Savin P7026 Manual do Utilizador descarregar pdf (Página 72)

PET-70216/ET-7026 User Manual, version 1.0.1 beta1, Page: 72

4.2. Modbus Message Structure

Modbus devices communicate using a master-slave (client-server) technique in which

only one device (the master/client) can initiate transactions (called queries). The other

devices (slaves/servers) respond by supplying the requested data to the master, or by

taking the action requested in the query.

A master‟s query will consist of a slave address (or broadcast address), a function

code defining the requested action, any required data, and an error checking field. A

slave‟s response consists of fields confirming the action taken, any data to be

returned, and an error checking field.

Address

(1 byte)

Function Code

(1 byte)

Data

(0 to 252 bytes)

Checksum

(2 bytes)

Address specifies the address of the receiver.

Function Code specifies the message type.

Data is the data block.

Checksum specifies the numerical check value for testing the validity of the

protocol.

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Savin P7026 Manual do Utilizador Página 72