H-4 MIRRORED BITS Communications Date Code 20011026
SEL-321/321-1 Instruction Manual
· Received data parity, framing, or overrun errors.
· Receive data redundancy error.
· Receive message identification error.
· No message received in the time three messages have been sent for MB protocol. No
message received in the time six messages have been sent for MB8 protocol.
While ROK is not asserted, the relay will:
· Enter the synchronization process described below.
· Prevent new data from being transferred to the pickup/dropout security counters.
Instead, the relay will send the default values (RXDFLT) to the security counter inputs:
1
0
The last valid value
The relay will assert ROK only after successful synchronization as described below and 2
consecutive messages pass all of the checks described above.
The user can combine ROK with other relay bits in SEL
OGIC
®
Control Equations.
After ROK is reasserted, received data may be delayed while passing through the security
counters described below.
Transfer of received data to RMB1 - RMB8 is supervised by the 8 user-programmable
pickup/dropout security counters (RMB1PU - RMB8DO) settable from 1 (allow every
occurrence to pass) to 8 (require 8 consecutive occurrences to pass). The pickup and dropout
security count settings are separate.
Note: Pickup/dropout security counters operate identically to pickup/dropout timers, except
they are set in counts of received messages instead of time. This is straightforward when
an SEL-321 Relay is talking to an SEL-321 Relay. For all of the following examples,
assume 38400 baud which allows a full message to be transmitted in 1/8 power system
cycle. An SEL-321 Relay talking to another SEL-321 Relay sends and receives
M
IRRORED BITS messages eight times per power system cycle. Therefore, a security
counter set to two counts will delay a bit by about 1/4 power system cycle. Complexity
increases when two relays of different processing rates are connected via M
IRRORED
BITS, for instance an SEL-321 Relay talking to an SEL-351Relay. The SEL-351 Relay
processes power system information each 1/4 power system cycle, but processes the
pickup/dropout security counters as messages are received. Since the SEL-351 Relay is
receiving messages from the SEL-321 Relay, it will receive two messages per 1/4-cycle
processing interval. Consequently, a counter set to two will further delay a bit by about
1/4 cycle. However, in that same example, a security counter set to two on the SEL-321
Relay will delay a bit by 1/2 cycle, because the SEL-321 Relay is only receiving new
M
IRRORED BITS messages each 1/4 cycle from the SEL-351 Relay.
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