J.2
SEL-311C Relay Instruction Manual Date Code 20060320
MIRRORED BITS Communications
Operation
Operation
Message
Transmission
All messages are transmitted without idle bits between characters. Idle bits are
allowed between messages.
➤ At 4800 baud, one message is transmitted each 1/2-power
system cycle
➤ At 9600 baud, one message is transmitted each 1/4-power
system cycle
➤ At 19200 and 38400 baud, one message is transmitted each 1/8-
power system cycle for the SEL-321 and 1/4 power system
cycle for the SEL-311C
Message Decoding
and Integrity Checks
The relay will deassert a user-accessible flag per channel (hereafter called
ROKx) upon failing any of the following received-data checks:
➤ Parity, framing, or overrun errors
➤ Receive data redundancy error
➤ Receive message identification error
➤ No message received in the time three messages have been sent
While ROKx is not asserted, the relay will prevent new data from being
transferred to the pickup dropout security counters described later. Instead, the
relay will send one of the following user selectable values (hereafter called
default values) to the security counter inputs:
➤ 1
➤ 0
➤ The last valid value
The user will be allowed to select one of the default values for each RMB.
Enter the synchronization process described below.
The relay will assert ROKx only after successful synchronization as described
below and two consecutive messages pass all of the data checks described
above. After ROKx is reasserted, received data may be delayed while passing
through the security counters described below.
Transfer of received data to RMB1x–RMB8x is supervised by eight user-
programmable pickup/dropout security counters settable from 1 (allow every
occurrence to pass) to at least 8 (require 8 consecutive occurrences to pass).
The pickup and dropout security count settings are separate.
A pickup/dropout security counter operates identically to a pickup/dropout
timer, except that it is set in counts of received messages instead of time. An
SEL-311C talking to another SEL-311C sends and receives M
IRRORED BITS
messages four times per power system cycle. Therefore, a security counter set
to two counts will delay a bit by about 1/2-power system cycle. Things get a
little more complicated when two relays of different processing rates are
connected via M
IRRORED BITS, such as an SEL-321 talking to an SEL-311C.
The SEL-321 processes power system information each 1/8-power system
cycle, but processes the pickup/dropout security counters as messages are
received. Since the SEL-321 is receiving messages from the SEL-311C, it will
receive a message per 1/4-cycle processing interval. So a counter set to two
will again delay a bit by about 1/2 cycle. However, in that same example, a
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