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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) ad-
dress, 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.
The Query:
The function code in the query tells the addressed slave device what kind of action to perform. The data bytes contain
any additional information that the slave will need to perform the function.
The data field must contain the information telling the slave which register to start at and how many registers to read.
The error check field provides a method for the slave to validate the integrity of the message contents.
The Response:
If the slave makes a normal response, the function code in the response is an echo of the function code in the query.
The data bytes contain the data collected by the slave, such as register values or status. If an error occurs, the func-
tion code is modified to indicate that the response is an error response, and the data bytes contain a code that de-
scribes the error.
The error check field allows the master to confirm that the message contents are valid.
2.4.
Serial Transmission Mode
The transmission mode defines the bit contents of message fields transmitted serially on the networks. It determines
how information will be packed into the message fields and decoded.
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