contain the value 0. The Modbus protocol will give an exception response if the client
tries to read out too few registers from the fault record structure.
Selection code 1: Reading the oldest unread record
When writing the selection code 1, the IED first checks the client. If the client has been
reading fault records before, the IED knows which internal fault record has been sent
to this specific client during the last reading. The IED then loads the next fault record,
that is the oldest unread, into the registers following the selection register. If this is the
first time the client reads fault records from the IED, the oldest fault record of the
Modbus fault record buffer is given to the client.
Selection code 2: Reading the oldest stored record
The selection code 2 always forces the fault record reading to go back to the oldest
fault record stored in the buffer. The oldest fault record is then loaded into the registers
following the selection register. After the client has read out this record, the next
record becomes the oldest unread. The client can continue by reading out the oldest
unread fault records again with the selection code 1.
Selection code -1...-99
A negative selection code, that is a 16 bit two's complement value, defines how many
records backwards from the newest fault record the reading is to be moved. For
example, the ten latest fault records can be read out at any time by first selecting -10,
reading out the record and then continuing with the selection code 1 to read out the
nine additional records
Selection code 3: Resetting the fault record read pointer
The write selection code 3 is not followed by a read operation. The selection 3 means
that there are no unread records in the Modbus fault record buffer left for the client in
question, that is, the buffer is cleared.. The next new fault record that is logged into the
Modbus fault record buffer becomes the first unread record for this specific client.
Selection code 4: Resetting SSR3 bit 9
The write selection 4 is not followed by any read operation. The selection code only
resets bit 9 in SSR3.
If the fault records are read by using two commands, the client can re-
read the given fault record registers as many times as it wants. As long
as no new selection write operation is performed, the contents of the
fault record registers are not changed.
3.4.10.3
Other fault record registers
Sequence number
Every fault record is given a sequence number. The sequence number runs from 1 to
65535 and then rolls over to one again. The client can check that the sequence numbers
Section 3
1MRS757645 B
Vendor-specific implementation
58
620 series
Communication Protocol Manual
