Microhard Systems MHX-920 User manual

Operating Manual
MHX-920
900 MHz Spread Spectrum OEM Transceiver
Revision 1.20, December 21, 2000
Microhard Systems Inc.
#110, 1144 - 29th Ave. N.E.
Calgary, Alberta T2E 7P1
Phone: [phone]
Fax: [phone]
www.microhardcorp.com

ii MHX-920 Operating Manual
MHX-920
900 MHz
Spread-Spectrum
Embedded Modem
WARNING
In order to comply with the FCC/IC
adopted RF exposure requirements, this
transmitter system will be installed by the
manufacturer's reseller professional.
Installation of all antennas must be
performed in a manner that will provide at
least 23 cm clearance from the front
radiating aperture, to any user or member
of the public.
EQUIPMENT LABELING
The manufacturer, product name, and FCC
and Industry Canada identifiers of this
product must appear on the outside label of
the end-user equipment.
l
This manual contains information of proprietary interest to
Microhard Systems Inc. It has been supplied in confidence to
purchasers and users of the MHX-920, and by accepting this
material the recipient agrees that the contents will not be copied
or reproduced, in whole or in part, without prior written consent
of Microhard Systems Inc.
Microhard Systems Inc. has made every effort to assure that this
document is accurate and complete. However, the company
reserves the right to make changes or enhancements to the
manual and/or the product described herein at any time and
without notice. Furthermore, Microhard Systems Inc. assumes
no liability resulting from any omissions in this document, or
out of the application or use of the device described herein.
Microhard Systems’ products are appropriate for home, office,
or industrial use, but are not authorized for utilization in
applications where failure could result in damage to property or
human injury or loss of life.
The electronic equipment described in this manual generates,
uses, and radiates radio frequency energy. Operation of this
equipment in a residential area may cause radio interference, in
which case the user, at his own expense, will be required to take
whatever measures necessary to correct the interference.
FCC Declaration of Conformity
This device complies with Part 15 of the FCC Rules.
Operation is subject to the following two conditions: (1) this
device may not cause harmful interference, and (2) this
device must accept any interference received including
interference that may caused undesired operation.
Microhard Systems Inc.’s products are warranted against all
failures which occur as a result of defective material or
workmanship within 12 months of purchase by the user. This
warranty does not extend to products that, in the opinion of
Microhard Systems Inc., have been subject to misuse, accidents,
lightning strikes, improper installation or application, nor shall
it extend to units which have, in Microhard Systems Inc.’s
opinion, been opened, tampered with or repaired by an
unauthorized facility.
Microhard Systems Inc.
Leaders in Wireless Telecom
#110, 1144 - 29th Ave. N.E.
Calgary, Alberta T2E 7P1
Phone: [phone]
Fax: [phone]
www.microhardcorp.com
© 2000 by Microhard Systems Inc., All Rights Reserved.
HyperTerminal is copyrighted by Hilgraeve Inc, and developed for Microsoft.
Microsoft and Windows are registered trademarks of Microsoft Corporation.
pcANYWHERE and Symantec are registered trademarks of Symantec Corp.
All other products mentioned in this document are trademarks or registered
trademarks of their respective holders.
Manual Revision 1.20, December 21, 2000.

iii MHX-920 Operating Manual
Contents
1. Introduction
1.0 Product Overview .............................................................................................................................................................................. 1
1.1 Features.............................................................................................................................................................................................. 1
1.2 About this Manual............................................................................................................................................................................. 2
2. Electrical/Physical
2.0 Functional Block Diagram................................................................................................................................................................. 3
2.1 Pinout................................................................................................................................................................................................. 4
2.2 LED Operation................................................................................................................................................................................... 6
2.3 DC Characteristics............................................................................................................................................................................. 8
2.4 AC Characteristics.............................................................................................................................................................................. 9
3. Modes of Operation ..................................................................................................................................................................................... 11
3.1 Data Mode ........................................................................................................................................................................................ 11
3.2 Command Mode ............................................................................................................................................................................... 12
3.2.1 AT Command Interface ....................................................................................................................................................... 13
3.3 Switching Between Command and Data Modes............................................................................................................................... 13
4. Configuration
4.1 Quick Start Approach ...................................................................................................................................................................... 15
4.2 AT Commands................................................................................................................................................................................. 16
A - Answer........................................................................................................................................................................................ 17
E - Command Echo.......................................................................................................................................................................... 17
I - Identification ............................................................................................................................................................................... 17
O - Online Mode.............................................................................................................................................................................. 17
Q - Quiet Mode................................................................................................................................................................................ 17
V - Result Codes Display................................................................................................................................................................. 17
W - Connection Result..................................................................................................................................................................... 18
Z - Reset Modem and load stored configuration.............................................................................................................................. 18
&C - DCD (Data Carrier Detect) ..................................................................................................................................................... 18
&D - DTR (Data Terminal Ready)................................................................................................................................................... 18
&F - Load Factory default configuration ......................................................................................................................................... 18
&K - Handshaking............................................................................................................................................................................ 18
&S - DSR (Data Set Ready)............................................................................................................................................................. 19
&V - View Configuration ................................................................................................................................................................. 19
&E - Framing Error Check................................................................................................................................................................ 19
&W - Write Configuration to memory............................................................................................................................................. 19
Sxxx? - Read S register value........................................................................................................................................................... 19
Sxxx=yyy - Set S register value ....................................................................................................................................................... 19
Command Result Codes................................................................................................................................................................... 20
4.3 S Registers....................................................................................................................................................................................... 21
S Register 2 - Escape Code............................................................................................................................................................... 21
S Register 3 - CR Control Code....................................................................................................................................................... 21
S Register 4 - Linefeed Control Code .............................................................................................................................................. 21
S Register 5 - Backspace Control Code........................................................................................................................................... 21
S Register 101 - Operating Mode..................................................................................................................................................... 22
SRegister 102 - Serial Baud Rate.................................................................................................................................................... 24
S Register 103 - Wireless Link Rate................................................................................................................................................ 24
S Register 104 - Network Address................................................................................................................................................... 25
S Register 105 - Unit Address.......................................................................................................................................................... 25
S Register 106 - Primary Hopping Pattern....................................................................................................................................... 25
S Register 206 - Secondary Hopping Pattern.................................................................................................................................... 25
S Register 107 - Encryption Key...................................................................................................................................................... 27
S Register 108 - Output Power Level............................................................................................................................................... 27
S Register 109 - Hopping Interval.................................................................................................................................................... 28
S Register 110 - Data Format............................................................................................................................................................ 29
S Register 111 - Packet Minimum Size ............................................................................................................................................ 29
S Register 112 - Packet Maximum Size............................................................................................................................................ 29
S Register 114 - Packet Size Control ................................................................................................................................................ 29
S Register 116 - Packet Character Timeout ...................................................................................................................................... 29
S Register 113 - Packet Retransmissions.......................................................................................................................................... 30
S Register 213 - Packet Retry Limit.................................................................................................................................................. 31
S Register 115 - Packet Repeat Interval........................................................................................................................................... 31
S Register 117 - Modbus Mode ........................................................................................................................................................ 32
S Register 118 - Roaming................................................................................................................................................................. 33
S Register 119 - Quick Enter to Command....................................................................................................................................... 33

iv MHX-920 Operating Manual
S Register 120 - RTS/DCD Framing................................................................................................................................................. 33
S Register 121 - DCD Timeout......................................................................................................................................................... 33
S Register 122 - Remote Control ...................................................................................................................................................... 34
S Register 123 - RSSI Reading......................................................................................................................................................... 34
4.4 Diagnostics, Statistics and Remote Control...................................................................................................................................... 35
4.4.1 Spectrum Analyzer Feature............................................................................................................................................................... 35
4.4.2 Statistics............................................................................................................................................................................................ 35
4.4.3 Remote Control and Diagnostics ...................................................................................................................................................... 36
5. Installation ................................................................................................................................................................................................... 39
5.1 Estimating the Gain Margin.............................................................................................................................................................. 39
5.2 Antennas and Cabling....................................................................................................................................................................... 41
5.2.1 Internal Cabling ................................................................................................................................................................................ 41
5.2.2 Installing External Cables, Antennas and Lightning Arrestors.......................................................................................................... 42
A. Modem Command Summary...................................................................................................................................................................... 45
B. Serial Interface............................................................................................................................................................................................ 47
C. Sample Schematic Diagram........................................................................................................................................................................ 49
D. Factory Default Settings.............................................................................................................................................................................. 51
EPerformance Tables...................................................................................................................................................................................... 53
F. Hopping Tables............................................................................................................................................................................................ 55
G. Technical Specifications............................................................................................................................................................................. 57
H. Mechanical Drawing.................................................................................................................................................................................... 59
I. Glossary...................................................................................................................................................................................................... 61

MHX-920 Operating Manual: Chapter 1 Introduction. 1
1. Introduction
1.0 Product Overview
The MHX-920 is a high-performance embedded wireless data transceiver.
Operating in the 902 - 928 MHz ISM band, this frequency-hopping spread-
spectrum module is capable of providing reliable wireless data transfer
between almost any type of equipment which uses an asynchronous serial
interface. The small-size and superior RF performance of this module make
it ideal for many applications. Typical uses for this module include:
n SCADA
n Traffic Control
n Remote Monitoring
n Fleet Management;
n Telemetry;
n Remote Camera/Robot Control;
n Security Systems; and,
n Display Signs.
While a pair of MHX-920 modules can link two terminal devices (“point-to-
point” operation), multiple modules can be used together to create a network
of various topologies, including “point-to-multipoint” and “repeater”
operation. Multiple independent networks can operate concurrently, so it is
possible for unrelated communications to take place in the same or a nearby
area without sacrificing privacy or reliability.
1.1 Features
Key features of the MHX-920 include:
ntransmission within a public, license-exempt band of the radio
spectrum1– this means that it can be used without access fees
(such as those incurred by cellular airtime);
na serial I/O data port with handshaking and hardware flow
control, allowing the MHX-920 to interface directly to any
equipment with an asynchronous serial interface.
1902-928 MHz, which is license-free within North America; may need to be factory-configured
differently for some countries.

2MHX-920 Operating Manual: Chapter 1 Introduction
n64 sets of user-selectable pseudo-random hopping patterns,
intelligently designed to offer the possibility of separately
operating multiple networks while providing security, reliability
and high tolerance to interference;
nencryption key with 65536 user-selectable values to maximize
security and privacy of communications;
nbuilt-in CRC-16 error detection and auto re-transmit to provide
100% accuracy and reliability of data;
nease of installation and use – the MHX-920 module uses a
subset of standard AT style commands, very similar to those
used by traditional telephone line modems.
While the typical application for the MHX-920 is to provide a short- to mid-
range wireless communications link between DTEs, it can be adapted to
almost any situation where an asynchronous serial interface is used and data
intercommunication is required.
1.2 About this Manual
This manual has been provided as a guide and reference for installing and
using MHX-920 wireless modem modules. The manual contains instructions,
suggestions, and information which will help you set up and achieve optimal
performance from your equipment using the MHX-920 module.
It is assumed that users of the MHX-920 module have either system
integration or system design experience. Chapter 2 details the
electrical/physical attributes of the module. Chapter 3 explains the different
modes of operation. Chapter 4 provides complete details of all configuration
parameters; and, Chapter 5 is an installation/deployment guide. The
Appendices, including the Glossary of Terms, are provided as informational
references which you may find useful throughout the use of this manual as
well as during the operation of the wireless modem.
Throughout the manual, you will encounter not only illustrations that further
elaborate on the accompanying text, but also several symbols which you
should be attentive to:
Caution or Warning: Usually advises against some action which could
result in undesired or detrimental consequences.
Point to Remember: Highlights a key feature, point, or step which is worth
noting, Keeping these in mind will make using the MHX-920 more useful
or easier to use.
Tip: An idea or suggestion is provided to improve efficiency or to make
something more useful.
With that in mind, enjoy extending the boundaries of your communications
with the MHX-920 module.

MHX-920 Operating Manual: Chapter 2 Electrical/Physical 3
2. Electrical/Physical
2.0 Functional Block Diagram
Antenna
Switch
Mixer
LNAMixer
IF Demod
Frequency
Synthesizer
PA
Comparator +
-
uC
8 bit data bus
UART
(DCE)
GAIN
A/D SRAM EEPROM
SRAM
CTS
DCD
DSR
RxD
DTR
TxD
RTS
\Config
\Reset
RSSI1-3
RXMODE
TXMODE
DVcc
AVcc
GND
Figure 1. Functional Block Diagram

4MHX-920 Operating Manual: Chapter 2 Electrical/Physical
2.1 Pinout
Figure 2 provides a top-view pinout drawing of the MHX-920 module. The
corner pins (1,20,21,40) are labeled directly on the module.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
36
35
34
33
32
31
30
29
28
27
26
25
24
23
22
21
AVcc
AVcc
DVcc
DVcc
DVcc
DVcc
DVcc
NC
\Config
\Reset
GND
GND
GND
GND
GND
GND
GND
NC
NC
NC
NC
NC
NC
NC
NC
Rx/SYNC
TxMODE
RSSI3
RSSI2
RSSI1
CTS
RTS
DSR
NC
DTR
TxD
MHX-910
19
20
NC
NC RxD DCD
37
38
39
40
Figure 2 - Pinout (Top View)
Table 1. Pin Description
Pin Name No. Description I/O
AVcc 1,2 Positive Supply for Radio Circuitry. See Section
2.3 for DC Characteristics I
\Config 9Leave unconnected. For factory use only. Do
not ground
CTS 28 RS-232 Clear to Send. Active low (TTL level)
output. See Appendix B for a complete description
of all RS-232 signals.
O
DCD 21 RS-232 Data Carrier Detect. Active low (TTL
level) output. O
DSR 26 RS-232 Data Set Ready. Active low (TTL level)
output. O
DTR 24 RS-232 Data Terminal Ready. Active low
(TTL level) input.
I
DVcc 3-7 Positive Supply for Logic circuitry and I/O
pins. See Section 2.3 for DC Characteristics I

MHX-920 Operating Manual: Chapter 2 Electrical/Physical 5
Table 1 (continued)
GND 11-17 Ground reference for logic, radio and I/O pins.
\Reset 10 Active low reset input to the module. See
Section 2.4 for timing information. I
RSSI1 29 Receive Signal Strength Indicator 1. This
output is the first of the three RSSI indicators
to become active high as the signal strength
increases. See Section 2.2 for details
O
RSSI2 30 Receive Signal Strength Indicator 2. This
output is the second RSSI indicator to become
active high as the signal strength increases.
See Section 2.2 for details.
O
RSSI3 31 Receive Signal Strength Indicator 3. This
output is the last RSSI indicator to become
active high as the signal strength increases.
See Section 2.2 for details.
O
RTS 27 RS-232 Request to Send. Active low (TTL
level) input.
I
RxD 22 RS-232 Receive Data. TTL level output. O
RX/SYNC 33 Active high output indicates receive and
synchronization status. See Section 2.2.
O
TxD 23 RS-232 Transmit Data. TTL level input. I
TXMODE 32 Active high output indicates module is
transmitting data over the RF channel. See
Section 2.2.
O

6MHX-920 Operating Manual: Chapter 2 Electrical/Physical
2.2 LED OPERATION
LED functionality is dependent on the mode of operation. Lines RX/SYNC,
TXMODE, and RSSI1,2 and 3 are designed to drive LED’s (active high).
Table 2 explains LED operation for the various modes.
Table 2. LED Operation
LED
MODE RX/Sync TXMode RSSI1,2,3
Power Up (S0=1, S119=1) off off blink 500ms
on/500ms off
Power Up (S0=1, S119=0) off off off
Power Up (S0=0) off off off
Command Mode off off off
Data Mode - Master on while receiving
valid data packets
from slaves and
repeaters in the
network
on for the first
portion of each
hop interval.
RSSI mode
based on all
received packets
See Table 3
Data Mode - Repeater
During Sync. Acquisition off off alternating 300ms on
Data Mode - Repeater
When Synchronized on for first portion of hop interval on for second portion of hop
interval
RSSI mode
based on packets
received from
Slaves*
See Table 3
Data Mode - Slave During
Sync. Acquisition off off alternating 300ms on
Data Mode - Slave
When Synchronized on on when transmitting a
packet.
RSSI mode
based on packets
received from the
Repeater or
Master with
which it
communicates
See Table 3
*If Slaves have been silent for 2 seconds, repeater will base its RSSI on
packets received from the Master.

MHX-920 Operating Manual: Chapter 2 Electrical/Physical 7
Signal strength, which is also reported in Register S123, is calculated based
on the last four valid received packets with correct CRC, and represented by
RSSI1, 2 and 3.
For slaves, packets are received on every single hop either from a repeater, or
the master.
When calculating RSSI, the master takes into consideration all packets
received from slaves and repeaters. Repeaters and slaves only transmit back
to the master when they have information to send. Therefore, if no data is
coming back to the master then RSSI will never get updated at the master,
and the LED’s will be off.
Table 3 - RSSI mode operation
Signal Strength
(dBm) RSSI1 RSSI2 RSSI3
-108 50% duty cycle off off
-101 on solid off off
-93 on solid 50% duty cycle off
-86 on solid on solid off
-79 on solid on solid 50% duty cycle
-71 on solid on solid on solid

8MHX-920 Operating Manual: Chapter 2 Electrical/Physical
IMPORTANT:
For best performance, it is
strongly recommended to use
a separate, linearly regulated
supply for Vcc Radio. Do
not directly feed a switching
power supply into Vcc
Radio.
For OEM design simplicity,
connect both the radio and
logic Vcc connections
together.
Caution: Using any other power
supply which does not provide the
proper voltage or current could
damage the MHX-920 module.
2.3 DC Characteristics
Sym Characteristic Min Typ Max Units
AVCC Radio Supply Voltage 4.9 5.0 5.5 V
DVCC Logic Supply Voltage 4.75 5.0 5.5 V
VPOT Power On Reset Threshold Voltage 1.8 22.2 V
VRST Reset Pin Threshold Voltage DVCC/
2V
AICCR Radio Supply Current in Receive Mode 96 107 117 mA
AICCT
0
Radio Supply Current at 1mW Transmit 68 108 119 mA
AICCT
1
Radio Supply Current at 10mW Transmit 111 123 135 mA
AICCT
2
Radio Supply Current at 100mW Transmit 157 174 191 mA
AICCT
3
Radio Supply Current at 1W Transmit 398 442 486 mA
DICC Logic Supply Current 95 105 115 mA
VIL Input Low Voltage (Pins 23,24,27) -0.5 .3DVCC V
VIH Input High Voltage (Pins 23,24,27) 0.6VCC VCC+.5 V
VOL Output Low Voltage (Pins 21,22,26,28-
33) 0.6 V
VOH Output High Voltage (Pins 21,22,26,28-
33) 4.2 V
ISRCESourcing Current (Pins 21,22,26,28-33) 10 mA

MHX-920 Operating Manual: Chapter 2 Electrical/Physical 9
2.4 AC Characteristics
Sym Characteristic Min Typ Max Units
TTOUT Reset Delay Time-Out Period 500 ms
TR2D Internal Reset to Data Mode* 0 5 s
TR2C Internal Reset to Command Mode 0ms
*Unit will enter into Command Mode upon power up if register S0=0. Unit will enter into Data
Mode upon power-up if register S0=1. When powering up into Data Mode, an additional delay
of 5 seconds is added if Register S119=1. See page 14 for details.
Figure 3 provides timing information for both power-up reset and the \Reset
line operation. A fixed internal reset delay timer of roughly 500ms is
triggered as the VPOT or VRST threshold is reached.
T
V
VDV
\Reset
Internal Reset
CC
TOUT
RST
POT
T
Data Mode
R2D
T
Command Mode
R2C
(Valid when S0=1)
(Valid when S0=0)
Figure 3. Reset Timing

10 MHX-920 Operating Manual: Chapter 3 Modes of Operation

MHX-920 Operating Manual: Chapter 3 Modes of Operation 11
3. Modes of Operation
The MHX-920 modem can be easily configured to meet a wide range of
needs and applications. The module is designed such that all communication
is through one serial port (Pins 21 to 28 on the module). This port has two
functions:
1. It provides the asynchronous interface with the host equipment for data
that is sent/received on the RF channel. When operating in this fashion,
the module is said to be in data mode.
2. It is also used for configuring and programming the module. When
operating in this fashion, the module is said to be in command mode.
In addition to data mode and command mode, there is a third mode of
operation called diagnostics mode. The module will always be in one of
these three modes.
3.1 Data Mode
Data mode is the normal operating mode of the MHX-920. When in data
mode, the MHX-920 is communicating with other MHX-920 modules, and
facilitating wireless asynchronous serial communication amongst two or more
terminal devices. There are three basic elements to any MHX-920
communications network:
• One module configured as the Master
• Zero or more modules configured as Repeaters
• One or more modules configured as Slaves
The function of the Master is to provide synchronization for the entire
network, and to control the flow of data. There is always one Master per
network. The Master is the ultimate destination for all data collected at the
various Repeater’s and Slave’s serial ports. With the network set up for
Point-to-Multipoint communication, all data received at the Master’s serial
port is transmitted to every Repeater and Slave in the network. The MHX-
920 is a frequency hopping transceiver, meaning that it “hops” to a new
frequency after a predetermined time interval. This time interval is a fixed
time set by the user, and can range from 8ms to 120ms. The MHX-920 hops
according to a pseudorandom pattern of 50 different channels.
When configured as a Slave, the MHX-920 searches for synchronization with
a Master. Network topologies consisting of a single Master and virtually any
combination of Slaves and Repeaters may be deployed. The functionality of
any particular MHX-920 can be configured as follows:

12 MHX-920 Operating Manual: Chapter 3 Modes of Operation
MS
Network 1
MSR MSR
Network 2
M
S
S
S
Network 3
MR
S
S
S
Network 4
MR
SR
S
Network 5
Figure 4 - Sample Network
Topologies. Virtually any
Combination of Slaves and
Repeaters May be Used.
nMaster Point-to-Point: The modem is configured to
communicate with a single Slave, either directly, or through one
or more Repeaters.
nMaster Point-to-Multipoint: The modem is configured to
communicate with one or more Slaves and/or Repeaters.
nSlave: The modem is configured to communicate with one
Master either directly or through one or more Repeaters..
nRepeater: The modem is configured to pass information from
either a Master or another Repeater onto subsequent Repeaters
and/or Slaves and vice versa. The Repeater also acts as a Slave
in the sense that, like a Slave, it passes information to/from its
serial port.
Examples of different network topologies are shown in Figure 4. Network 1
shows Point-to-Point communication between a Master and Slave. Network
2 makes use of a Repeater to communicate with the Slave. Network 3
illustrates a simple Point-to-Multipoint network with no Repeaters. Networks
4 and 5 gives examples of Point-to-Multipoint networks consisting of both
Repeaters and Slaves. There is effectively no restriction to the number of
Repeaters and Slaves that can be added to a network. As seen in Network 4,
a Master can communicate directly with both Slaves and Repeaters.
3.2 Command Mode
The MHX-920 firmware has been designed to allow the user to customize the
operation of the modem through an AT Command Interface. This interface is
ideal for direct interface with another microcontroller or for higher level
Windows-based software applications, but also contains user-friendly built-in
register descriptions. These descriptions make it easy for the user to
configure the unit by manually inputting AT Commands and modifying S-
Register parameters, using any standard terminal program. The MHX-series
development board is a useful tool for familiarizing yourself with the various
operating parameters and user interface. Reference schematics for the
development board can be found in Appendix G. To access the MHX-920’s
command mode using the development board:
1. Insert the module into the socket with the antenna connector towards the
edge of the board.
2. Attach the supplied antenna.
3. Connect a straight through serial cable between the DB9 connector and
the serial port on your PC
4. Run any terminal application program such as Hyperterminal
5. Set the serial port to 9600 baud, 8N1
6. Apply power to the development board
7. While the three RSSI LED’s are blinking, type ‘mhx’ (you have about 5
seconds to do this). The modem should respond with ‘OK’.
8. Type ‘AT&V <ENTER>’

MHX-920 Operating Manual: Chapter 3 Modes of Operation 13
3.2.1 AT Command Interface
At this point you should see a menu similar to the following appear:
BAUD = 9600
E1 Q0 V1 W0
DCD &C1 DTR &D0 Framing &E0 Handshaking &K3 DSR &S1
S0=1 S2=43 S3=13 S4=10 S5=8
Operating Mode S101=1 Serial Baud Rate S102=7
Wireless Link Rate S103=2 Network Address S104=1
Unit Address S105=1 Hop Pattern S106=0
Encryption Key S107=1 Output Power S108=2
Hop Interval S109=4 Data Format S110=1
Packet Min Size S111=1 Packet Max Size S112=43
Packet Retransmissions S113=1 Quick enter to command S119=1
Packet Repeat Interval S115=1 Character Timeout, ms S116=8
RTS/DCD Framing, ms S120=0 DCD Timeout, ms S121=0
Secondary Hop Pattern S206=2 Packet Retry Limit S213=2
Average RSSI value S123= -0 dBm Modbus Mode S117=0
Roaming S118=0 Packet Size Control S114=0
Remote Control S122=0
OK
The MHX-920 is controlled through an AT Command line interface using a
command set which is very similar to a traditional Hayes telephone modem
command set.
All line entries must be preceded by the characters ‘AT’. The characters
‘AT’ are known as the attention characters and must be typed at the
beginning of each command line. For example, to change the operating
mode, type:
ATS101=2 <ENTER>
The modem should respond with ’OK.’ The above command will set the
operating mode to Master Point-to-Point.
Register settings are not immediately stored to non-volatile memory,
therefore if the modem is powered down at this point, the Operating Mode
would revert to its previous value. To store any recently updated command
registers, the following “write” command must be entered.
AT&W <ENTER>
3.3 Switching Between Command and Data Modes
Your modem must be in command mode for it to execute a command. If
you send characters when the modem is in data mode, the modem transmits
the characters over the air.
Depending on its settings, the modem will either power up in command mode
or data mode. Normally, when first received from the factory, the unit will
power up into data mode. During the first five seconds after power-up, the
user is given the opportunity to avoid entering into data mode but instead
enter into command mode by typing ‘mhx’.

14 MHX-920 Operating Manual: Chapter 3 Modes of Operation
DATA
MODE
POWER-UP
SEQUENCE
COMMAND
MODE USER TYPES 'mhx'
5 sec elapses
or
characters
other than
'mhx'
entered by
the user
(ATA or ATO Command)
DTR or
Escape Sequence
Figure 5A. S0=1, S119=1
(factory default)
DATA
MODE
POWER-UP
SEQUENCE
COMMAND
MODE
500 msec
(ATA or ATO Command)
DTR or
Escape Sequence
Figure 5B. S0=1, S119=0
DATA
MODE
POWER-UP
SEQUENCE
COMMAND
MODE
500 msec
(ATA or ATO Command)
DTR or
Escape Sequence
Figure 5C. S0=0
The escape sequence will not
be accepted unless both the
MHX-920 and the terminal
are set to the same baud rate
The terminal must be set for 9600 baud 8N1 in order for the modem to accept
these characters. If ‘mhx’ is typed incorrectly, the modem will immediately
enter into data mode. If the five seconds elapses without any response from
the user, the modem will go into data mode.
In command mode, the module “autobauds,” meaning that it will adapt to the
baud rate of the DTE equipment to which it is connected. Therefore, when in
command mode, you may change the baud rate of your equipment, and the
MHX-920 will automatically adjust to this baud rate once an AT string is
issued. The new baud rate is stored in register S102. Several baud rates
ranging from 2400 to 115200 may be selected.
You can place the modem into Data Mode from Command Mode either by:
• Issuing the answer command (ATA <ENTER>); or,
• Issuing the online command (ATO <ENTER>).
The modem will now attempt to communicate with other MHX-920 modules.
While in Data Mode, the modem will communicate through the serial port at
the same baud rate as was last used in Command Mode2.
To return to Command Mode, you can either:
• Send the escape sequence. (The escape sequence consists of 1 second of
inactivity, followed by the characters ‘+++’ followed by another second
of inactivity.); or,
• Toggle the DTR line (depending on the &D parameter see pg 11).
The escape sequence must be issued at the baud rate that the modem has been
set to. If the modem is set to 19200 baud, and the escape sequence is issued
at 9600 baud, for example, the modem will not recognize it, and will not go
into Command Mode.
Figure 5 provides a state diagram for power-up, command mode, and data
mode. Note that there are three different variants of the state diagram which
depend on the values of registers S0 and S119. See the appropriate sections
for more details about these registers. The factory defaults are S0=1 and
S119=1.
2It is possible to enter into Data Mode at a different baud rate from what is
currently being used in Command Mode by issuing the command ATS102=x,
where x is one of the valid baud rates. Care must be taken when setting the
baud rate in this manner. If you issue another AT string after attempting to
set the baud rate using ATS102 <ENTER>, the modem will again autobaud
and automatically revert to the baud rate of the host equipment. For example,
if your equipment is running at 9600 baud and you wish to set up the modem
to run at 19200 baud, the following command line entry would be suitable:
ATS102=5&WA <ENTER>
The first part (S102=5) sets the baud rate to 19200. The next characters
(&W) write this baud rate to memory. The last character (A) puts the modem
into Data Mode. Once in Data Mode, the modem is unable to autobaud, and
is fixed at 19200 baud. By combining several commands into one command
line entry, and then immediately putting the modem online, the modem is not
given a chance to autobaud back to 9600.

MHX-920 Operating Manual: Chapter 4 Configuration 15
4. Configuration
Warning: After testing the units
for correct operation using the
quick-start approach, be sure to
modify some of the security
parameters such as Network
Address and Encryption Key, to
avoid unintentional
communication with other users of
MHX-920 products..
MS
Network 1
MSR MSR
Network 2
Figure 6. Basic Networks
This chapter provides a detailed description of the various operating
parameters of the MHX-920. Section 4.1 provides a quick-start approach
which outlines the minimum requirements for establishing communication
between two MHX-920 modules. The settings will not necessarily provide
optimal performance for your application, but will verify that the modules are
functioning correctly.
Section 4.2 describes the AT Command interface, and the various AT
Commands. Section 4.3 covers all S-Register parameters which affect the
operation of the modem, and Section 4.4 provides a description of all
diagnostic features of the modem.
4.1 Quick Start Approach
There are several parameters that must be set in order to establish
communication between a pair of MHX-920 modules.
The MHX-920 is equipped with four standard factory default settings.
Instead of manually configuring each individual operating parameter, a global
command may be used to quickly configure the modem for a particular type
of operation. For example, to quickly implement Network 1, Factory default
1 would be applied to the Master, and Factory default 2 would be applied to
the Slave. To quickly set up Network 2, apply Factory 1 to the Master,
Factory 3 to the Repeater, and Factory 4 to the Slave. These defaults will get
you started and only ensure that a link can be established, but do not
necessarily provide the best performance. Optimization of the
communications link is discussed in later sections.
To implement the basic network illustrated in Figure 6, Network 1,
1. Insert the module into the development board socket with the antenna
connector towards the edge of the board.
2. Attach the supplied antenna.
3. Connect a straight through serial cable between the DB9 connector and
the serial port on your PC
4. Run any terminal application program such as Hyperterminal and set the
terminal application’s serial port settings to 9600 baud, 8N1
5. Apply power to the development board
6. While the three RSSI LED’s are blinking, type ‘mhx’ (you have about 5
seconds to do this). The modem should respond with ‘OK’.
7. Configure the unit to Factory Setting 1 by typing AT&F1 <return>. This puts
the unit into Master Point-to-point mode.
8. Store these settings to memory by typing AT&W <return>.
9. Put the modem into Data Mode by typing ATA (or ATO) <return>
10. Perform above steps for the second unit, using Factory Setting 2 instead of
Factory Setting 1. This will configure the second unit as a Slave.

16 MHX-920 Operating Manual: Chapter 4 Configuration
The escape sequence will not
be accepted unless both the
MHX-920 and the terminal
are set to the same baud rate
The units should now be communicating. Remember, the parameters defined
by Factory Settings 1 and 2 will likely not be the most ideal for your
application, but will quickly allow you to test the units. A complete summary
of the settings defined by all four factory settings can be found in Appendix
C. Factory Default Settings.
Settings are not immediately stored in non-volatile memory, therefore, the
command &W is issued to store the current configuration into non-volatile
memory. Settings are retained even after powering down. All user selectable
parameters for the MHX-920 are described in detail in Sections 4.2 and 4.3:
Checking the Link
To check if the units are communicating, observe the LED indicators on the
development board which houses the Slave unit. If the link is good, up to
three RSSI LEDs on the Slave modem should be active along with the
RX/Sync LED, and if the link is absent (due to a fault at one end or another,
such as misconfiguration), the LED’s will be in either “scanning mode” or
inactive. See Section 2.2 for complete LED operation.
Characters typed at the Master terminal should appear at the Slave’s terminal,
and vice versa. Also, verify that the RX LED blinks as packets of data are
received at the Master modem. As data is sent from Slave to Master, the RX
indicator should blink on as correct packets of data are received. At this
point, the Master’s RSSI LED’s should become active. It is recommended
that if the MHX-920 will be deployed in the field where large distances
separate the units, the modems should be configured and tested in close
proximity (e.g., in the same room) first to ensure a good link can be
established and settings are correct. This will facilitate troubleshooting,
should problems arise.
4.2 AT Commands
Several AT Commands are supported by the MHX-920. These commands
affect the operation of the modem in command mode and the transition
between data and command modes. More commands and S-Register settings
are discussed in Sections 4.3 and 4.4.
To make the command line more readable, you can insert as many spaces as
desired. The command line holds up to 16 characters, not including the AT
prefix. If you want to send more than one command line, wait for a response
before entering the AT prefix at the start of the next command line.
To re-execute the previous command, enter A/. The modem will execute the
previous command line.
When in Command Mode, the modem “autobauds”, meaning that it will
automatically adjust to the baud rate of the terminal. You may change the
terminal baud rate while in Command Mode without losing communication
with the modem.
For the AT command protocol, an escape sequence consists of three
consecutive escape codes preceded and followed by at least 1 second of
inactivity. Typically, the ‘+’ character is used as the escape code.
+++ preceded and followed by 1 second of inactivity