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750/760 FEEDER MANAGEMENT RELAY – COMMUNICATIONS GUIDE
READING TRACE MEMORY
CHAPTER 4: MODBUS MEMORY MAP
26. Finally, the value 27 is written to the selector followed by reading the data for this
event. All the data for the new events has now been retrieved by the SCADA system so it
resumes polling the Number of Events register.
The data for an event should be straightforward to interpret from the information given in
the Memory Map except for the ‘Cause of Event’ register. Data format F24 describes the
event cause register which contains several ‘fields’ of information. The ‘Event Type’ field
determines how to interpret the remainder of the register. Interpreting the event cause
register is most easily described with several examples.
Reading Trace Memory
All Trace Memory waveform data can be read from Modbus registers found in the address
range 2100h to 215Fh. Familiarity with the Trace Memory settings is required to
understand this section; refer to the
Trace Memory
section in Chapter 5 of the instruction
manual for details.
The ‘Number of Trace Memory Triggers Since Last Clear’ register is incremented by one
every time a new Trace Memory is triggered. This register is cleared to zero when Trace
Memory is cleared. When a new trigger occurs, the associated waveform data is assigned
a ‘Trace Memory Number’ which is equal to the incremented value of this register; the
newest data will have a number equal to the Number of Trace Memory Triggers. This
register can be used to determine if any new data has been captured by periodically
reading it to see if the value has changed; if the Number of Trace Memory Triggers has
increased then there is new data available.
The Trace Memory Buffer Organization setpoint determines the number of samples that
are captured per channel for each separate trigger. This setpoint also determines how
many data triggers are stored in memory. For example if the setting is 4x1024 then there
are 1024 samples per channel per trigger and the last three data triggers can be read from
memory. Note that only 64 samples of one data channel for a single waveform can be
read from the Modbus memory map in a single data packet. The ‘Trace Memory Selectors’
registers determine which waveform data can be read from the memory map.
The ‘Trace Memory Number Selector’ determines which Trace Memory can be read. For
example, to read the data for Trace Memory number 3, the value 3 must first be written to
this register. Data for Trace Memory number 3 can now be read from the ‘Trace Memory
Information’ registers at addresses 2111h to 2119h. These registers include a trigger cause
(see the previous section for a description of the data format), a trigger time and date
Event Cause
Register Value
Event Cause Interpretation
1003h
Breaker Opened
3501h
A-C Phase Time Overcurrent 1 Trip
8501h
A-C Phase Time Overcurrent 1 Dropout
4212h
B Overvoltage 1 Alarm
6514h
User Input A Asserted via Contact Input
7009h
Clock Not Set