These operating instructions assume that the installed controller supports the interfaces in this document and that all the requirements stipulated in the

controller, as well as the frequency converter, are strictly observed, along with all limitations therein.

7.6.2. What the User Should Already Know

The Modbus RTU (Remote Terminal Unit) is designed to communicate with any controller that supports the interfaces defined in this document. It is

assumed that the user has full knowledge of the capabilities and limitations of the controller.

7.6.3. Modbus RTU Overview

Regardless of the type of physical communication networks, the Modbus RTU Overview describes the process a controller uses to request access to

another device. This includes i.a. how it will respond to requests from another device, and how errors will be detected and reported. It also establishes

a common format for the layout and contents of message fields.

During communications over a Modbus RTU network, the protocol determines how each controller will learn its device address, recognise a message

addressed to it, determine the kind of action to be taken, and extract any data or other information contained in the message. If a reply is required, the

controller will construct the reply message and send it.

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.

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. No responses are 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 protocol. It contains fields confirming the action taken, any data to

be returned, and an error-checking field. If an error occurs 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 in response, or a time-out will occur.

7.7. Network Configuration