RTU And ASCII

What is the difference between Modbus ASCII and Modbus RTU? I want to get all difference between both Modbus? Is there any official website where I can get information about the Modbus ASCII and Modbus RTU? I think both are protocol. So is it right or not, tell me about in details?

Hello sir, can any one show me difference between RTU and ASCII? I want to know the difference between modbus RTU and ASCII?

[Kiran Chandar]

RTU and ASCII are two transmission modes of MODBUS. These transmission modes determine the way in which the MODBUS messages are coded. In ASCII format, the messages are readable, whereas in RTU the messages are in binary coding and cannot be read while monitoring.

Data formats of RTU and ASCII:
To review, we can see that each byte of data can be represented in different ways.
RTU ASCII
Binary (base2): 10101110 2 characters 0 and 1
Decimal (base10): 174 10 characters 0 through 9
Hexadecimal (base 16): AE 16 characters 0 through F
ASCII (base 256): 256 different characters

Byte Size

In Modbus RTU each byte is sent as a string of 8 binary characters framed with a start bit, and a stop bit, making each byte 10 bits.

In Modbus ASCII, the number of data bits is reduced from 8 to 7. A parity bit is added before the stop bit which keeps the actual byte size at 10 bits.

ASCII vs RTU example:

Using the same example shown above to request registers 40108 to 40110 from slave address 17.

11 03 00 6B 00 03

The complete ASCII request is made by first adding the message delimiting characters. A colon is added to the start of the message, the LRC, carriage return and line feed are added to the end:

: 1 1 0 3 0 0 6 B 0 0 0 3 7 E CR LF

Each character is now treated as an ASCII character and replaced with it's hex value to give the final message.

3A 3131 3033 3030 3642 3030 3033 3745 0D 0A

This Modbus ASCII request size is 17 bytes (170 bits)

The equivalent Modbus RTU message would be:

11 03 00 6B 00 03 76 87

This Modbus RTU request size is 8 bytes (80 bits)

RTU and ASCIi