G.2
SEL-734 Meter Instruction Manual Date Code 20090730
MIRRORED BITS Communications
Operation
Message Decoding
and Integrity Checks
The meter 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 meter will:
1. Prevent new data from being transferred to the pickup/dropout
security counters described later. Instead, the meter will send
one of the following user selectable values (hereafter called
default values) to the security counter inputs:
➢ 1
➢ 0
➢ The last valid value
You will be allowed to select one of the default values for each
RMB.
2. Enter the synchronization process described below.
The meter 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
8 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-734 communicating with another SEL-734 sends and receives
MIRRORED BITS messages 4 times per power system cycle. Therefore, a
security counter set to 2 counts will delay by about a half power system cycle.
Things become more complicated when two meters of different processing
rates are connected via M
IRRORED BITS, as in the case of an SEL-321
communicating with an SEL-734. The SEL-321 processes power system
information each 1/8 power system cycle, but processes the pickup/dropout
security counters as messages are received. Because the SEL-321 is receiving
messages from the SEL-734, it will receive a message per 1/4-cycle
processing interval. So a counter set to two will again delay by about a
half cycle. However, in that same example, a security counter set to two on the
SEL-734 will delay by 1/4-cycle, because the SEL-734 is receiving new
M
IRRORED BITS messages each 1/8 cycle from the SEL-321.
Synchronization
When a node detects a communications error, it deasserts ROKx and transmits
an attention message, which includes the TX_ID setting for that node.
When a node receives an attention message, it checks whether its TX_ID is
included.
If its own TX_ID is included along with at least one other TX_ID, the node
transmits data.
Courtesy of NationalSwitchgear.com
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