Comtech EF Data CDM-625A - Manual (Page 314)

CDM-625A Advanced Satellite Modem MN-CDM625A
Serial-based Remote Product Management Revision 3
8–2
In the other direction, on the other pair each Target has a tri-state 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, the address of the intended recipient Target is included in a
framed 'packet' of data. 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 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.
8.3 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.
8.4 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 Universal Asynchronous Receiver/Transmitter (UART). The asynchronous character format is 8-N-1 (8 data bits, no parity, 1 stop
bit). The baud rate may vary from 1200 thru 57600 baud.
All data is transmitted in framed packets. The Controller is in charge of the process of monitor and control, and 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.