Comtech EF Data CDM-625A - Manual (Page 313)

8–1
Chapter 8. SERIAL-BASED REMOTE PRODUCT
MANAGEMENT
8.1 Overview
The CDM-625A Advanced Satellite Modem serial remote product management interface is an electrical interface that is either an
EIA-485 multi-drop bus (for the control of multiple devices) or an EIA-232 connection (for the control of a single device). The interface
transmits data in asynchronous serial form, using ASCII characters. This data consists of control and status information, transmitted in
packets of variable length in accordance with the structure and protocol explained later in this appendix.
8.2 EIA-485
For applications where multiple devices are to be monitored and controlled, a full-duplex (or 4-wire plus ground) EIA-485 is preferred.
Half-duplex (2-wire plus ground) EIA-485 is possible, but is not preferred. In full-duplex EIA-485 communication, there are two
separate, isolated, independent, differential-mode twisted pairs, each handling serial data in different directions.
It is assumed that a 'Controller' device (a PC or dumb terminal) transmits data in a broadcast mode via one of the pairs. Multiple
'Target' devices are connected to this pair, and all simultaneously receive data from the Controller. The Controller is the only device
with a line-driver connected to this pair – the Target devices have only line-receivers connected.