Reading The Event Recorder - GE Feeder Management Relay 750 Instruction Manual

7 COMMUNICATIONS
The following table shows the format of the master and slave packets for a master device sending the Store Time operation
code to all slave devices on the communications link as required by step three of the procedure.
MASTER QUERY MESSAGE:
SLAVE ADDRESS
FUNCTION CODE
OPERATION CODE: high order byte, low order byte
CODE VALUE: high order byte, low order byte
CRC: low order byte, high order byte
SLAVE RESPONSE:
No response from slave
All Event Recorder data can be read from Modbus registers found in the address range 2000h to 20FFh.
The 'Number of Events Since Last Clear' register at address 2001h is incremented by one every time a new event occurs.
The register is cleared to zero when the Event Recorder is cleared. When a new event occurs, it is assigned an 'event num-
ber' which is equal to the incremented value of this register; the newest event will have an event number equal to the Num-
ber of Events. This register can be used to determine if any new events have occurred by periodically reading the register
to see if the value has changed. If the Number of Events has increased then there are new events available.
Only the data for a single event can be read from the Modbus memory map in a single data packet. The 'Event Number
Selector' register at address 2000h selects the event number for which data can be read from the memory map. For exam-
ple, to read the data for event number 123, the value 123 must first be written to this register. All the data for event number
123 can now be read from the 'Event Record Data' registers at addresses 2010h to 201Dh. Only the last 128 events are
actually stored in the relay's memory. Attempting to retrieve data for elder events that are not stored will result in a Modbus
exception response when writing to the 'Event Number Selector'.
The following example illustrates how information can be retrieved from the Event Recorder:
A SCADA system polls the Number of Events register once every minute. It now reads a value of 27 from the register when
previously the value was 24 which means that three new events have occurred. The SCADA system writes a value of 25 to
the Event Number Selector register. It then reads the data for event number 25 from the Event Record Data registers and
stores the data to permanent memory for retrieval by an operator. The system now writes the value 26 to the selector and
then reads the data for event number 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.
EVENT CAUSE EVENT CAUSE INTERPRETATION
REGISTER VALUE
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
GE Multilin
Courtesy of NationalSwitchgear.com
EXAMPLE (HEX):
11 query message for slave 11
05 execute operation
00 04 set time
FF 00 perform operation
CC 2A computed cyclic redundancy check
EXAMPLE (HEX):
750/760 Feeder Management Relay
7.3 MODBUS OPERATIONS

7.3.4 READING THE EVENT RECORDER

7
7-9