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Modbus ASCII vs Modbus RTU

Data formats

To review, we can see that each byte of data can be represented in different ways.

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

Message Delimiting

In Modbus RTU, bytes are sent consecutively with no space in between them with a 3-1/2 character space between messages for a delimiter.  This allows the software to know when a new message is starting.

Any delay between bytes will cause Modbus RTU to interpret it as the start of a new message. This keeps Modbus RTU from working properly with modems.

Modbus ASCII marks the start of each message with a colon character " : " (hex 3A).
The end of each message is terminated with the carriage return and line feed characters (hex 0D and 0A). This allows the space between bytes to be variable making it suitable for transmission through some modems.

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.

Split Data bytes

In Modbus ASCII, each data byte is split into the two bytes representing the two ASCII characters in the Hexadecimal value. For example,

Modbus Mode

data (ASCII)

data (hex) data (binary)

Modbus RTU

AE 1010 1110

Modbus ASCII

A , E

41 , 45 100 0001 , 100 0101

The range of data bytes in Modbus RTU can be any characters from 00 to FF.
The range of data bytes in Modbus ASCII represent only the 16 hexadecimal characters. Therefore, every data byte in Modbus ASCII must be one of these 16:

ASCII

hex binary   ASCII hex binary

0

30 011 0000   8 38 011 1000

1

31 011 0001  

9

39 011 1001

2

32 011 0010  

A

41 100 0001

3

33 011 0011  

B

42 100 0010

4

34 011 0100  

C

43 100 0011

5

35 011 0101  

D

44 100 0100

6

36 011 0110  

E

45 100 0101

7

37 011 0111  

F

46 100 0110

Error Checksum - LRC Calculation

As mentioned earlier, each Modbus RTU message is terminate with two error checking bytes called a CRC or Cyclic Redundancy Check. Similarly, Modbus ASCII is terminated with an error checking byte called an LRC or Longitudinal Redundancy Check.

Here is a spreadsheet CRC calculator for messages up to 16 bytes.
To download a copy, right click and select Save Target As...

The calculation of the LRC is much easier than the CRC.
To calculate the LRC:
   1.  Add up all the data bytes in the message (before converting to ASCII and without the initial colon and final CR/LF).
   2.  Throw away any bits that carry over 8 bits.
   3.  Make the result negative (by twos compliment) to get the LRC byte.

The sum of the resulting byte stream with the LRC will then be 0 since adding the negative subtotal will make the final total zero.

For example, this command shows the data bytes required to request the content of analog output holding registers 40108 to 40110 from the slave device with address 17.

11 03 00 6B 00 03

Add the bytes:

decimal

hex binary

17

11 0001 0001

3

03 0000 0011

0

00 0000 0000

107

6B 0110 1011

0

00 0000 0000

3

03 0000 0011

total:

decimal

hex binary
130 82 1000 0010

two's compliment (make negative) to get the LRC

decimal

hex

binary
-130 7E 0111 1110

total of the entire message including the LRC

decimal

hex

binary
0 00 0000 0000

The single hex LRC byte is 7E

The LRC byte is then split into two ASCII bytes with the rest of the data.

The two ASCII bytes for the LRC are then 37 and 45

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)

 

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