ABB REM 610 - Page 80

The event record contains a sequence number which makes it possible for the master
to determine whether one or several unread event records have been deleted due to
overflow by comparing it to the sequence number of the previously read event
record.
The slave keeps track of which event record is currently the oldest unread. The
master can continue reading event records for as long as Status register 3 indicates
that there are unread records.
*
Special case 1: If there are no unread event records, the contents of the last read
record is returned. If the buffer is empty, however, the registers contain only
zeros. This is the only time when sequence number zero appears.
*
Special case 2: If the master tries to read the next unread event record without
entering selection code 1 again, the contents of the last read record is returned.
Selection code 2: reading the oldest stored record
By resetting the read pointer using selection code 2, the master can read the oldest
stored event record. After this, the master can continue reading the following
records using selection code 1, irrespective of whether they have been read before.
Resetting the read pointer does not affect the sequence number of the
event record.
Selection code -1...-99
With selection code -1...-99, the master can move backwards from the newest event
as many events as defined by the selection code and read that specific event record.
After this, the master can continue reading the following records using selection
code 1, irrespective of whether they have been read before.
*
Special case: If there is not as many events in the buffer as specified by the
selection code, the oldest stored event is read.
Selection code 3
The Modbus event buffer is cleared with selection code 3. Clearing the buffer does
not require any read operation to follow.
Digital inputs
As the master may not detect the state changes of all digital signals when scanning,
an additional change detect (CD) target bit is created for every instantaneous target
point; see the example below.
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REM 610REM 610
Motor Protection Relay
Technical Reference Manual - ANSI Version
1MRS755537