Sel 551 - SEL ASCII Protocol

5.5
Date Code 20110408 Instruction Manual SEL-551 Relay
Serial Port Communications and Commands
Communications Protocol
Based upon the port PROTOCOL setting, the relay activates either SEL
ASCII, SEL LMD, or Modbus RTU protocol for the given port. SEL Fast
Meter and SEL Compressed ASCII commands are always active for the given
port if the corresponding protocol is SEL ASCII or LMD. SEL Fast Meter and
SEL Compressed ASCII are not available on the port if the protocol is
Modbus RTU.
SEL ASCII Protocol
SEL ASCII protocol is designed for manual and automatic communications.
1. All commands received by the relay must be of the form:
<command><CR> or <command><CRLF>
A command transmitted to the relay should consist of the
command followed by either a CR (carriage return) or a CRLF
(carriage return and line feed). You may truncate commands to
the first three characters. For example, EVENT 1 <Enter>
would become EVE 1 <Enter>. Upper- and lower-case
characters may be used without distinction, except in
passwords.
2. The relay transmits all messages in the following format:
<STX><MESSAGE LINE 1><CRLF>
<MESSAGE LINE 2><CRLF>
•
•
•
<LAST MESSAGE LINE><CRLF>< ETX>
Each message begins with the start-of-transmission character
(ASCII 02) and ends with the end-of-transmission character
(ASCII 03). Each line of the message ends with a carriage
return and line feed.
3. The relay implements XON/XOFF flow control.
The relay transmits XON (ASCII hex 11) and asserts the RTS
output (if hardware handshaking is enabled) when the relay
input buffer drops below 25 percent full.
The relay transmits XOFF (ASCII hex 13) when the buffer is
over 75 percent full. If hardware handshaking is enabled, the
relay deasserts the RTS output when the buffer is
approximately 95 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 the XON/XOFF protocol to control the relay
during data transmission. When the relay receives XOFF
during transmission, it pauses until it receives an XON
character. If there is no message in progress when the relay
receives XOFF, it blocks transmission of any message
presented to its buffer. Messages will be accepted after the
relay receives XON.
NOTE: The <Enter> key on most
keyboards is configured to send the
ASCII character 13 (^M) for a carriage
return. This manual instructs you to
press the Enter key after commands,
which should send the proper ASCII
code to the relay.