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750/760 FEEDER MANAGEMENT RELAY – COMMUNICATIONS GUIDE
EXCEPTION RESPONSES
CHAPTER 3: MODBUS FUNCTIONS
Exception Responses
Programming or operation errors happen because of illegal data in a packet, hardware or
software problems in the slave device, etc. These errors result in an exception response
from the slave. The GE Multilin 750/760 implements the error codes listed below. The slave
detecting one of these errors sends a response packet to the master with the high-order
bit of the function code set to 1. The following table shows the format of the master and
slave packets. The example shows a master device sending the unsupported function
code 39h to slave device 11.
Data Starting Address
2
11 00
data starting at 1100h
Number of Setpoints
2
00 02
2 setpoints = 4 bytes total
CRC (low, high)
2
46 64
computed CRC error code
Slave Response
Bytes
Example
Description
Master Transmission
Bytes
Example
Description
Slave Address
1
11
message for slave 11
Function Code
1
39
unsupported function code
CRC (low, high)
2
CD F2
computed CRC error code
Slave Response
Bytes
Example
Description
Slave Address
1
11
message from slave 11
Function Code
1
B9
return unsupported function
code
Data Starting Address
2
11 00
data starting at 1100h
Number of Setpoints
2
00 02
2 setpoints = 4 bytes total
CRC (low, high)
2
93
computed CRC error code
Table 3: Exception Response Error Codes
Error
Code
Modbus Definition
GE Multilin Implementation
01
Illegal Function
The function code of the master query message is not supported by the
slave.
02
Illegal Data Address
The address referenced in the data field of the master query message is not
an address supported by the slave.
03
Illegal Data Value
The value referenced in the data field of the master query message is not
allowable in the addressed slave location.
04
Failure In Associated Device
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 750/760.
* Some Modbus implementations may not support these exception responses