Comtech EF Data CDM-700 - Manual (Page 152)

CDM-700 High Speed Satellite Modem Revision 5
Remote Control MN/CDM700.IOM
11–2
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.
11.3 RS-232
This a much simpler configuration in which the controller device is connected directly to the
target via a two-wire-plus-ground connection. Controller-to-target data is carried, via RS-232
electrical levels, on one conductor, and target-to-controller data is carried in the other direction on
the other conductor.
11.4 Basic Protocol
Whether in RS-232 or RS-485 mode, all data is transmitted as asynchronous serial characters,
suitable for transmission and reception by a UART. In this case, the asynchronous character
format is 8N1. The baud rate may vary between 2400 and 57,600 baud.
All data is transmitted in framed packets. The controller is assumed to be a PC or ASCII dumb
terminal, which is in charge of the process of monitor and control. The controller is the only
device which is permitted to initiate, at will, the transmission of data. Targets are only permitted
to transmit when they have been specifically instructed to do so by the controller.
All bytes within a packet are printable ASCII characters, less than ASCII code 127. In this
context, the Carriage Return and Line Feed characters are considered printable.
All messages from controller to target require a response (with one exception). This will be either to
return data which has been requested by the controller, or to acknowledge reception of an instruction
to change the configuration of the target. The exception to this is when the controller broadcasts a
message (such as Set time/date) using Address 0, when the target is set to RS-485 mode.