Date Code 20111215 Reference Manual SEL-421 Relay
Section 5
R.Reference Manual
SEL Communications Protocols
This section describes features of the SEL-421 Relay communications
protocols and includes the following topics:
➤ Serial Port Hardware Protocol on page R.5.1
➤ Software Protocol Selections on page R.5.2
➤ Protocol Active When Setting PROTO := SEL on page R.5.3
➤ Virtual File Interface on page R.5.11
➤ SEL MIRRORED BITS Communications on page R.5.15
➤ SEL Distributed Port Switch Protocol (LMD) on page R.5.21
➤ SEL-2600A RTD Module Operation on page R.5.23
Serial Port Hardware Protocol
The serial ports comply with the EIA/TIA-232 Standard, commonly referred
to as EIA-232 (formerly known as RS-232). The serial ports support RTS/CTS
hardware flow control. See also Software Flow Control on page R.5.7.
Hardware Flow
Control
Hardware handshaking is one form of flow control that two serial devices use
to prevent input buffer information overflow and loss of characters. To support
hardware handshaking, connect the RTS output pin of each device to the CTS
input pin of the other device. To enable hardware handshaking, use the SET P
command (or front-panel {SET} pushbutton sequence) to set RTSCTS := Y.
Disable hardware handshaking by setting RTSCTS := N. Table 5. 1 shows
actions the relay takes for the RTSCTS setting values and the conditions
relevant to hardware flow control.
Table 5.1 Hardware Handshaking
Setting
RTSCTS Value
Condition Relay Action
N All Assert RTS output pin and
ignore CTS input pin.
Y Normal input reception Assert RTS output pin.
Y Local input buffer is close to full Deassert RTS pin to signal
remote device to stop transmit-
ting.
Y Normal transmission Sense CTS input is asserted,
transmit normally.
Y Remote device buffer is close to
full, so remote device deasserts RTS
Sense CTS input is deasserted,
stop transmitting
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