7.2
SEL-751A Relay Instruction Manual Date Code 20100129
Communications
Communications Interfaces
Be sure to evaluate the installation and communications necessary to integrate
with existing devices before ordering your SEL-751A. For example, consider
the fiber-optic interface in noisy installations or for large communications
distances. Following is general information on possible applications of the
different interfaces.
Serial (EIA-232 and
EIA-485) Port
Use the EIA-232 port for communications distances of up to 15 m (50 feet) in
low noise environment. Use the optional EIA-485 port for communications
distances up to 1200 m (4000 feet) 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 will 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-751A Relay
Some of the SEL devices available for integration or communication system
robustness are included in the following list:
➤ SEL Communications Processors (SEL-2032, SEL-2030,
SEL-2020)
➤ SEL-2800 series fiber-optic transceivers
➤ SEL-2890 Ethernet Transceiver
➤ SEL-3010 Event Messenger
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 listed below:
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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