ABB Modbus AN00198 Introduction Manual

Motion Control Products
Application note
Integrated Modbus support
Modbus is an industry standard protocol that allows a
variety of automation devices (such as Programmable
Logic Controllers and Human Machine Interfaces) to
communicate with each other. The Modbus protocol
defines a simple protocol data unit (PDU) that is
independent of the underlying communication layers.
Inclusion of Modbus protocol support as standard
makes integration of ABB products with any other
Modbus RTU/TCP device as simple as possible.
Introduction
Controllers communicate (via RS232/422/485 Serial or Ethernet) using a Client / Server topology, in which only one device (the
Client) can initiate transactions (called 'queries'). The other devices (the Servers) respond by supplying the requested data to
the Client, or by taking the action requested in the query. Typical Client devices include host processors and programming
panels. Typical Servers include programmable controllers.
The Client can address individual Servers, or can initiate a broadcast message to all Servers. Servers return a message (called
a 'response') to queries that are addressed to them individually. Responses are not returned to broadcast queries from the
Client.
The Modbus protocol establishes the format for the Client'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 Server's response message is also
constructed using the 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 Server is unable to perform the requested action, the
Server will construct an error message and send it as its response.
ABB Motion control products
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AN00198
Rev E (EN)