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Appendix A: Modbus Protocol PPC-2000 User’s Guide
270 Watlow Anafaze Doc.# 30002-00 Rev 2.3
Figure A.1 Query—Response Cycle
Query
The function code in the query tells the addressed slave device
what kind of action to perform. The data bytes contain any
additional information that the slave will need to perform 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 telling the slave which
register to start at and how many registers 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 response, 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
Device Address
Function Code
Eight-Bit
Data Bytes
Error Check
Query Message from Master
Response Message from Slave