Schweitzer Engineering Laboratories SEL-421-4, Instruction Manual

Page: 1304 / 1518

15.34
SEL-400 Series Relays Instruction Manual Date Code 20171006
Communications Interfaces
SEL M
IRRORED
B
ITS
Communication
Message Decoding and Integrity Checks
The relay provides indication of the status of each M
IRRORED
B
ITS
communica-
tions channel, with element bits ROKA and ROKB. During normal operation, the
relay sets the ROK
c bit. The relay clears the bit upon detecting any of the follow-
ing conditions:
➤
Parity, framing, or overrun errors
➤
Receive data redundancy error
➤
Receive message identification error
➤
No message received in the time three messages have been sent
The relay will assert 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 not set, 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 ,
specify the default value with setting RMB n FL, as follows:
➤
1
➤
0
➤
P (to use last valid value)
Individual pickup and dropout security counters supervise the movement of each
received data bit into the corresponding RMB
n element. You can set each pickup/
dropout security counter from 1 to 8. A setting of 1 causes a security counter to
pass every occurrence, while a setting of 8 causes a counter to wait for eight con-
secutive occurrences in the received data before updating the data bits. The
pickup and dropout security count settings are separate. Control the security
count settings with the settings RMB
n PU and RMB n DO.
A pickup/dropout security counter operates identically to a pickup/dropout timer,
except that the counter uses units of counted received messages instead of time.
An SEL relay communicating with another SEL relay typically sends and
receives M
IRRORED
B
ITS
communications messages eight times per power sys-
tem cycle. Therefore, a security counter set to two counts will delay a bit by
approximately 1/4 of a power system cycle. Reference
Table 15.26 for the mes-
sage rates based on the settings. You must consider the impact of the security
counter settings in the receiving device to determine the channel timing perfor-
mance.
Channel Synchronization
When an SEL relay detects a communications error, it deasserts ROKA or
ROKB. The relay transmits an attention message until it receives an attention
message that includes a match to the TX_ID setting value. If the attention mes-
sage is successful, the relay has properly synchronized and data transmission will
resume. If the attention message is not successful, the relay will repeat the atten-
tion message until it is successful.
Loopback Testing
Use the LOOP command to verify the communications channel. In this mode,
the relay expects the transmitted data to be looped back to the relay to test the
data transmissions, including communications data. At the remote end, jumper