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8. Modbus protocol
8. 12. Exception responses
An exception response is returned by a slave when it is unable to perform the request which is addressed to it.
Format of an exception response:
Example: Writing the value 1 to the status word (EtA) = logic address 3201 (which cannot be written, because in “read-only" mode) on
slave 4
Request:
Response:
Slave
no.
Response
code
Error
code
CRC16
Lo Hi
1 byte 1 byte 1 byte 2 bytes
Response
code:
request code + H’80.
Error code: 1 = The function requested is not recognized by the slave.
2 = The addresses indicated in the request do not exist in the slave.
3 = The values indicated in the request are not permitted on the slave.
4 = The slave has started to execute the request but cannot continue to process it completely.
6 = The requested write operation has been refused because the drive is in "Forced local" mode.
7 = The requested write operation has been refused because the motor is running (configuration parameters).
Slave no. Request
code
No. of 1
st
word No. of words No. of
bytes
Value of 1
st
word CRC CRC
Hi Lo Hi Lo Hi Lo Lo Hi
4 100C8100010200018AD1
Slave no. Response
code
Error code CRC
Lo
CRC
Hi
490005C01