Chapter Getting Started, Introduction

Chapter 1 Getting Started

Introduction

This manual describes the function and application of the Radio Modem Interface Module (A2400). The A2400 provides an intelligent interface between radio modems available from many manufacturers and devices designed to operate on a bi-directional RS-485 serial bus. Although the A2400 has been designed specifically for our family of industrial I/O modules, it may also be used with other RS-485 devices.

Figure 1.1 depicts a typical application that incorporates A2400’s. In many data acquisition situations, the sensor data is inaccessible to the host computer due to large distances or the lack of telephone facilities to incorporate conventional dial-up modems. In some cases, sensor data may have to be monitored full time and the cost of telephone service can be prohibitive. For these and a multitude of other reasons the use of a radio link can be the best solution.

Unfortunately, radio modems are designed for computer-to-computer com- munications and require a certain amount of intelligence at each radio site in order to construct useful systems. The cost of a local computer at each radio can easily make the concept impractical. The Radio Modem Interface Module (A2400) fills the need for a low-cost intelligent interface between the radio modem and the RS-485 data acquisition devices.

In a typical system as shown in Figure 1.1, there is one host or master computer and any number of slave sites. The master radio transmitter and the slave receivers communicate on the same radio frequency, and of course, the slave transmitters and the master receiver are tuned to the same frequency. While it is common to use two frequencies for simultaneous transmitting and receiving, it is possible to use one frequency for all communications. In an idle condition all slave transmitters are turned off. Each slave site is assigned a unique address so that the master may direct commands to a particular site. To initiate a communications sequence, the master will transmit a command by radio which is received by all the slaves. The transmitted command contains an address which directs the command to a particular slave site. The slave site that matches the address will respond to the command. At this time, the addressed slave site will turn on its radio transmitter and communicate back to the master in response to the command. Once the response is complete the slave will turn off the transmitter and wait for a new command. To avoid interference, only one slave transmitter can be on at any given time. The primary function of the