VACON CX
Modbus user’s manual
Page
7
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3.5
Exception responses
If the slave receives the query without a communication error, but cannot handle it, the slave will return an
exception response informing the master of the nature of the error. The exception response codes are
listed below.
Code
Name
Description
01
ILLEGAL FUNCTION
The message function requested is not recognized by the
slave.
02
ILLEGAL DATA ADDRESS
The received data address is not an allowable address for
the slave.
03
ILLEGAL DATA VALUE
The received data value is not an allowable value for the
slave.
04
SLAVE DEVICE ERROR
An unrecoverable error occurred while the slave was
attempting to perform the requested action,
06
SLAVE DEVICE BUSY
The message was received without error, but the slave was
engaged in processing a long duration program command
In an exception response, the slave sets a most-significant bit (MSB) of the function code to 1. The slave
returns an exception code in the data field.
Example:
Query:
01
01
04
2E
00
01
CRC16
Slave
address
Function
Starting
address HI
Starting
address LO
Number of
bits HI
Number of
bits LO
2 bytes
Exception Response:
01
81
02
CRC16
Slave
address
Function
Response
code
2 bytes
