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CDM-Qx Satellite Modem Revision 4
Remote Control MN/CDMQx.IOM
C-2
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.
a. Controller-to-target pair has one line driver (controller), and all targets have
line-receivers.
b. Target-to-controller pair has one line receiver (controller), and all targets have
tri-state drivers.
C.2 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.
C.3 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 fixed at 8N1. The baud rate may vary between 1200
and 38,400 baud.
All data is transmitted in framed packets. The controller is assumed to be a PC, 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.