v Input

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asynchronous into the radio serial port). If the base transmits continuously at a higher rate
than this, unless the default settings are changed, the transmit buffer will eventually
overflow. To allow a higher base throughput, either increase the base slot size or the hop
duration or both. A similar analysis needs to be performed for the remote radios. Refer to
Section 2.2.3 TDMA Mode for the remote throughput calculation.
4.4 Power-On Reset Requirements
The WIT2420 has an internal reset circuit that provides 100ms for power to the module to be
applied and stabilized as well as removed. If the host cannot guarantee that power will be
within the specified voltage range within 100ms of power being applied and be completely
removed within 100ms of dropping below 3.3 volts, the host must apply a Reset signal to the
module on pin 10 of the power/data connector. The operation of the microprocessor in the
WIT2420 is undefined for voltages below 2.7 volts. If such supply voltages are present for
more than 100ms, non-volatile configuration and program parameters can be overwritten and
corrupted. This is particularly important in battery operated systems where a low battery
condition may present a “brown out” voltage for an indefinite period of time. The circuit
schematic below will provide a Reset signal to the module whenever the supply voltage falls
below 2.98 volts. Note that the radio is not specified to operate below 3.3 volts.
JP1
WIT2410
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
This example is only for VCC voltages between 3.3V and 5.5V
U1
DS1816A
1
2
3RST
VCC
GND
VCC