Comtech EF Data SLM-5650A - Manual (Page 354)

SLM-5650A Satellite Modem Revision 8
Appendix C MN-SLM5650A
C–2
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 Target s have line-receivers.
• Target-to-Controller pair has one line receiver (Controller), and all Targets have Tri-State drivers.
C.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.
C.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 format supported 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 EIA-485 mode.