Modbus protocol

RTU

In RTU mode the protocol starts with a pause of 3.5 characters, and finishes in the same way (illustrated in the diagram with T1-T2-T3-T4).
The characters permitted for transmission in all fields are hexadecimal 0... 9, A..., F.

Start

Slave address

Function

Data

CRC

END

t1-t2-t3-t4

1 byte

1 byte

n Bytes

2 bytes

t1-t2-t3-t4

ASCII

In ASCII mode the telegram starts with a colon (":", character 0x3A) and ends with a carriage return line feed ("CRLF", characters 0x0D and 0x0A).
The characters transferred are represented in the ASCII code.

Start

Slave address

Function

Data

LRC

END

1 byte 0x3A

2 bytes

2 bytes

n Bytes

2 bytes

2 bytes 0x0D, 0x0A