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1-4. Modbus Message RTU Framing
In RTU mode, messages (request or response) frames are separated by a silent
interval of at least 3.5 character times. Each character is 11 bits (1 start, 8 bits data,
1 bit parity and 1 stop). 2 stops bits is required if no parity is used.
Modbus RTU Message Frame
The entire message frame must be transmitted as a continuous stream of characters. If
a silent interval of more than 1.5 character times occurs between 2 characters, the
frame is declared incomplete and should be discarded by the receiver.
For baud rates greater than 19200 Bps, fixed values for the 2 timers should be used: it
is recommended to use a value of 750μs for the inter-character time-out (t1.5) and a
value of 1.750ms for inter-frame delay (t3.5).
The CRC field is appended as the last field of the message. The LSB is appended
first followed by the MSB. The CRC calculation is started by first pre-loading a 16
bits register to all 1s.
Reading the value of 2 successive commands/registers is not supported.
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