5.4
SEL-551 Relay Instruction Manual Date Code 20110408
Serial Port Communications and Commands
Communications Protocol
For long-distance communications up to 500 meters and for electrical
isolation of communications ports, use the SEL-2800 or SEL-2810 Fiber-
Optic Transceivers. Contact SEL for more details on these devices.
Communications Protocol
Serial communications with the relay includes hardware and software
protocols.
Hardware Protocol
SEL-551 relays equipped with an EIA-232 port support RTS/CTS hardware
handshaking. RTS/CTS handshaking is not supported on relays equipped with
just the EIA-485 port.
To enable hardware handshaking, use the SET P command (or front-panel
{SET} pushbutton) to set RTS_CTS = Y. Disable hardware handshaking by
setting RTS_CTS = N.
➤ If RTS_CTS = N, the relay permanently asserts the RTS line.
➤ If RTS_CTS = Y, the relay deasserts RTS when it is unable to
receive characters.
➤ If RTS_CTS = Y, the relay does not send characters until the
CTS input is asserted.
Software Protocols
Software protocols consist of:
➤ Standard SEL ASCII
➤ SEL Distributed Port Switch Protocol (LMD)
➤ SEL Fast Meter
➤ SEL Compressed ASCII
➤ Modbus
®
RTU
Table 5.1 Serial Communications Port Pin Function Definitions
Pin Function Definition
N/C No Connection
+5 V dc 5 Volt DC Power Connection
RXD, RX Receive Data
TXD, TX Transmit Data
IRIG-B IRIG-B Time-Code Input
GND Ground
SHIELD Shielded Ground
RTS Request To Send
CTS Clear To Send
DCD Data Carrier Detect
DTR Data Terminal Ready
DSR Data Set Ready
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