Sel 751A Instruction Manual Download

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I.4

SEL-751A Relay

Instruction Manual

Date Code 20100129

IRRORED

ITS

Communications

Operation

In summary, when a relay detects an error, it transmits an attention message
until it receives an attention message with its own TX_ID included. If three or
four relays are connected in a ring topology, the attention message will go all
the way around the loop until the originating relay receives it. The message
then dies and data transmission resumes. This method of synchronization
allows the relays to reliably determine which byte is the first byte of the
message. It also forces unsynchronized UARTs to become resynchronized. On
the down side, this method takes down the entire loop for a receive error at any
relay in the loop. This decreases availability. It also makes one-way
communications impossible.

Loopback Testing

Use the

LOOP

command to enable loopback testing. In the loopback mode,

you loop the transmit port to the receive port of the same relay to verify
transmission messages. While in loopback mode, ROK

is deasserted, and

another user accessible Relay Word bit, LBOK

(Loop Back OK) asserts and

deasserts based on the received data checks (see the

Section 7:

Communications

for the ACSII commands).

Channel Monitoring

Based on the results of data checks (described above), the relay collects
information regarding the 255 most recent communications errors. Each
record contains at least the following fields:

➤

DATE—Date when the dropout occurred

➤

TIME—Time when the dropout occurred

➤

RECOVERY_DATE—Date when the channel returned to
service (if the channel is currently failed, it is displayed and
included in the calculations, as if its recovery were to occur at
the time the report was requested)

➤

RECOVERY_TIME—Time when the channel returned to
service (if the channel is currently failed, it is displayed and
included in the calculations, as if its recovery were to occur at
the time the report was requested)

➤

DURATION—Time elapsed during dropout

➤

CAUSE—Reason for dropout (see

Message Decoding and

Integrity Checks on page I.2

)

There is a single record for each outage, but an outage can evolve. For
example, the initial cause could be a data disagreement, but framing errors can
extend the outage. If the channel is currently failed, it is displayed and
included in the calculations, as if its recovery were to occur at the time the
report was requested.

NOTE:

Combine error conditions

including RBADA, RBADB, CBADA, and
CBADB with other alarm conditions
using SEL

OGIC

control equations. You

can use these alarm conditions to
program the relay to take appropriate
action when it detects a
communications channel failure.

When the duration of an outage on Channel A or B exceeds a user-definable
threshold, the relay asserts a user-accessible Relay Word bit, RBADA or
RBADB. When channel unavailability exceeds a user-definable threshold for
Channel A or B, the relay asserts a user-accessible Relay Word bit, CBADA or
CBADB. Use the

COMM

command to generate a long or summary report of

the communications errors.

Use the RBADPU setting to determine how long a channel error must last
before the meter element RBADA is asserted. RBADA is deasserted when the
channel error is corrected. RBADPU is accurate to ±1 second.

Sel 751A, Instruction Manual

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