Date Code 20011205
M
IRRORED
B
ITS
Communications
H-3
SEL-311A Instruction Manual
In summary, when a node detects an error, it transmits attention until it receives an attention with
its own TX_ID included. If three or four relays are connected in a ring topology, then the
attention message will go all the way around the loop, and will eventually be received by the
originating node. It will then be killed, and data transmission will resume. This method of
synchronization allows the relays to reliably determine which byte is the first byte of the
message. It also forces mis-synchronized UARTs to become re-synchronized. On the down side,
this method takes down the entire loop for a receive error at any node in the loop. This decreases
availability and makes one-way communications impossible.
Loopback Testing
Use the
LOO
command to enable loopback testing.
While in loopback mode, ROK
x
is deasserted, and another user-accessible flag, LBOK
x
will
assert and deassert based on the received data checks.
Channel Monitoring
Based on the results of data checks described above, the relay will collect information regarding
the 255 most recent communications errors. Each record will contain at least the following
fields:
·
Dropout Time/Date
·
Pickup Time/Date
·
Time elapsed during dropout
·
Reason for dropout (See Message Decoding and Integrity Checks)
Use the COMM command to generate a long or summary report of the communications errors.
There is only a single record for each outage, but an outage can evolve. For example, the initial
cause could be a data disagreement, but the outage can be perpetuated by framing errors. If the
channel is presently down, the COMM record will only show the initial cause, but the COMM
summary will display the present cause of failure.
When the duration of an outage exceeds a user-settable threshold, the relay will assert a user-
accessible flag, hereafter called RBAD
x
. Note: The user will typically combine RBAD
x
with
other alarm conditions using SEL
OGIC
control equations.
When channel unavailability exceeds a user-settable threshold, the relay will assert a user
accessible flag, hereafter called CBAD
x
. Note: The user will typically combine CBAD
x
with
other alarm conditions using SEL
OGIC
control equations.
Содержание SEL-311A
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