Comtech EF Data MBT-4000 - Manual (Page 45)

A–1
Appendix A. REMOTE CONTROL
A.1 Overview
This appendix describes the protocol and message command set for remote monitor and control
of the MBT-4000 Multi-Band Transceiver System (more specifically, the BUC-4000 and
BDC-4000 modules).
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), 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.
A.2 RS-485
For applications where multiple devices are to be monitored and controlled, a full-duplex (or 4-
wire plus ground) RS-485 is preferred. Half-duplex (2-wire plus ground) RS-485 is possible, but
is not preferred. In full-duplex RS-485 communications, 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-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, 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.
RS-485 (full duplex) summary:
•
Two differential pairs – one pair for Controller-to-Target, one pair for Target-to-Controller.
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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.