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Modbus RTU Message Format

02/13 Modbus® RTU Serial Communications User Manual 3

General Modbus RTU message format

Query message format

[Slave Address, Function Code, Function code dependent data, CRC 16]

Response message format

[Slave Address, Function Code*, Function code dependent data, CRC 16]

* If an error is detected in a valid message the response function code is modified by adding 80 (hex) and the

function code dependent data is replaced by an exception response code as described in Section 4 - Modbus RTU

Exception Codes.

Between messages, the RS-485 link is in a high impedance state. During this time receiving devices are

more susceptible to noise generated false start of messages. Although noise-generated messages are

rejected due to address, framing, and CRC checking, they can cause the loss of a good message when they

are included in the message stream. In the slave, the transmitting device enables its transmitter line driver

and forces an idle line state onto the link for three character time slots prior to transmitting. This action

forces termination of any noise generated messages and improves message frame synchronization.

2.2 Modbus RTU Data Layer

The data layer includes:

• Diagnostic loopback,

• Function code recognition / rejection,

• Busy / repoll,

• Data error code generation

Errors detected by the data layer are rejected and the slave responds to the polling device with a Modbus-

type status exception error. A summary of the Modbus status exception codes is listed in

Section 4 - Modbus RTU Exception Codes

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Siemens NAN 67 Series Manual de usuario Pagina 9