Appendix C. Remote Control; Overview - Comtech EF Data CRS-300 Installation And Operation Manual

Appendix C. REMOTE CONTROL

C.1

Overview

This appendix describes the protocol and message command set for remote monitor and control
of the CRS-300 1:10 Redundancy Switch. 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.
Note: The Remote Control pin-out is shown in Table 6-2 in Chapter 6. CONNECTOR
PINOUTS for EIA-232 and EIA-485.
C.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. Many '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.
EIA-485 (full duplex) summary:
•
Two differential pairs – one pair for Controller-to-Target, one pair for Target-to-Controller.
•
Controller-to-Target pair has one line driver (Controller), and all Targets have line-receivers.
•
Target-to-Controller pair has one line receiver (Controller), and all Targets have Tri-State drivers.
C–1