G.3
Date Code 20090730 Instruction Manual SEL-734 Meter
MIRRORED BITS Communications
Operation
If its own TX_ID is not included, the node deasserts ROKx, includes its
TX_ID in the attention message, and transmits the new attention message.
If its own TX_ID is the only TX_ID included, the meter assumes the message
is corrupted unless the loopback mode has been enabled. If loopback is not
enabled, the node deasserts ROKx and transmits the attention message with its
TX_ID included. If loopback is enabled, the meter transmits data.
In summary, when a node detects an error, it transmits an attention message
until it receives an attention message with its own TX_ID included. If three or
four meters are connected in a ring topology, the attention message will go all
the way around the loop until it is received by the originating node. The
message then dies, and data transmission resumes.
This method of synchronization allows the meters 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 node in the loop. This decreases availability. It
also makes one-way communications impossible.
Loopback Testing
Use the LOOP command to enable loopback testing.
While in loopback mode, ROKx is deasserted, and another user-accessible
flag, LBOKx, asserts and deasserts according to received data checks.
Channel Monitoring
Based on the results of data checks described above, the meter 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 on page G.2)
Use the COM 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 COM
record will only show the initial cause, but the COM summary will display the
present cause of failure.
When the duration of an outage exceeds a user-settable threshold, the meter
asserts a user-accessible flag called RBADx.
You would typically combine RBADx with other alarm conditions through use
of SEL
OGIC control equations.
When channel unavailability exceeds a user-settable threshold, the meter
asserts a user-accessible flag called CBADx.
You would typically combine CBADx with other alarm conditions through use
of SEL
OGIC control equations.
Courtesy of NationalSwitchgear.com
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