Watlow CLS200, MLS300 and CAS200 - Manual (Page 27)

Chapter 2: Modbus-RTU Protocol
Communications Specification 21
Chapter 2: Modbus-RTU Protocol
Overview
Transactions on Modbus-RTU Networks
Standard Modbus-RTU ports use an EIA/TIA-232C- or EIA/TIA-485/
422-compatible serial interface that defines connector pinouts, cabling,
signal levels, transmission baud rates, and parity checking.
Controllers communicate using a master-slave technique, in which only
one device (the master) can initiate transactions (called “queries”). The
other devices (slaves) respond by supplying the requested data to the
master, or by taking the action requested in the query. Typical master
devices include host processors and programming panels.
The master can address individual slaves, or initiate a broadcast
message to all slaves. Slaves return a message (called a “response”) to
queries that are addressed to them individually. Responses are not
returned to broadcast queries from the master.
The Modbus-RTU protocol establishes the format for the master’s query
by placing into it the device (or broadcast) address, a function code
defining the requested action, any data to be sent, and an error-checking
field. The slave’s response message is also constructed using Modbus-
RTU 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.