Schweitzer Engineering Laboratories SEL-2020, Instruction Manual

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Date Code 20010518 Appendix C C-1
SEL-2020 Instruction Manual
APPENDIX C: LMD PROTOCOL
This protocol permits multiple SEL devices to share a common communications channel. It is
appropriate for low-cost, low-speed port switching applications where updating a real-time
database is not a requirement.
S
ETTINGS
Use the SET P command to activate the multidrop protocol. Change the PROTOCOL port
setting from the default SEL to LMD to reveal the following settings:
ADDRESS: Two character ASCII address. The range is "01" to "99". The
default is "01".
PREFIX: One character to precede the address. This should be a character
which does not occur in the course of other communications with the
relay. Valid choices are one of the following: "@" "#" "$" "%" "&".
The default is "@".
SETTLE TIME: Time in seconds that transmission is delayed after the request to
send (RTS line) asserts. This delay accommodates transmitters with
a slow rise time.
O
PERATION
1. The device ignores all input from this port until it detects the prefix character and the two-
byte address.
2. The device then asserts the RTS line, which you can use to key a serial data transmitter. The
port enables echo and message transmission. If the port has received an XOFF character, the
device performs as if it received an XON.
3. Wait until you receive a prompt before entering commands to avoid losing echoed characters
while the external transmitter is warming up.
4. Until the device connection terminates, you can use the standard commands that are available
when PROTOCOL is set to SEL.
5. The QUIT command terminates the connection. If no data are sent to the device before the
port timeup period, it automatically terminates the connection.
6. Enter the sequence CTRL-X QUIT <CR> before entering the prefix character if all devices
in the multidrop network do not have the same prefix setting.
Note: You can use the SET P command to change the port settings to return to SEL protocol.