Schweitzer Engineering Laboratories SEL-351 - Synchronization

I-2 MIRRORED BITS™ Date Code 20001006
SEL-351 Instruction Manual
While ROKx is not asserted, the relay will:
1. Prevent new data from being transferred to the pickup dropout security counters
described later. Instead, the relay will send one of the following user selectable
values (hereafter called default values) to the security counter inputs:
• 1
• 0
• The last valid value
The user will be allowed to select one of the default values for each RMB.
2. Enter the synchronization process described below.
The relay will assert ROKx only after successful synchronization as described below and 2
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-351 talking to another SEL-351 sends
and receives MIRRORED BITS messages 4 times per power system cycle. Therefore, a security
counter set to 2 counts will delay a bit by about 1/2 power system cycle. Things get a little more
complicated when two relays of different processing rates are connected via MIRRORED BITS. For
instance a SEL-321 talking to a SEL-351. The SEL-321 processes power system information each
1/8 power system cycle, but processes the pickup/dropout security counters as messages are
received. Since the SEL-321 is receiving messages from the SEL-351, it will receive a message
per 1/4 cycle processing interval. So a counter set to two will again delay a bit by about 1/2 cycle.
However, in that same example, a security counter set to two on the SEL-351 will delay a bit by
1/4 cycle, because the SEL-351 is receiving new MIRRORED 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 its TX_ID setting.
When a node receives an attention message, it checks to see if its TX_ID is included.
If its own TX_ID is included and at least one other TX_ID is included, the node transmits data.
If its own TX_ID is not included, the node deasserts ROKx, includes its TX_ID in the attention
message, and transmits the new attention message.
If its own TX_ID is the only TX_ID included, the relay assumes the message is corrupted unless
the loop back mode has been enabled. If loop back is not enabled, the node deasserts ROKx and
transmits the attention message with its TX_ID included. If loop back is enabled, the relay
transmits data.
Courtesy of NationalSwitchgear.com