Exception Responses - GE 745 TRANSFORMER MANAGEMENT RELAY Instruction Manual

8.2 MODBUS PROTOCOL
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 8–3: 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
8
CRC: low order byte
CRC: high order byte
Field:
SLAVE ADDRESS
FUNCTION CODE
ERROR CODE
CRC: low order byte
CRC: high order byte
8-10
GE POWER MANAGEMENT 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
Power Management 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
Example (hex):
11 response message from slave 11
B9 return unsupported fn. code with high-order bit set
01 illegal function
93
95
745 Transformer Management Relay
8 COMMUNICATIONS

8.2.13 EXCEPTION RESPONSES

GE Power Management