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Modbus RTU Message Format

2 Modbus® RTU Serial Communications User Manual 02/13

2. Modbus RTU Message Format

Table 2-1 Modbus RTU Message Formats

Coding system

8 bit binary

Number of data bits per

character

10 Bits

start bits - 1

data bits - 8

parity bits - 0

stop bits - 1

Parity

Not used

Bit transfer rate

300, 600, 1200, 2400, 4800, 9600, 19200, 38400 Selectable

NOTE: Not all instruments support all Baud Rates.

Duplex

Half duplex Transceiver or TX/RX

Error checking

CRC (cyclic redundancy check)

Polynomial

(CRC-16 10100000000001)

Bit transfer order

LSB first

End of message

Idle line for 3.5 or more characters (>1.82 msec for 19200).

2.1 Modbus RTU Link Layer

The link layer includes the following properties/behaviors:

 Slave address recognition,

 Start / End of Frame detection,

 CRC-16 generation / checking,

 Transmit / receive message time-out,

 Buffer overflow detection,

 Framing error detection,

 Idle line detection.

Errors detected by the physical layer in messages received by the slave are ignored and the physical layer

automatically restarts by initiating a new receive on the next idle line detection.

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Siemens NAN 67 Series Manual de usuario Pagina 8