C.1 Overview - Comtech EF Data SLM-5650A Installation And Operation Manual

C.1 Overview
This appendix describes the protocol and message command set for remote monitor and control of the SLM-5650A Satellite Modem.
The electrical interface is either an EIA-485 multi-drop bus (for the control of many devices) or 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.
C.2 EIA-485
For applications where multiple devices are to be monitored and controlled, a full-duplex (or 4-wire) EIA-485 is preferred. Half-duplex
(2-wire) 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 there is a 'Controller' device (a PC or dumb terminal), which transmits data in a broadcast
mode via one of the pairs. Many 'Target' devices are connected to this pair, which all simultaneously receive data from the Controller.
The Controller is the only device with a line-driver connected to this pair – the Target devices only have line-receivers connected.
In the other direction, on the other pair each Target has a Tri-Stateable 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 in a framed 'packet' of data, the address of the intended recipient
Target is included. 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.
Appendix C. REMOTE CONTROL
C–1