J.3
Date Code 20060320 Instruction Manual SEL-311C Relay
MIRRORED BITS Communications
Operation
security counter set to two on the SEL-311C will delay a bit by 1/4 cycle,
because the SEL-311C is receiving new M
IRRORED BITS messages each
1/8 cycle from the SEL-321.
Synchronization
When a node detects a communications error, it deasserts ROKx and transmits
an attention message, which includes its TXID setting.
When a node receives an attention message, it checks to see if its TXID is
included.
If its own TXID is included and at least one other TXID is included, the node
transmits data.
If its own TXID is not included, the node deasserts ROKx, includes its TXID
in the attention message, and transmits the new attention message.
If its own TXID is the only TXID included, the relay assumes the message is
corrupted unless the loopback mode has been enabled. If loop back is not
enabled, the node deasserts ROKx and transmits the attention message with its
TXID included. If loop back is enabled, the relay transmits data.
In summary, when a node detects an error, it transmits attention until it
receives an attention with its own TXID 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, ROKx is deasserted, and another user-accessible
flag, LBOKx 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 Table J.1)
Table J.1 Error Types Reported by the Communications Report (Sheet 1 of 2)
Error Type Description
Parity error Data failed UART parity check.
Underrun Three M
IRRORED BITS messages transmitted without one being
received.
Overrun UART data buffer overrun.
Re-sync The M
IRRORED BITS device at the other end of the link detected an error.
Data error Received data was not self-consistent, or the address was wrong.
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