Watlow CPC400 - Manual (Page 198)

Appendix A: Modbus Protocol CPC400 Series User’s Guide
184 Watlow Anafaze Doc. 0600-2900-2000
curred in receipt of the message, or if the slave is unable to
perform the requested action, the slave will construct an
error message and send it as its response.
Figure A.1 Query - Response Cycle
Query
The function code in the query tells the addressed slave de-
vice what kind of action to perform. The data bytes contain
any additional information that the slave will need to per-
form the function. For example, function code 03 will query
the slave to read holding registers and respond with their
contents. The data field must contain the information tell-
ing the slave which register to start at and how many reg-
isters to read. The error check field provides a method for
the slave to validate the integrity of the message contents.
Response
If the slave makes a normal response, the function code in
the response is an echo of the function code in the query.
The data bytes contain the data collected by the slave, such
as register values or status. If an error occurs, the function
code is modified to indicate that the response is an error re-
sponse, and the data bytes contain a code that describes the
error. The error check field allows the master to confirm
that the message contents are valid.
Device Address
Function Code
Eight-Bit
Data Bytes
Error Check
Query Message from Master
Response Message from Slave
Device Address
Function Code
Eight-Bit
Data Bytes
Error Check