7.16
SEL-751 Relay Instruction Manual Date Code 20170927
Communications
Communications Protocols
SNTP Accuracy Considerations
The accuracy of the SNTP server and the networking environment limit SNTP
time synchronization accuracy. You can achieve the highest degree of SNTP
time synchronization by minimizing the number of switches and routers
between the SNTP server and the SEL-751. You can also use network
monitoring software to ensure that average and worst-case network bandwidth
use is moderate.
When installed on a network configured with one Ethernet switch between the
SEL-751 and the SNTP server, and when using ESNTP = UNICAST or
MANYCAST, the relay time synchronization error with the SNTP server is
typically less than ±1 millisecond.
SEL ASCII Protocol
and Commands
Message Format
SEL ASCII protocol is designed for manual and automatic communication.
All commands the relay receives must be of the following form:
A command transmitted to the relay consists of the command followed by
either a CR (carriage return) or a CRLF (carriage return and line feed). You
can truncate commands to the first three characters. For example,
EVENT 1 <Enter> becomes EVE 1 <Enter>. Use upper- and lowercase
characters without distinction, except in passwords.
The relay transmits all messages in the following format:
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.
Software Flow Control
The relay implements XON/XOFF flow control. 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 the relay input buffer. Messages are
accepted after the relay receives XON.
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 more than
75 percent full. If hardware handshaking is enabled, the relay deasserts the
RTS output when the buffer is approximately 95 percent full. Automatic
<command><CR> or <command><CRLF>
<STX><MESSAGE LINE 1><CRLF>
<MESSAGE LINE 2><CRLF>
•
•
•
<LAST MESSAGE LINE><CRLF><ETX>
NOTE: The <Enter> key on most
keyboards is configured to send the
ASCII character 13 (<Ctrl+M>) for a
carriage return. This manual instructs
you to press the <Enter> key after
commands to send the proper ASCII
code to the SEL-751.
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