Mocomtech SLM-5650 - Manual (Page 124)

SLM-5650 Satellite Modem Revision 1
Remote Control MN/SLM5650.IOM
B–2
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.
EIA 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.
B.3 EIA-232
This 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 EIA-232 electrical
levels, on one conductor, and target-to-controller data is carried in the other direction on the other
conductor.
B.4 Basic Protocol
Whether in EIA-232 or EIA-485 mode, all data is transmitted as asynchronous serial characters,
suitable for transmission and reception by a UART. In this case, the asynchronous character
formats include 7O2, 7E2, and 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 EIA-485
mode.