Schweitzer Engineering Laboratories SEL-251 - Communications Protocol

Date Code 970103 Communications 3-3
SEL-251, -2, -3 Instruction Manual
Table 3.2: SEL-251 Plug-In Connector Model
Relay Serial Port Connector Pin Assignments
Pin
Port 1,
Port 2R Port 2F Description
1 +5 Vdc N/C +5 Vdc available if JMP12 installed
2 RXD RXD Receive data input.
3 TXD TXD Transmit data output.
4 +IRIG-B N/C +IRIG-B available if JMP14 installed
5 GND GND
6 -IRIG-B N/C -IRIG-B available if JMP13 installed
7 RTS RTS The relay asserts this line under normal conditions.
When its received-data buffer is full, the line is deas-
serted, and asserts again when the buffer has sufficient
room to receive more data. Connected devices should
monitor RTS (usually with their CTS input) and stop
transmission whenever the line deasserts. If transmis-
sion continues, data may be lost.
8 CTS CTS The relay monitors CTS, and transmits characters only
if CTS is asserted.
9 GND GND Ground for ground wires and shields
COMMUNICATIONS PROTOCOL
Communications protocol consists of hardware and software features. Hardware protocol
includes the control line functions described above. The following software 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>
Thus, a command transmitted to the relay should consist of the command followed by either
a carriage return or a carriage return and line feed. You may truncate commands to the first
three characters. Thus, EVENT 1 <ENTER> would become EVE 1 <ENTER>. Upper
and lower case characters may be used without distinction, except in passwords.
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.