ABB NMBA-01 Installation And Startup Manual page 40

efesotomasyon.com - Control Techniques,emerson,saftronics -ac drive-servo motor
Appendix B – Modbus Protocol
B-2
The Modbus protocol establishes the format for the master's query by
placing into it the device (or broadcast) address, a function code defin-
ing the requested action, any data to be sent, and an error-checking
field. The slave's response message is also constructed using Modbus
protocol. It contains fields confirming the action taken, any data to be
returned, and an error-checking field. If an error occurred 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.
Query message from Master
Device Address
Function Code
Eight-Bit
Data Bytes
Error Check
Figure B-1 Master-Slave Query-Response Cycle
The Query: The function code in the query tells the addressed slave
device what kind of action to perform. The data bytes contain any addi-
tional 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 infor-
mation telling the slave which register to start at and how many regis-
ters to read. The error check field provides a method for the slave to
validate the integrity of the message contents.
The 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 val-
ues or status. If an error occurs, the function code is modified to indi-
cate 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
Response message from Slave
NMBA-01 Installation and Start-up Guide