Appendix A. Remote Control; General - Comtech EF Data LBC-4000 Installation And Operation Manual

Appendix A. Remote Control

A.1 G
ENERAL
This section describes the protocol and message command set for remote monitor and
control of the L-Band converter.
The electrical interface is either an RS-485 multi-drop bus (for the control of many
devices) or an RS-232 connection (for the control of a single device). Data is transmitted
in asynchronous serial form, using ASCII characters. Control and status information is
transmitted in packets of variable length.
A.1.1 RS-485
For applications where multiple devices are to be monitored and controlled, a full-duplex
(or 4-wire) RS-485 is preferred. Half-duplex (2-wire) RS-485 is possible, but is not
preferred.
In full duplex RS-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 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 the intended one 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.
A-1