Schweitzer Engineering Laboratories SEL-787, Инструкция по эксплуатации, Страница: 524 / 564

I.2
SEL-787 Relay Instruction Manual Date Code 20081022
M
IRRORED
B
ITS
Communications
Operation
Transmitting at longer intervals for baud rates above 9600 avoids overflowing
relays that receive M
IRRORED
B
ITS
at a slower rate.
Message Reception
Overview
During synchronized M
IRRORED
B
ITS
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 (RMB
n , where n = 1 through 8) to the corresponding pickup and dropout
security counters, that in turn set or clear the RMB n A and RMB n B relay
element bits.
Message Decoding
and Integrity Checks
Set the RX_ID of the local SEL-787 to match the TX_ID of the remote
SEL-787. The SEL-787 provides indication of the status of each M
IRRORED
B
ITS
communications channel with Relay Word bits ROKA (receive OK) and
ROKB. During normal operation, the relay sets the ROK c ( c = A or B). Upon
detecting any of the following conditions, the relay clears the ROK c bit when:
➤
The relay is disabled.
➤
M
IRRORED
B
ITS
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 = MB
c , or seven messages have been sent when
PROTO = MB8 c .
➤
Loopback is enabled.
The relay asserts ROK c only after successful synchronization as described
below and two consecutive messages pass all of the data checks described
above. After ROK
c is reasserted, received data may be delayed while passing
through the security counters described below.
While ROK c 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
RMB
n , 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
B
ITS
in the Relay
Word bits (see Appendix J: Relay Word Bits ). Table I.2 is an extract of
Table J.1 on page J.1 , showing the positions of the M
IRRORED
B
ITS
.
Table I.1 Number of M
IRRORED
B
ITS
Messages for Different Baud
Baud Rate Transmission Rate of M
IRRORED
B
ITS
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)