Comtech EF Data CDM-625 - Manual (Page 465)

D–1
Appendix D. REMOTE CONTROL
D.1 Overview
The CDM-625 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.
D.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.
In the other direction, on the other pair each Target has a tri-state line driver connected, and the Controller has a line-receiver
connected. All the line drivers are held in high-impedance mode until one (and only one) Target transmits back to the Controller. Each
Target has a unique address, and each time the Controller transmits, the address of the intended recipient Target is included in a framed
'packet' of data. All of the Targets receive the packet, but only one (the intended) will reply. The Target enables its output line driver and
transmits its return data packet back to the Controller in the other direction, on the physically separate pair.