Omega A2400 User Manual
Page 17
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