15.27
Date Code 20171006 Instruction Manual SEL-400 Series Relays
Communications Interfaces
SEL Protocol
Software Flow Control
Software handshaking is a form of flow control that two serial devices use to pre-
vent input buffer overflow and loss of characters. The relay uses XON and XOFF
control characters to implement software flow control for ASCII commands.
The relay transmits the XOFF character when the input buffer is more than 75
percent full. The connected device should monitor the data it receives for the
XOFF character to prevent relay input buffer overflow. The external device
should suspend transmission at the end of a message in progress when it receives
the XOFF character. When the relay has processed the input buffer so that the
buffer is less than 25 percent full, the relay transmits an XON character. The
external device should resume normal transmission after receiving the XON
character.
The relay also uses XON/XOFF flow control to delay data transmission to avoid
overflow of the input buffer in a connected device. When the relay receives an
XOFF character during transmission, it pauses transmission at the end of the
message in progress. If there is no message in progress when the relay receives
the XOFF character, it blocks transmission of any subsequent message. Normal
transmission resumes after the relay receives an XON character.
Automatic Messages
If you enable automatic messages, AUTO = Y, the relay issues a message any
time the relay turns on, asserts a self-test, changes to another settings group, or
triggers an event. For virtual ports, the relay issues automatic messages only if
the connection is active. Automatic messages contain the following information:
➤ Power-up: When you turn on the relay, the message provides the
terminal ID and the present date and time.
➤ Self-test failure: When the relay detects an internal failure, the
automatic message is the same as the relay response to the STATUS
command.
➤ Group switch: Whenever a settings group change occurs, the
message contains the relay ID, terminal ID, present date and time,
and the selected settings group.
➤ Events: When the relay triggers an event, the automatic message is
the same as the relay response to the SUMMARY command.
Timeout
Use the TIMEOUT setting to set the idle time for each port. Idle time is the
period when no ASCII characters are transmitted and received (interleaved Fast
Messages do not affect the idle time). When the idle time exceeds the TIMEOUT
setting, the following takes place:
➤ The access level changes to Access Level 0.
➤ The front-panel targets reset to TAR 0 if the port had previously
remapped the targets.
➤ Virtual connections are disconnected.
➤ The software flow control state changes to XON.
When set to OFF, the port never times out.
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