Omega A2400 manual Chapter Communications, Command $1RD Response *+99999.99

Chapter 3

Communications

Introduction

The A2400 modules have been carefully designed to be easy to interface to all radio modems and many leased-line modems. All communications to and from the modules are performed with printable ASCII characters. This allows the information to be processed with string functions common to most high- level languages such as BASIC. The ASCII format makes system debugging easy with a dumb terminal.

This system allows multiple modules to be connected through the A2400 to a modem with a single 4-wire cable. Up to 32 RS-485 modules may be strung together on one cable; 121 with repeaters. The modules communicate with the host on a polling system; that is, each module responds to its own unique address and must be interrogated by the host. A module can never initiate a communications sequence. A simple command/response protocol must be strictly observed to avoid communications collisions and data errors.

Communication to the A2400 modules is performed with two- or three- character ASCII command codes such as RD for Read Data. A complete description of all commands is given in the Chapter 4. A typical command/ response sequence would look like this:

Command: $1RD

Response: *+99999.99

A command/response sequence is not complete until a valid response is received. The host may not initiate a new command until the response from a previous command is complete. Failure to observe this rule will result in communications collisions. A valid response can be in one of three forms:

1)a normal response indicated by a ‘ * ‘ prompt

2)an error message indicated by a ‘ ? ‘ prompt

3)a communications time-out error

When a module receives a valid command, it must interpret the command, perform the desired function, and then communicate the response back to the host. Each command has an associated delay time in which the module is busy calculating the response. If the host does not receive a response in an appropriate amount of time specified in Table 3.1, a communications time-out error has occurred. After the communications time-out it is assumed that no response data is forthcoming. This error usually results when an