7.2
SEL-751 Relay Instruction Manual Date Code 20170927
Communications
Communications Interfaces
Serial (EIA-232 and
EIA-485) Port
Use the EIA-232 port for communications distances as far as 15 m (50 ft) in
low noise environments. Use the optional EIA-485 port for communications
distances as far as 1200 m (4000 ft) maximum distance (to achieve this
performance, ensure proper line termination at the receiver).
To connect a PC serial port to the relay front-panel serial port and enter relay
commands, you need the following:
➤ A personal computer equipped with one available EIA-232
serial port
➤ A communications cable to connect the computer serial port to
the relay serial ports
➤ Terminal emulation software to control the computer serial port
➤ An SEL-751 Relay
Some of the SEL devices available for integration or communications system
robustness are included in the following list:
➤ SEL communications processors (SEL-2032, SEL-2030,
SEL-2020), SEL-3530 RTAC Real-Time Automation
Controller
➤ SEL-2800 series fiber-optic transceivers
➤ SEL-2890 Ethernet Transceiver
➤ SEL-3010 Event Messenger
➤ SEL-2505 Remote I/O Module (with SEL-2812 compatible ST
fiber-optic port) for connection to relay fiber-optic serial Port 2,
or use SEL-2505 with EIA-232 (DB-9) serial port to connect to
EIA-232 Port 3 on the relay
A variety of terminal emulation programs on personal computers can
communicate with the relay. For the best display, use VT-100 terminal
emulation or the closest variation.
The default settings for all EIA-232 serial ports are as follows:
Baud Rate = 9600
Data Bits = 8
Parity = N
Stop Bits = 1
To change the port settings, use the SET P command (see Section 6: Settings)
or the front-panel. Section 8: Front-Panel Operations provides details on
making settings with the front panel.
Hardware Flow Control
All EIA-232 serial ports support RTS/CTS hardware handshaking (hardware
flow control). To enable hardware handshaking, use the SET P command or
front-panel PORT submenu to set RTSCTS = Y. Disable hardware
handshaking by setting RTSCTS := N.
➤ If RTSCTS := N, the relay permanently asserts the RTS line.
➤ If RTSCTS := Y, the relay deasserts RTS when it is unable to
receive characters.
➤ If RTSCTS := Y, the relay does not send characters until the
CTS input is asserted.
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