Reading The Event Recorder; Reading Trace Memory - GE 745 TRANSFORMER MANAGEMENT RELAY Instruction Manual

8 COMMUNICATIONS 8.2 MODBUS PROTOCOL

8.2.14 READING THE EVENT RECORDER

All Event Recorder data can be read from Modbus registers found in the address range 0800h - 0FFFh.
The 'Total Number of Events Since Last Clear' register at address 0804h 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, the event is assigned an 'event number' which is equal to the incremented value of this register. The
newest event will have an event number equal to the Total Number 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 Total Number of Events has increased, then new events have occurred.
Only the data for a single event can be read from the Modbus memory map in a single data packet. The 'Event
Record Selector Index' register at address 0805h selects the event number whose data can be read from the
memory map. For example, 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 Recorder Data' registers at
addresses 0830h to 0866h. Only the last 128 events are actually stored in the relay's memory. Attempting to
retrieve data for older events that are not stored will result in a Modbus exception response when writing to the
'Event Record Selector Index'.
The following example illustrates how information can be retrieved from the Event Recorder:
A SCADA system polls the Total 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 during the
last minute. The SCADA system writes a value of 25 to the Event Record Selector Index register. It then reads
the data for event number 25 from the Event Recorder Data registers and stores the data to permanent mem-
ory for retrieval by an operator. The SCADA system now writes the value 26 to the selector and then reads the
data for event number 26. Finally, the SCADA system writes the value 27 to the selector and then reads 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 Total Number of Events register.

8.2.15 READING TRACE MEMORY

All Trace Memory data can be read from Modbus registers found in the address range 4000h - 47FFh.
The 'Total Number of Trace Triggers Since Last Clear' register at address 4004h is incremented by one every
time a new trace memory waveform capture is triggered. The register is cleared to zero when the Trace Mem-
ory is cleared. When a new trigger occurs, the captured trace memory buffer is assigned a 'trigger number'
which is equal to the incremented value of this register. The newest captured buffer will have a trigger number
equal to the Total Number of Trace Triggers. This register can be used to determine if any new triggers have
occurred by periodically reading the register to see if the value has changed. If the Total Number of Trace Trig-
gers has increased, then new trace memory waveform captures have occurred.
Only the data for a single channel of a single trace memory buffer can be read from the Modbus memory map
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at a time. The 'Trace Buffer Selector Index' register at address 4005h selects the trace memory buffer, and the
'Trace Channel Selector Index' register at address 4006h selects the trace memory channel, whose waveform
data can be read from the memory map. For example, to read the waveform data for the 'Winding 1 Phase C
Current' of trace memory buffer 5, the value 5 must be written to the Trace Buffer Selector Index, and the value
2 (as per data format F65) must be written to the Trace Channel Selector Index. All the captured waveform
data for buffer 5, channel 'Winding 1 Phase C Current' can now be read from the 'Trace Memory Data' registers
at addresses 4010h to 4416h. Only the trace memory buffers for the last 3 trace memory triggers are actually
stored in the relay's memory. Attempting to retrieve data for older triggers that are not stored will result in a
Modbus exception response when writing to the 'Trace Buffer Selector Index'.
The following example illustrates how information can be retrieved from the Trace Memory:
GE Power Management 745 Transformer Management Relay 8-11