Specifications

62
Schneider Electric
The Modbus Protocol (continued)
Control of messages received by the slave
Contents of an exception response
Example:
b Query.
b Response.
Note: Exception responses 5 and 7 are related to Jbus functions 13 and 14.
The master transmits a query indicating:
b Slave number.
b Function code.
b Parameters of the function.
It calculates and transmits the contents of
the control word (CRC 16).
When the slave receives the query
message, it stores the message in memory,
calculates the CRC and compares it with
the CRC 16 received.
b If the message received is incorrect
(CRC 16 frames not equal) the slave does
not respond.
b If the message received is correct but the
slave cannot process it (bad address,
incorrect data, etc.) it sends an exception
response.
E52356
Master
CRC 16
CRC
calculation
CRC 16 comparison
Slave
Slave no.
Function
Data
zone
E52357E52358
1
CRC 16
1 byte 1 byte 1 byte 2 bytes
Slave no.
(1 to FF)
Exception code
1. - Unknown function code
2. - Incorrect address
3. - Incorrect data
4. - PLC not ready
5. - Acknowledgement
7. - No acknowledgement
8. - Write error
9. - Zone overlap
Function code received
and most significant bit at 1.
MSB* LSB*
190
0
0
0
CRC 16
LSB* MSB*
Important: the least significant byte (LSB)
of CRC 16 is transmitted first.
E52359
1891
CRC 16
LSB* MSB*
Guide to Integration
Modbus / Jbus