Exception Responses; Reading The Event Recorder - GE TRANSFORMER MANAGEMENT RELAY 745 Instruction Manual

7.3 MODBUS FUNCTIONS
Programming or operation errors happen because of illegal data in a message, hardware or software problems in the slave
device, etc. These errors result in an exception response from the slave. The slave detecting one of these errors sends a
response message to the master consisting of slave address, function code, error code, and CRC. To indicate that the
response is a notification of an error, the high order bit of the function code is set to 1.
Table 7–3: MODBUS ERROR CODES
ERROR MODBUS DEFINITION
CODE
01 ILLEGAL FUNCTION
02 ILLEGAL DATA ADDRESS
03 ILLEGAL DATA VALUE
04 FAILURE IN ASSOCIATED
DEVICE
05* ACKNOWLEDGE
06* BUSY, REJECTED MESSAGE
07* NAK - NEGATIVE
ACKNOWLEDGE
Some implementations of Modbus may not support these exception responses.
NOTE
MESSAGE FORMAT AND EXAMPLE:
Request to slave device 11 to perform unsupported function code 39h.
MASTER QUERY MESSAGE:
SLAVE ADDRESS
FUNCTION CODE
CRC: low order byte, high order byte
SLAVE RESPONSE:
7
SLAVE ADDRESS
FUNCTION CODE
ERROR CODE
CRC: low order byte, high order byte
All Event Recorder data can be read from Modbus registers found in the address range 0800h to 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 reg-
ister 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'.
7-8
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GE MULTILIN IMPLEMENTATION
The function code of the master query message is not a function code supported by the
slave.
The address referenced in the data field of the master query message is not an address
supported by the slave.
The value referenced in the data field of the master query message is not allowable in
the addressed slave location.
An external device connected to the addressed slave device has failed and the data
requested cannot be sent. This response will be returned if a GE Multilin device
connected to the RS485 external device port of the 745 has failed to respond to the 745.
The addressed slave device has accepted and is processing a long duration command.
Poll for status.
The message was received without error, but the slave device is engaged in processing
a long duration command. Retransmit later, when the slave device may be free.
The message was received without error, but the request could not be performed,
because this version of the 745 does not have the requested operation available.
EXAMPLE (HEX):
11 query message for slave 11
39 unsupported function code – error
CD F2 CRC computed by master
EXAMPLE (HEX):
11 response message from slave 11
B9 return unsupported function code with high-order bit set
01 illegal function
93 CRC computed by slave
745 Transformer Management Relay
7 COMMUNICATIONS

7.3.2 EXCEPTION RESPONSES

7.3.3 READING THE EVENT RECORDER

GE Multilin