Transactions On Modbus Networks - ABB RVT Serie Instrucciones De Instalación Y De Funcionamiento

2.3 Transactions on Modbus Networks

Modbus protocol uses a master–slave technique, in which only one device (the master)
can initiate transactions (called 'queries'). The other devices (the 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. Typical
slaves include programmable controllers.
The master can address individual slaves, or can 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 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 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.
8 Modbus protocol overview ç RVT communication