User's Manual

905U-D Radio Modem Module User Manual
Page 8 © May 2000
Chapter One INTRODUCTION
1.1 General
The 905U-D 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 905U-D 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 905U-D 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 905U-D unit can receive data from the host device and transmit this data by radio to
another (or several) 905U-D module. The other module will recreate the serial data and
output it as either a RS232 or RS485 serial signal. The 905U-D unit provides two-way
communications - each module can accept serial data and also output serial data.
The 905U-D module includes power supply, microprocessor controller, serial input/output
circuits and a UHF radio transceiver - no external electronics are required. The 905U-D 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 905U-D
modules in many countries. See Chapter Five Specifications for details. The units are
configured from a terminal using Hayes commands or by using switches under the plastic
cover on the front of the unit.
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 905U-D
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 905U-D unit
and output as RS485 data to another host device.
The 905U-D 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 905U-D 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 905U-D will accept a variety of 7 or 8 data bit characters
• Serial Data Rate - between 75 and 38400 bits/sec
• Radio Data Rate - between 1200 and 9600 bits/sec
• Operating mode - transparent mode or controlled mode .
The operation of the 905U-D radio modem is relatively simple. As data is received at the
serial port, the data is transmitted on the radio channel. Up to 520 bytes of data can be