Section 3
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Vendor-specific implementation
40
615/620 series ANSI
Communication Protocol Manual
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.
