Appendix A. Remote Control; Overview; Basic Protocol - Comtech EF Data CDM-740 Installation And Operation Manual

Appendix A. REMOTE CONTROL

A.1

Overview

This appendix describes the protocol and message command set for remote monitor and control
(M&C) of the CDM-740 Satellite Modem.
The electrical interface is an EIA-232 connection (for the control of a single device), and data is
transmitted in asynchronous serial form, using ASCII characters. Control and status information is
transmitted in packets, of variable length, in accordance with the structure and protocol defined in
later sections.
A.2 EIA-232
This is a configuration in which the 'Controller' device is connected directly to the 'Target'
device via a two-wire-plus-ground connection. Controller-to-Target data is carried, via EIA-232
electrical levels, on one conductor, and Target-to-Controller data is carried in the other direction
on the other conductor.
A.3

Basic Protocol

All data is transmitted as asynchronous serial characters, suitable for transmission and reception
by a UART. The character format should be 8N1 (eight data bits, no parity, one stop bit). The
baud rate may vary between 1200 and 38400 baud.
All data is transmitted in framed packets. The Controller is assumed to be a PC or ASCII dumb
terminal, which is in charge of the process of monitor and control. The Controller is the only
device that is permitted to initiate, at will, the transmission of data. Targets are only permitted to
transmit when they have been specifically instructed to do so by the Controller.
All bytes within a packet are printable ASCII characters, less than ASCII code 127. In this
context, the Carriage Return and Line Feed characters are considered printable.
With one exception, all messages from Controller-to-Target require a response – this will be either to
return data that has been requested by the Controller, or to acknowledge reception of an instruction to
change the configuration of the Target. The exception to this is when the Controller broadcasts a
message (such as Set time/date) using Address 0, when the Target is set to EIA-485 mode.
A–1