7.7
Date Code 20080918
Instruction Manual
SEL-749M Relay
Communications
SEL ASCII Protocol Details
Software
Flow Control
The relay implements XON/XOFF flow control. You can use the XON/XOFF
protocol to control the relay during data transmission. When the relay receives
XOFF during transmission, it pauses until it receives an XON character. If
there is no message in progress when the relay receives XOFF, it blocks
transmission of any message presented to the relay input buffer. Messages will
be accepted after the relay receives XON.
The relay transmits XON (ASCII hex 11) and asserts the RTS output (if
hardware handshaking is enabled) when the relay input buffer drops below
25 percent full.
The relay transmits XOFF (ASCII hex 13) when the buffer is more than
75 percent full. If hardware handshaking is enabled, the relay deasserts the
RTS output when the buffer is approximately 95 percent full. Automatic
transmission sources should monitor for the XOFF character to avoid
overwriting the buffer. Transmission should terminate at the end of the
message in progress when XOFF is received and can resume when the relay
sends XON.
The CAN character (ASCII hex 18) aborts a pending transmission. This is
useful for terminating an unwanted transmission.
You can send control characters from most keyboards with the following
keystrokes:
XOFF
: <Ctrl+S>
(hold down the
<Ctrl>
key and press
S
)
XON:
<Ctrl+Q>
(hold down the
<Ctrl>
key and press
Q
)
CAN:
<Ctrl+X>
(hold down the
<Ctrl>
key and press
X
)
Automatic Messages
When the serial port AUTO setting is Y, the relay sends automatic messages to
indicate specific conditions. The automatic messages are described in
.
Access Levels
Commands can be issued to the SEL-749M via the serial port to view
metering values, change relay settings, etc. The available serial port
commands are listed in the
SEL-749M Relay Command Summary
at the end of
this manual. These commands can be accessed only from the corresponding
access level, as shown in the
SEL-749M Relay Command Summary
. The
access levels are:
➤
Access Level 0 (the lowest access level)
➤
Access Level 1
➤
Access Level 2 (the highest access level)
Table 7.4
Serial Port Automatic Messages
Condition
Description
Power Up
The relay sends a message containing the present date and
time, Relay and Terminal Identifiers, and the Access Level 0
prompt when the relay is turned on.
Event Trigger
The relay sends an event summary each time an event report
is triggered. See
.
Self-Test Warning or
Failure
The SEL-749M sends a status report each time a self-test
warning or failure condition is detected. See
mand (Relay Self-Test Status) on page U.7.24
.
Содержание SEL-749M
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