General; Message Structure; Start Character - Comtech EF Data SDM-650B Installation And Operation Manual

Remote Control Operation
B.1.1 General
Remote controls and status information are transferred via an RS-485 (optional RS-232-C)
serial communications link.
Commands and data are transferred on the remote control communications link as
US ASCII-encoded character strings.
The remote communications link is operated in a half-duplex mode.
Communications on the remote link are initiated by a remote controller or terminal. The
modem never transmits data on the link unless it is commanded to do so.
B.1.2 Message Structure
The ASCII character format used requires 11 bits/character:
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Messages on the remote link fall into the categories of commands and responses.
Commands are messages which are transmitted to a satellite modem, while responses are
messages returned by a satellite modem in response to a command.
The general message structure is as follows:
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B.1.2.1 Start Character
A single character precedes all messages transmitted on the remote link. This character flags
the start of a message. This character is:
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B–2
1 start bit
7 information bits
1 parity bit (odd/even)
2 stop bits

Start Character

Device Address
Command/Response
End of Message Character
"<" for commands
">" for responses
SDM-650B Satellite Modem
Rev. 6