Sel 751A - Message Reception Overview; Message Decoding and Integrity Checks; Table I.1 Number of MIRRORED BITS Messages for Different Baud

I.2
SEL-751A Relay Instruction Manual Date Code 20100129
MIRRORED BITS Communications
Operation
Transmitting at longer intervals for baud rates above 9600 avoids overflowing
relays that receive M
IRRORED BITS at a slower rate.
Message Reception
Overview
During synchronized MIRRORED BITS communications with the
communications channel in normal state, the relay decodes and checks each
received message. If the message is valid, the relay sends each received logic
bit (RMBn, where n = 1 through 8) to the corresponding pickup and dropout
security counters, that in turn set or clear the RMBnA and RMBnB relay
element bits.
Message Decoding
and Integrity Checks
Set the RX_ID of the local SEL-751A to match the TX_ID of the remote
SEL-751A. The SEL-751A provides indication of the status of each
M
IRRORED BITS communications channel with Relay Word bits ROKA
(receive OK) and ROKB. During normal operation, the relay sets the ROKc
(c = A or B). Upon detecting any of the following conditions, the relay clears
the ROKc bit when:
➤ The relay is disabled.
➤ MIRRORED BITS are not enabled.
➤ Parity, framing, or overrun errors.
➤ Receive message identification error.
➤ No message received in the time three messages have been sent
when PROTO = MBc, or seven messages have been sent when
PROTO = MB8c.
➤ Loopback is enabled.
The relay asserts ROKc only after successful synchronization as described
below and two consecutive messages pass all of the data checks described
above. After ROKc is reasserted, received data may be delayed while passing
through the security counters described below.
While ROKc is deasserted, the relay does not transfer new RMB data to the
pickup-dropout security counters described below. Instead, the relay sends one
of the user-definable default values to the security counter inputs. For each
RMBn, use the RXDFLT setting to determine the default state the M
IRRORED
B
ITS should use in place of received data if an error condition is detected. The
setting is a mask of 1s, 0s, and/or Xs (for RMB1A–RMB8A), where X
represents the most recently received valid value. The positions of the 1s and
0s correspond to the respective positions of the M
IRRORED BITS in the Relay
Word bits (see Appendix J: Relay Word Bits). Table I.2 is an extract of
Appendix J: Relay Word Bits, showing the positions of the M
IRRORED BITS.
Ta b le I . 1 N u m b e r of M IRRORED BITS Messages for Different Baud
Baud Rate Transmission Rate of MIRRORED BITS Packets
2400 15 ms
4800 7.5 ms
9600 4 times a power system cycle (automatic pacing mode)
19200 4 times a power system cycle (automatic pacing mode)
38400 4 times a power system cycle (automatic pacing mode)