MTL WMO-900S User manual

WMO-900S
Radio Modem
User Guide
INM WMO900S

WMO-900S Radio Modem
Page 2 July 2010
Thank you for your selection of the WMO-900S radio modem.
We trust it will give you many years of valuable service.
ATTENTION!
Incorrect termination of supply wires may
cause internal damage and will void warranty.
To ensure your WMO-900S enjoys a long life,
double check ALL your connections with the user’s manual
before turning on the power.
THIS EQUIMENT IS SUITABLE FOR USE IN CLASS 1 DIVISION 2, GROUPS A, B, C
& D OR NON-HAZARDOUS LOCATIONS.
CAUTION:
For continued protection against risk of fire, replace the internal module fuse only with the same
type and rating.
CAUTION:
To comply with FCC RF Exposure requirements in section 1.1310 of the FCC Rules, antennas
used with this device must be installed to provide a separation distance of at least 20 cm from all
persons to satisfy RF exposure compliance.
DO NOT:
operate the transmitter when someone is within 20 cm of the antenna
operate the transmitter unless all RF connectors are secure and any open connectors are properly
terminated.
operate the equipment near electrical blasting caps or in an explosive atmosphere
All equipment must be properly grounded for safe operations. All equipment should be serviced
only by a qualified technician.

User Manual
INM WMO900S_v1.9a.doc Page 3
FCC Notice:
This user’s manual is for the MTL WMO-900S radio telemetry module. This device complies
with Part 15.247 of the FCC Rules.
Operation is subject to the following two conditions:
This device may not cause harmful interference and
This device must accept any interference received, including interference that may cause
undesired operation.
This device must be operated as supplied by Measurement Technology Limited. Any changes or
modifications made to the device without the written consent of Measurement Technology
Limited (MTL) may void the user’s authority to operate the device.
End user products that have this device embedded must be supplied with non-standard antenna
connectors, and antennas available from vendors specified by Measurement Technology Limited
(MTL). Please contact MTL for end user antenna and connector recommendations.
Notices: Safety
Exposure to RF energy is an important safety consideration. The FCC has adopted a safety
standard for human exposure to radio frequency electromagnetic energy emitted by FCC regulated
equipment as a result of its actions in Docket 93-62 and OET Bulletin 65 Edition 97-01.
Important Notice
MTL products are designed to be used in industrial environments by experienced industrial
engineering personnel with adequate knowledge of safety design considerations.
MTL radio products are used on unprotected license-free radio bands with radio noise and
interference. The products are designed to operate in the presence of noise and interference,
however in an extreme case, radio noise and interference could cause product operation delays or
operation failure. Like all industrial electronic products, MTL products can fail in a variety of
modes due to misuse, age, or malfunction. We recommend that users and designers design
systems using design techniques intended to prevent personal injury or damage during product
operation, and provide failure tolerant systems to prevent personal injury or damage in the event
of product failure. Designers must warn users of the equipment or systems if adequate protection
against failure has not been included in the system design. Designers must include this Important
Notice in operating procedures and system manuals.
These products should not be used in non-industrial applications, or life-support systems, without
consulting MTL first.
1. A radio license is not required in some countries, provided the module is installed using
the aerial and equipment configuration described in the WMO-900S Installation Guide.
Check with your local distributor for further information on regulations.
2. Operation is authorized by the radio frequency regulatory authority in your country on a
non-protection basis. Although all care is taken in the design of these units, there is no
responsibility taken for sources of external interference. Systems should be designed to be
tolerant of these operational delays.

WMO-900S Radio Modem
Page 4 July 2010
3. To avoid the risk of electrocution, the aerial, aerial cable, serial cables and all terminals of
the WMO-900S module should be electrically protected. To provide maximum surge and
lightning protection, the module should be connected to a suitable earth and the aerial,
aerial cable, serial cables and the module should be installed as recommended in the
Installation Guide.
4. To avoid accidents during maintenance or adjustment of remotely controlled equipment,
all equipment should be first disconnected from the WMO-900S module during these
adjustments. Equipment should carry clear markings to indicate remote or automatic
operation. E.g. "This equipment is remotely controlled and may start without warning.
Isolate at the switchboard before attempting adjustments."
5. The WMO-900S module is not suitable for use in explosive environments without
additional protection.
How to Use This Manual
To receive the maximum benefit from your WMO-900S product, please read the Introduction,
Installation and Operation chapters of this manual thoroughly before putting the WMO-900S to
work.
Chapter Four Configuration details the configurations available and explains the diverse
operation of the product in detail.
Chapter Five Specifications details the features of the product and lists the standards to which the
product is approved.
Chapter Six Troubleshooting will help if your system has problems.
The WMO-900S Installation Guide supplied is an installation drawing appropriate for most
applications.

User Manual
INM WMO900S_v1.9a.doc Page 5
CONTENTS
CHAPTER ONEINTRODUCTION...............................................................................7
1.1GENERAL ........................................................................................................................7
1.2TRANSPARENT MODE .....................................................................................................8
1.3CONTROLLED MODE.......................................................................................................9
1.4REPEATER UNITS ..........................................................................................................10
CHAPTER TWOINSTALLATION...............................................................................11
2.1GENERAL ......................................................................................................................11
2.2ANTENNA INSTALLATION .............................................................................................11
2.2.1Dipole and Collinear antennas.................................................................................13
2.2.2Yagi antennas...........................................................................................................13
2.3POWER SUPPLY.............................................................................................................15
2.4SERIAL CONNECTIONS ..................................................................................................15
2.4.1 RS232 Serial Port........................................................................................................15
2.4.2 RS485 Serial Port........................................................................................................16
CHAPTER THREEOPERATION.................................................................................18
3.1POWER-UP AND NORMAL OPERATION ..........................................................................18
3.2SERIAL AND RADIO DATA.............................................................................................18
3.2.1Character Type.........................................................................................................19
3.2.2Serial Data Rate.......................................................................................................19
3.2.3Radio Data Rate.......................................................................................................19
3.3ADDRESSING.................................................................................................................20
3.4TRANSPARENT MODE ...................................................................................................21
3.5TRANSPARENT MODE REPEATERS ................................................................................21
3.6CONTROLLED MODE.....................................................................................................23
3.6.1 Auto-Connect Controlled Mode .................................................................................25
3.6.2 Low Power Auto-Connect Mode...............................................................................26
3.6.3 Single-Connect Controlled Mode..............................................................................26
3.6.4 Fast Operation Controlled Mode ...............................................................................26
3.6.5 CTS/RTS Flow Control .............................................................................................26
3.6.6 Intermediate Repeaters ...............................................................................................26
3.7WHAT OPERATING MODE TO USE ?..............................................................................27
3.7.1Transparent or Controlled Mode?............................................................................27
3.7.2Error Check ?...........................................................................................................28
3.8SERIAL /RADIO RATES .................................................................................................28
3.9RADIO INTERFERENCE ..................................................................................................28
3.10OPERATING PROBLEMS.................................................................................................30
CHAPTER FOURCONFIGURATION...........................................................................31
4.1BEFORE CONFIGURING..................................................................................................31
4.2ADDRESSING.................................................................................................................32
4.3DEFAULT CONFIGURATION ...........................................................................................32
4.4CONFIGURATION PROGRAM..........................................................................................32

WMO-900S Radio Modem
Page 6 July 2010
4.4.1Transparent Mode....................................................................................................34
4.4.2Controlled Mode......................................................................................................34
4.4.3Other Parameters......................................................................................................36
4.5HAYES COMMANDS ......................................................................................................38
4.6CONFIGURATION EXAMPLES.........................................................................................43
4.7SWITCH CONFIGURATION .............................................................................................45
CHAPTER FIVESPECIFICATIONS..............................................................................47
CHAPTER SIXTROUBLESHOOTING.........................................................................49
6.1DIAGNOSTICS CHART....................................................................................................49
6.2TEST FUNCTIONS ..........................................................................................................49
6.2.1Radio Testing - AT&Tx...........................................................................................49
6.2.2Bit Error Rate Test (BER) .......................................................................................50
6.2.3On-line diagnostics..................................................................................................51
APPENDIX A HAYES COMMANDS.............................................................................52

User Manual
INM WMO900S_v1.9a.doc Page 7
Chapter One INTRODUCTION
1.1 General
The WMO-900S radio modem module has been designed to provide flexible and reliable
radio modem functions, at an economical price. Radio modems transmit serial data over a
long distance via radio. The serial data is not changed - the output data is the same as the
input data. Although the WMO-900S is intended to be simple in its application, it also
provides many sophisticated features. This manual should be read carefully to ensure that the
modules are configured and installed to give reliable performance.
Each WMO-900S module will connect to a host device by RS232 or RS485 serial
connection. Examples of host devices are PLC’s, data loggers, intelligent transducers and
computers. The WMO-900S unit can receive data from the host device and transmit this data
by radio to another (or several) WMO-900S module. The other module will recreate the
serial data and output it as either a RS232 or RS485 serial signal. The WMO-900S unit
provides two-way communications - each module can accept serial data and also output serial
data.
The WMO-900S module includes power supply, microprocessor controller, serial
input/output circuits and a 900MHz frequency-hopping spread-spectrum radio transceiver -
no external electronics are required. The WMO-900S radio frequency has been selected to
meet the requirements of unlicensed operation for remote monitoring and control of
equipment. That is, a radio licence is not required for the WMO-900S modules in many
countries. See Chapter Five Specifications for details. The units are configured from a PC
using a “free-ware” configuration package, or from a PC terminal using Hayes commands.
RS232 is an electrical standard format for a full
duplex point-to-point serial connection. RS485
is an electrical standard format for a half-duplex
multidrop serial connection. Up to 32 devices
can communicate on a common RS485 serial
bus. Each WMO-900S unit can only connect to
one serial signal - either RS232 or RS485.
However different modules in the same system
can connect to different types of serial signals. For example, RS232 data from one host
device can be transmitted to a remote WMO-900S unit and output as RS485 data to another
host device.
The WMO-900S has been designed to be flexible enough to cover a wide range of
applications. The user is able to configure many different parameters such that the WMO-
900S unit will connect reliably to different types of host devices. Before the radio modem
can be used, these parameters must be configured. Some of these parameters are :-
Character type - the WMO-900S will accept a variety of 7 or 8 data bit characters
Serial Data Rate - between 1200 and 115200 bits/sec
Radio Data Rate - 19200, 57600 or 115200 bits/sec
Operating mode - transparent mode or controlled mode .
HOST
900S
RS232
HOST
900S
RS485

WMO-900S Radio Modem
Page 8 July 2010
The operation of the WMO-900S radio modem is relatively simple. As data is received at the
serial port, the data is transmitted on the radio channel. Up to 530 bytes of data can be
transmitted in one transmission. The radio transmission commences when the first data byte
is received, and ends when there are no more data bytes in the input buffer, or when the
number of bytes transmitted equals the maximum message length (user configurable - default
530 bytes). If more than 530 bytes is input, the WMO-900S unit will transmit the first 530
bytes, then the next 530 bytes, and so on until all of the data has been transmitted.
Because the radio data rate could be less than the input serial data rate, an input memory
buffer of 2Kbytes is provided. The RS232 connection provides CTS control to prevent the
buffer overflowing. There are no data flow control signals for RS485.
A radio channel cannot provide as secure a data channel as a wired connection. The WMO-
900S uses a radio band with a low level of natural or industrial noise, however there is a
chance of interference from other users of the unlicensed radio channel. We recommend that
the flow of data over the radio channel is controlled by using error detection and
“handshaking” - that is, returning an acknowledgment transmission if a data packet is
received on the radio channel without error. This function can be performed by either the
host devices or the WMO-900S modules. The modules may be configured by the user to
operate in one of two modes. In transparent mode, it is assumed that the host devices control
the flow of data. In controlled mode, the WMO-900S units control the flow of data.
1.2 Transparent Mode
The default configuration of the WMO-
900S modem is transparent mode - the
modules are set in this mode at the factory.
In transparent mode, the WMO-900S
provides no control of the data
transmissions (no error correction). Input
data is simply transmitted by radio and
every other WMO-900S unit in that system
which receives the transmission will output
the data. This mode relies on the host
devices to perform the “handshaking”
function, and re-transmitting serial data if the data is corrupted (no “handshake”). It also
relies on the host devices to include any addressing necessary in the data. In this mode,
modules are not configured with a unit address. Data is “broadcast” - every other WMO-
900S in the system will receive the data and output the data to their individual host devices.
The user may configure the WMO-900S modems to add error checking to each data packet
transmitted - if error checking is configured, data will not be output if it is received without a
correct error-check. This feature provides additional protection against corruption of the data
during the radio transmission. If error-checking is not configured, then the data received by
radio will be output without checking for errors.
HOST
900S
DATA
HOST
900S
DATA
HOST
900S
DATA

User Manual
INM WMO900S_v1.9a.doc Page 9
Transparent mode is “point-to-multipoint” communications, suitable for a host device which
is able to communicate on a multi-drop “bus” type network. An example of an application is
the use of radio modems to extend a PLC RS485 network. The serial messages from the
PLC’s already include PLC addressing and error detection/correction to control the flow of
data.
1.3 Controlled Mode
“Controlled mode” provides “point-to-
point” communications similar to telephone
modems. In controlled mode, the flow of
data is controlled by the WMO-900S units.
Each WMO-900S unit is configured with an
address by the user, and a destination
address for the data to be transmitted to.
Data is transmitted addressed to the
destination module, and only this module
will output the serial data. The source
module will add an error-check (16 bit
CRC) to the data transmitted by radio. The destination module will process the error-check,
and if correct, it will transmit an acknowledgment message (ACK) back to the source
module. If the source module does not receive a ACK, it will re-transmit the data. The
source module will attempt to transmit the data up to five times, until an acknowledgment
(ACK) is received. If an acknowledgment is still not received, then the DCD signal on the
RS232 port will be reset and a alarm message can be sent to the host via the serial port.
An example of an application using controlled mode would be a radio modem link between
an intelligent gas analyzer and a monitoring computer system. Intelligent transducers do not
normally provide addressing or error checking functions - these would be provided by the
WMO-900S modules.
In controlled mode, the destination address may be pre-set, or set on-line by the host device
using “Hayes” commands. Hayes commands are a standard set of commands used with
conventional telephone modems. An example of an application that would use Hayes
command to set destination addresses would be a central computer polling data loggers for
periodic information.
PLC
900S
900S
RS485
PLC
PLC
900S
PLC
HOST
900S
DATA
HOST
900S
NO DATA
HOST
900S
DATA
#1 #2
#3

WMO-900S Radio Modem
Page 10 July 2010
1.4 Repeater Units
A WMO-900S unit may be used as a repeater to re-transmit radio messages. The purpose of
a repeater unit is to extend radio range.
In transparent mode, there can be an unlimited number of repeaters, however with some
conditions (refer to Section 3.5). The repeater in transparent mode will repeat every
transmission it receives.
In controlled mode, up to five repeaters may be configured for any transmission path. The
repeaters are configured by address.
PLC
900S
900S
RS485
PLC
PLC
900S
PLC
REPEATER
900S 900S 900S 900S
900S 900S 900S 900S

User Manual
INM WMO900S_v1.9a.doc Page 11
Chapter Two INSTALLATION
2.1 General
The WMO-900S module is housed in a rugged aluminium case, suitable for DIN-rail
mounting. Terminals will accept wires up to 12 gauge (2.5 sqmm) in size.
All connections to the module must be SELV. Normal 110-240V mains supply should
not be connected to any terminal of the WMO-900S module. Refer to Section 2.3 Power
Supply.
WARNING – EXPLOSION HAZARD: Do not connect or disconnect this equipment
while circuit is live unless the area is known to be non-hazardous.
Before installing a new system, it is preferable to bench test the complete system.
Configuration problems are easier to recognize when the system units are adjacent.
Following installation, the most common problem is poor communications caused by
incorrectly installed antennas, or radio interference on the same channel, or the radio path
being inadequate. If the radio path is a problem (i.e. the path is too long, or obstructions are
in the way), then higher performance antennas or a higher mounting point for the antenna
may rectify the problem. Alternately, use an intermediate WMO-900S Module as a repeater.
The WMO-900S Installation Guide supplied provides an installation drawing appropriate to
most applications. Further information is detailed below.
Each WMO-900S module should be effectively earthed via the "GND" terminal on the
module - this is to ensure that the surge protection circuits inside the WMO-900S module are
effective.
2.2 Antenna Installation
The WMO-900S module will operate reliably over large distances. The distance which may
be reliably achieved will vary with each application - depending on the type and location of
antennas, the degree of radio interference, and obstructions (such as hills or trees) to the radio
path. Typical reliable distances are :
USA/Canada 15 miles 6dB net gain antenna configuration permitted. (4W ERP)
Australia/NZ 12 km unity gain antenna configuration. (1W ERP)
Longer distances can be achieved if one antenna is mounted on top of a hill.
To achieve the maximum transmission distance, the antennas should be raised above
intermediate obstructions so the radio path is true “line of sight”. Because of the curvature of
the earth, the antennas will need to be elevated at least 15 feet (5 metres) above ground for
paths greater than 3 miles (5 km). The modules will operate reliably with some obstruction
of the radio path, although the reliable distance will be reduced. Obstructions close to either
antenna will have more of a blocking affect than obstructions in the middle of the radio path.
For example, a group of trees around the antenna is a larger obstruction than a group of trees
further away from the antenna. The WMO-900S modules provide a test feature which
displays the radio signal strength.
Line-of-sight paths are only necessary in order to obtain the maximum range. Obstructions
will reduce the range, but may still provide a reliable path. A larger amount of obstruction
can be tolerated for shorter distances and, for very short ones, it is even possible to mount the

WMO-900S Radio Modem
Page 12 July 2010
antenna inside a building. An obstructed path requires testing to determine whether the path
will be reliable - refer to Section 6 of this manual.
Where it is not possible to achieve reliable communications between two WMO-900S
modules, a third one may be used to receive the message and re-transmit it. This module is
referred to as a repeater. This module could also have a host device connected to it.
An antenna should be connected to the module via 50 ohm coaxial cable (eg RG58, Cellfoil
or RG213) terminated with a male SMA connector. The higher the antenna is mounted, the
greater will be the transmission range; however, as the length of coaxial cable increases so do
the cable losses. For use on unlicensed frequency channels, there are several types of
suitable antennas, but it is important to choose one carefully in order to avoid contravening
the maximum permitted power limit on the unlicensed channel. If in doubt, refer to an
authorised service provider.
The net gain of an antenna/cable configuration is the gain of the antenna (in dBi) less the loss
in the coaxial cable (in dB).
The maximum net gain of the antenna/cable configuration permitted is:
Country Max. gain (dB)
USA / Canada 6
Australia / New Zealand 0
The gains and losses of typical antennas are:
Antenna Gain (dB)
Dipole with integral 15’ cable 0
5dBi Collinear (3dBd) 5
8dBi Collinear (6dBd) 8
6 element Yagi 10
9 element Yagi 12
16 element Yagi 15
Cable type Loss (dB per 30 ft / 10 m)
RG58 -5
RG213 -2.5
Cellfoil -3
The net gain of the antenna/cable configuration is determined by adding the antenna gain and
the cable loss. For example, a 6 element Yagi with 70 feet (20 metres) of Cellfoil has a net
gain of 4dB (10dB – 6dB).
Connections between the antenna and coaxial cable should be carefully taped to prevent
ingress of moisture. Moisture ingress in the coaxial cable is a common cause for problems
with radio systems, as it greatly increases the radio losses. We recommend that the
connection be taped, firstly with a layer of PVC Tape, then with a vulcanising tape such as
“3M 23 tape”, and finally with another layer of PVC UV Stabilised insulating tape. The first
layer of tape allows the joint to be easily inspected when trouble shooting as the vulcanising
seal can be easily removed.
Where antennas are mounted on elevated masts, the masts should be effectively earthed to
avoid lightning surges. For high lightning risk areas, surge suppression devices between the
module and the antenna are recommended. If the antenna is not already shielded from

User Manual
INM WMO900S_v1.9a.doc Page 13
lightning strike by an adjacent earthed structure, a lightning rod may be installed above the
antenna to provide shielding.
2.2.1 Dipole and Collinear antennas.
A collinear antenna transmits the same amount of radio power in all directions - as such that
are easy to install and use. The dipole antenna with integral 15 ‘cable does not require any
additional coaxial cable, however a cable must be used with the collinear antennas.
Collinear and dipole antennas should be mounted vertically, preferably 3 feet (1 metre) away
from a wall or mast to obtain maximum range.
2.2.2 Yagi antennas.
A Yagi antenna provides high gain in the forward direction, but lower gain in other
directions. This may be used to compensate for coaxial cable loss for installations with
marginal radio path.
The Yagi gain also acts on the receiver, so adding Yagi antennas at both ends of a link
provides a double improvement.
Yagi antennas are directional. That is, they have positive gain to the front of the antenna, but
negative gain in other directions. Hence Yagi antennas should be installed with the central
beam horizontal and must be pointed exactly in the direction of transmission to benefit from
the gain of the antenna. The Yagi antennas may be installed with the elements in a vertical
plane (vertically polarised) or in a horizontal plane (horizontally polarised). For a two station
installation, with both modules using Yagi antennas, horizontal polarisation is recommended.
If there are more than two stations transmitting to a common station, then the Yagi antennas
should have vertical polarisation, and the common (or “central” station should have a
collinear (non-directional) antenna.
1m minimum
COLINEAR
ANTENNA
MAST
EARTH STAKE
IF GROUND CONDITIONS ARE
POOR, INSTALL MORE THAN
INSTALL AERIAL ABOVE
LOCAL OBSTRUCTIONS
ANT
900S
SURGE
ARRESTOR
(OPTIONAL) COAXIAL CABLE
WEATHERPROOF
CONNECTORS WITH
“3M 23” TAPE
STRESS RELIEF LOOP
PROVIDE GOOD
GROUND
CONNECTION TO
MAST, MODULE
AND SURGE
ARRESTOR
GND

WMO-900S Radio Modem
Page 14 July 2010
Also note that Yagi antennas normally have a drain hole on the folded element - the drain
hole should be located on the bottom of the installed antenna.
900S
Antenna installed
with drain holes
down
Coax feed
loo
p
ed
90
o

User Manual
INM WMO900S_v1.9a.doc Page 15
2.3 Power Supply
The WMO-900S module is powered by a 10 - 30VDC or 13 – 24VAC supply. The power
supply should be rated at 1 Amp and be CSA Certified Class 2.
WARNING – EXPLOSION HAZARD: Do not connect or disconnect while the circuit is
live unless the area is known to be non-hazardous
For DC supplies, the negative side of the supply
is connected to "COM" and may be connected to
“ground”. The supply negative is connected to
the “GND” terminal internally. The positive side
of the supply must not be connected to earth.
The DC supply may be a floating supply or
negatively grounded.
The power requirement of the WMO-900S units
is 80mA at 12VDC or 50mA at 24VDC. The
power requirement in low power mode is 20mA
at 12VDC.
2.4 Serial Connections
2.4.1 RS232 Serial Port
The serial port is a 9 pin DB9 female and provides for connection to a host device as well as
a PC terminal for configuration, field testing and for factory testing. This port is internally
shared with the RS485 - ensure that the RS485 is disconnected before attempting to use the
RS232 port. Communication is via standard RS232 signals. The WMO-900S is configured
as DCE equipment with the pinout detailed below.
900S
DB9
MALE
DTE HOST
DB9
FEMALE
900S
DB9
MALE
DCE HOST
DB9
MALE
Hardware handshaking using the CTS/RTS lines is provided. The CTS/RTS lines may be
used to reflect the status of the local unit’s input buffer, or may be configured to reflect the
status of CTS/RTS lines at the remote site. The WMO-900S does not support XON/XOFF.
Example cable drawings for connection to a DTE host (a PC) or another DCE host (or modem)
are detailed above.
900S
10 – 30 +
VDC -
+
-
900S
13 - 24
VAC
+
-

WMO-900S Radio Modem
Page 16 July 2010
DB9 Connector Pinout
Pin Name Direction Function
1 DCD Out Data carrier detect –
- on when link is established in controlled mode
- on always in transparent mode
2 RD Out Transmit Data – Serial Data Output
3 TD In Receive Data – Serial Data Input
4 DTR In Data Terminal Ready - DTR can be configured to initiate low power
mode, or to force a link disconnection (“hang up” in controlled mode.
5 SG Signal Ground
6 DSR Out Data Set Ready - always high when unit is powered on.
7 RTS In Request to Send - hardware flow control configurable
8 CTS Out Clear to send - hardware flow control configurable
9 RI Ring indicator - indicates another module is attempting to connect in
controlled mode.
2.4.2 RS485 Serial Port
The RS485 port provides for communication between the WMO-900S unit and its host device
using a multi-drop cable. Up to 32 devices may be connected in each multi-drop network. Note
that the RS485 port is shared internally with the RS232 port - make sure that the RS232 port is
disconnected before using the RS485 port.
As the RS485 communication medium is shared, only one of the units on the RS485 cable
may send data at any one time. Thus communication protocols based on the RS-485
standard require some type of arbitration.
RS485 is a balanced, differential standard but it is recommended that shielded, twisted pair
cable be used to interconnect modules to reduce potential RFI. It is important to maintain the
polarity of the two RS485 wires. An RS485 network should be wired as indicated in the
diagram below and terminated at each end of the network with a 120 ohm resistor. On-board
120 ohm resistors are provided and may be engaged by operating the single DIP switch in the
end plate next to the RS485 terminals. The DIP switch should be in the “1” or “on” position
to connect the resistor. If the module is not at one end of the RS485 cable, the switch should
be off.

User Manual
INM WMO900S_v1.9a.doc Page 17
HOST WMO-900S HOST
RS485 CONNECTIONS

WMO-900S Radio Modem
Page 18 July 2010
Chapter Three OPERATION
3.1 Power-up and Normal Operation
When power is initially connected to the WMO-900S module, the module will perform
internal diagnostics to check its functions. The following table details the status of the
indicating LEDs on the front panel under normal operating conditions.
LED Indicator Condition Meaning
OK On Normal Operation
Radio RX GREEN flash
RED flash
Radio receiving data
Weak radio signal
Radio TX Flash Radio Transmitting
Serial RX GREEN flash
RED flash
Serial Port Receiving
CTS low
Serial TX GREEN flash Serial Port Transmitting
DCD On Transparent mode - always on
Controlled mode - on when
communications link is established
DCD Off Communications failure or link not
established
Other conditions indicating a fault are described in Chapter Six Troubleshooting.
3.2 Serial and Radio Data
The WMO-900S module provides a full-duplex RS232 serial port and half-duplex RS485
serial port - only one serial port can be used at any one time. The radio communications is
half-duplex - this means that the WMO-900S operates at half duplex. Many applications use
full duplex RS232 communications but do not require full duplex - the protocol used operates
at half-duplex and will operate with the WMO-900S without problems. If an application
really requires full duplex communications, then the WMO-900S should not be used.
Data input at the serial port is placed into the input buffer. This buffer will store 2Kbytes of
data, and CTS/RTS control can be configured on the RS232 port to prevent overflow.
When the WMO-900S unit detects data in the input buffer, it initiates a radio message. The
radio message will end when the number of transmitted bytes reaches the maximum message
length (configurable by the user), or if the input buffer becomes empty.
If the configured serial data rate is the same or more than the radio data rate, then data is
transmitted as soon as it enters the input buffer - data “streams” from the input buffer to the

User Manual
INM WMO900S_v1.9a.doc Page 19
radio port. If the serial rate is less than the radio rate, then the transmission will be delayed
for a period to allow sufficient data to build up in the input buffer to avoid the radio emptying
the input buffer before a complete serial message has been input. The WMO-900S will
calculate the amount of delay depending on the difference between the serial and radio rates.
The radio transmission will stop when the input buffer is empty or when the radio has
transmitted the maximum number of bytes (user configurable - maximum 530 bytes). If there
is still data in the input buffer, the WMO-900S will start another radio transmission.
If error checking is configured, then a 16 bit CRC error-check is added to the end of the
transmitted data packet. The receiving module will receive the full data packet and check the
CRC before outputting the data.
The maximum size of the data packet is configurable by the user (maximum is 530 bytes). If
the data input to the WMO-900S is less than the maximum size, then the WMO-900S will
transmit the actual data input. If the data exceeds the maximum packet size, then the WMO-
900S will transmit multiple packets until all of the data is transmitted.
Because of radio start-up delays, the effective radio data rate will be lower than the
transmitted data rate. If you are sending large blocks of data, and the serial rate is equal or
more than the radio rate, we recommend that you use CTS/RTS flow control to prevent the
input buffer from overflowing. If you use the maximum data rate of 115,200 bits/sec, the
best effective overall rate is approx 65,700 bits/sec using a serial speed of 76,800 bits/sec. If
you use a serial speed of 115,200, the effective overall rate will be lower. If you are required
to use a serial speed of 115,200 bits/sec, then we suggest that you configure the maximum
packet size to 60 bytes - this will give faster performance than the default setting of 530
bytes.
3.2.1 Character Type
The WMO-900S may be configured by the user to recognize the following types of
characters - 7 or 8 data bits, even or odd or no parity, 1 or 2 stop bits.
Most applications will require the character type to be the same at each WMO-900S modem
in the system. Nevertheless, the character type may be configured to be different at different
WMO-900S modems. Data is transmitted by radio as an eight-bit byte without stop or start
bits. If the input data is 7 data bits, then the byte transmitted by radio comprises the 7 bits
plus a zero bit. Input characters with 8 bits are transmitted as just the 8 data bits, with no
parity. Because the data is transmitted without parity, the user may configure CRC error
checking to be added to each transmitted data packet. Data is output at the destination
module based on the character type configured at that module - that is, the start/stop bits and
parity is added to the radio data.
3.2.2 Serial Data Rate
The communications baud rates supported on both the RS232 serial port and the RS485 serial
port are 1200, 2400, 4800, 9600, 14400, 19200, 28800, 31250, 38400, 57600, 76800 and 115200
baud - the user selects one of these rates during the configuration of the modem.
3.2.3 Radio Data Rate
The data is transmitted by radio as direct modulated synchronous data at 19200, 57600 or
115200 bits/second. The user must configure the radio data rate at each WMO-900S module.
The configured radio data rates must be the same for each module in a system.

WMO-900S Radio Modem
Page 20 July 2010
At 19200 and 57600 bits/sec, the WMO-900S adds forward error correction in the
transmitted data. This is added automatically and is different to the configurable CRC error-
check.. The radio range at 19200 baud is better than 57600 or 115200. The expected range
at 57600 is 60% of maximum and at 115200 is 30%.
The radio message includes the following :-
A 30 msec leading sequence of alternating 1’s and 0’s provides the receiving unit
with time to capture and lock onto the incoming signal. This time is reduced to 5msec for the
“fast” controlled mode option (point-to-point only).
A system address is superimposed on each message to provide discrimination
between different WMO-900S systems on the same radio channel. Each WMO-900S unit in
the same system must be configured with the same system address - refer Section 4,
Configuration. Although other WMO-900S modules may hear the radio transmissions,
because they have a different system address, the radio transmission is ignored and no serial
data is output.
In transparent mode, a group address is included, and in controlled mode, unit
addressing is included.
An error-check (16 bit CRC) may be configured by the user.
Up to 530 bytes of data may be transmitted in a message - the maximum message size is
configurable between 10 and 530 bytes. The data consists of a sequence of 8 bit bytes. Start,
stop and parity bits are not transmitted, but they are re-generated at the receiving unit (if
configured).
A “transmit delay” time and a “receive delay” time may also be configured. These parameters
may be used to fine tune and give priority to different WMO-900S units in a system.
After each message is transmitted, a WMO-900S unit will not transmit another message
during the transmit delay time. This could be used to allow a reply message to be received
before the next message is sent. This delay is a different delay to the delay introduces by the
WMO-900S if the serial data rate is less than the radio data rate.
After a message is received, a message will not be transmitted during the receive delay time.
This could be used to delay a reply message until other messages have been sent.
3.3 Addressing
A WMO-900S network comprises modules with the same "system" address. Only modules
with the same system address will communicate with each other. This feature allows more
than one system to operate in the same area on the same radio channel. We recommend that
you select a random number for the system address.
In transparent mode, each module is also configured with a “group” address. A system may
comprise several groups or sub-systems. Only modules with the same system and group
address will communicate directly with each other - but modules with different group
addresses can communicate via a repeater. The group address is used for repeater operation
in transparent mode. The group address is not used in controlled mode.