ABB Relion 620 Series Protocol Manual page 56

Section 3
Vendor-specific implementation
50
Event records can be read by using two commands, function 6 for the
write operation and function 3 for the read operation, or by using
function 23 that includes write and read operations in the same
transaction.
If event records are read by using two commands, the positive
confirmation to the write select operation tells the client that an event
record has been loaded for reading. Another way to detect the positive
confirmation is by monitoring the state of SSR3 bit 8.
Selection code 1: Reading the oldest unread record
When writing the selection code 1, the IED first checks the client. If the client has read
events before, the IED knows which internal event has been sent to this specific client
during the last reading. The IED then loads the next event, that is the oldest unread,
into the next 11 registers. If this is the first time the client reads events from the IED,
the oldest event of the Modbus event buffer is loaded into the 11 event record registers.
Selection code 2: Reading the oldest stored record
Selection code 2 always forces the event reading to go back to the oldest event in the
Modbus event buffer. The oldest event record is then loaded into the 11 event record
registers. After the client has read out this record, the next record becomes the oldest
unread. The client can continue with the selection code 1 by reading out the oldest
unread event record again.
Selection code -1...-499
A negative selection code, that is a 16 bit two's complement value, defines how many
records backwards from the newest event the event record reading is to be moved. For
example, the ten latest events could be read out at any time by first selecting -10,
reading out the event and then continuing with the selection code 1 to read out the nine
additional event records. There can be 500 event records altogether.
Selection code 3: Resetting the event read pointer
The write selection 3 is not followed by a read operation. The selection 3 means that
there are no unread records in the Modbus event buffer left for the client in question,
that is, the buffer is cleared. The next new event that is logged into the Modbus event
buffer becomes the first unread record for this specific client.
Selection code 4: Resetting SSR3 bit 8
The write selection 4 is not followed by a read operation. The selection code only
resets the bit 8 in SSR3.
If event records are read by using two commands, the client can re-
read the 11 event record registers as many times as it wants. As long
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620 series
Communication Protocol Manual