7.8
SEL-387-0, -5, -6 Relay Instruction Manual Date Code 20170601
Serial Port Communications and Commands
Communications Protocol
2. The relay transmits all messages in the following format:
<STX><CR><LF>
<MESSAGE LINE 1><CR><LF>
<MESSAGE LINE 2><CR><LF>
.
.
.
LAST MESSAGE LINE><CR><LF>
<ETX> <PROMPT>
Each message begins with the start of transmission character
STX (ASCII character 02) and ends with the end of
transmission character ETX (ASCII character 03).
3. The relay indicates how full its receive buffer is through an
XON/XOFF protocol.
The relay transmits XON (ASCII hex 11) when the buffer
drops below 40 percent full.
The relay transmits XOFF (ASCII hex 13) when the buffer is
over 80 percent full. Automatic transmission sources should
monitor for the XOFF character so they do not overwrite the
buffer. Transmission should terminate at the end of the message
in progress when XOFF is received and may resume when the
relay sends XON.
4. You can use an XON/XOFF procedure to control the relay
during data transmission. When the relay receives an XOFF
command during transmission, it pauses until it receives an
XON command. If there is no message in progress when the
relay receives an XOFF command, it blocks transmission of
any message presented to its buffer.
The CAN character (ASCII hex 18) aborts a pending
transmission. This is useful in terminating an unwanted
transmission.
5. Control characters can be sent from most keyboards with the
following keystrokes:
XON: <Ctrl+Q> (hold down the Control key and press Q)
XOFF: <Ctrl+S> (hold down the Control key and press S)
CAN: <Ctrl+X> (hold down the Control key and press X)
SEL Distributed Port Switch Protocol
The SEL Distributed Port Switch Protocol (LMD) permits multiple SEL
relays to share a common communications channel. Select the protocol by
setting PROTOCOL = LMD, a SET P setting. See Appendix C: SEL
Distributed Port Switch Protocol (LMD) for more information.
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