Editing Modbus (section)

=== Modbus RTU ===
Modbus RTU (Remote Terminal Unit), which is the most common implementation available for Modbus, makes use of a compact, binary representation of the data for protocol communication. The RTU format follows the commands/data with a [[cyclic redundancy check]] checksum as an error check mechanism to ensure the reliability of data. A Modbus RTU message must be transmitted continuously without inter-character hesitations. Modbus messages are framed (separated) by idle (silent) periods. Each byte (8 bits) of data is sent as 11 bits:<ref name="Drury2009" />{{Sfn|MODBUS over Serial Line protocol|2006|p=12|ps=}}

* 1 start bit
* 8 bit data/message, least significant bit sent first
* 1 bit parity
* 1 stop bit

The default is even parity, while odd or no parity may be implemented as additional options.{{Sfn|MODBUS over Serial Line protocol|2006|p=12|ps=}}

A Modbus RTU frame then will be:{{Sfn|MODBUS over Serial Line protocol|2006|p=13|ps=, "2.5.1.1 MODBUS Message RTU Framing"}}
{| class="wikitable" style="float:middle; margin:0 0 1em 1em;"
!Slave address
!Function Code
!Data
!CRC
|-
|1 byte
|1 byte
|0 – 252 bytes
|2 bytes: 1 CRC low byte and 1 CRC high byte
|}
The CRC calculation is widely known as CRC-16-MODBUS, whose polynomial is ''x''<sup>16</sup> + ''x''<sup>15</sup> + ''x''<sup>2</sup> + 1 (normal hexadecimal algebraic polynomial being <code>8005</code> and reversed <code>A001</code>).{{Sfn|MODBUS over Serial Line protocol|2006|p=39|ps=}}

Example of a Modbus RTU frame in hexadecimal: <code>01 04 02 FF FF B8 80</code> (CRC-16-MODBUS calculation for the 5 bytes from <code>01</code> to <code>FF</code> gives <code>80B8</code>, which is transmitted least significant byte first).

To ensure frame integrity during the transmission, the time interval between two frames must be at least the transmission time of 3.5 characters, and the time interval between two consecutive characters must be no more than the transmission time of 1.5 characters.{{Sfn|MODBUS over Serial Line protocol|2006|p=13|ps=, "2.5.1.1 MODBUS Message RTU Framing"}} For example, with the default data rate of 19200 bit/s, the transmission times of 3.5 (t3.5) and 1.5 (t1.5) 11-bit characters are:

<math>t3.5 = 3.5*\left( \frac{11*1000}{19200} \right)= 2.005 ms</math>

<math>t1.5 = 1.5*\left( \frac{11*10^6}{19200} \right)= 859.375 \mu s</math>

For higher data rates, Modbus RTU recommends to use the fixed values 750&nbsp;μs for t1.5 and 1.750 ms for t3.5.{{Sfn|MODBUS over Serial Line protocol|2006|p=13|ps=, "2.5.1.1 MODBUS Message RTU Framing"}}