Schweitzer Engineering Laboratories SEL-734, Инструкция по эксплуатации, Страница: 306 / 344

G.2
SEL-734 Meter Instruction Manual Date Code 20090730
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Communications
Operation
Message Decoding
and Integrity Checks
The meter will deassert a user-accessible flag per channel (hereafter called
ROK
x ) 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 ROK x 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 ROK x only after successful synchronization (as
described below) and two consecutive messages pass all of the data checks
described above. After ROK
x is reasserted, received data may be delayed
while passing through the security counters described below.
Transfer of received data to RMB1 x –RMB8 x 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
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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
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, 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
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messages each 1/8 cycle from the SEL-321.
Synchronization
When a node detects a communications error, it deasserts ROK x 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