Comtech EF Data CRS-200 REDUNDANCY SWITCH - Manual (Page 45)

CRS-200 COMTECH COMMUNICATIONS CORPORATION
CRS-200 REDUNDANCY SWITCH - OPERATIONS MANUAL PAGE 43
12 REMOTE CONTROL
12.1 INTRODUCTION
This section describes the protocol and message command set for remote monitor and
control of the CRS-200 Redundancy Switch.
The electrical interface is either an RS485 multi-drop bus (for the control of many devices)
or an RS232 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.
12.2 RS485
For applications where multiple devices are to be monitored and controlled, a full-duplex
(or 4-wire plus ground) RS485 is preferred. Half-duplex (2-wire plus ground) RS485 is
possible, but is not preferred.
In full-duplex RS485 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), that transmits data, in a broadcast
mode, via one of the pairs. Many ‘target’ devices are connected to this pair, that 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.
RS 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.