R.5.7
Date Code 20111215 Reference Manual SEL-421 Relay
SEL Communications Protocols
Protocol Active When Setting PROTO := SEL
➤ yyyy is the 4-byte hex ASCII representation of the checksum.
Every checksum is followed by a new line indication
(<CR><LF>).
Software Flow Control
Software handshaking is a form of flow control that two serial devices use to
prevent 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.
Interleaved ASCII and
Binary Messages
SEL relays have two separate data streams that share the same physical serial
port. Human data communications with the relay consist of ASCII character
commands and reports that you view using a terminal or terminal emulation
package. The binary data streams can interrupt the ASCII data stream to
obtain information; the ASCII data stream continues after the interruption.
This mechanism uses a single communications channel for ASCII
communication (transmission of an event report, for example) interleaved
with short bursts of binary data to support fast acquisition of metering data.
The device connected to the other end of the link requires software that uses
the separate data streams to exploit this feature. However, you do not need a
device to interleave data streams in order to use the binary or ASCII
commands. Note that XON, XOFF, and CAN operations operate on only the
ASCII data stream.
An example of using these interleaved data streams is when the SEL-421
communicates with an SEL Communications Processor. The communications
processor performs auto-configuration by using a single data stream and SEL
Compressed ASCII and binary messages. In subsequent operations, the
communications processor uses the binary data stream for Fast Meter, Fast
Operate, and Fast SER messages to populate a local database and to perform
SCADA operations. At the same time, you can use the binary data stream to
connect transparently to the SEL-421 and use the ASCII data stream for
commands and responses.
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