Comtech EF Data CDM-570 - Manual (Page 352)

CDM-570/570L Satellite Modem with Optional IP Module Revision 12
Appendix D MN/CDM570L.IOM
D–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.
EIA-485 (full duplex) Summary:
Two differential pairs One pair for Controller-to-Target, one pair for Target-to-Controller.
Controller-to-Target pair One line driver (Controller), and all Targets have line receivers.
Target-to-Controller Pair One line receiver (Controller), and all Targets have Tri-State drivers.
D.3 EIA-232
This is 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 EIA-232 electrical levels on one conductor, and Target -to-Controller data is carried in
the other direction on the other conductor.
D.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 1200 and 38400
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 that 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 that 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.