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Modbus Agile

02/2011

Protocol

7.3.10.1.2

Frame Description

Slave

Address

Function

Code

Data

CRC

1 Byte

0 to 252 Bytes

2 Bytes

(CRC-Lo, CRC-Hi)

The maximum size of a MODBUS RTU frame is 256 bytes.

7.3.10.1.3

Modbus Message RTU Framing

A MODBUS message is placed by the transmitting device into a frame that has a known beginning and

ending point. This allows devices that receive a new frame to begin at the start of the message, and

to know when the message is completed. Partial messages must be detected and errors must be set

as a result. In RTU mode, message frames are separated by a silent interval of at least 3.5 character
times.

MODBUS Message

Start

Address

Function Data

CRC

End

>= 3,5

Char

8 Bits

N x 8 Bits

16 Bits

>= 3,5

Char

The entire message frame must be transmitted as a contiguous stream of characters.
If a silent interval of more than 1.5 character times occurs between two characters, the message
frame is declared incomplete and is discarded by the inverter.

7.3.10.1.4

CRC Checking

The RTU mode includes an error–checking field that is based on a Cyclical Redundancy Checking

(CRC) method performed on the message contents.
The CRC field checks the contents of the entire message. It is applied regardless of any parity check-

ing method used for the individual characters of the message.
The CRC field contains a 16–bit value implemented as two 8–bit bytes.
The CRC field is appended to the message as the last field in the message. It is appended in the se-

quence "low-order byte", "high-order byte". The CRC high–order byte is the last byte to be sent in the

message.
The CRC value is calculated by the sending device, which appends the CRC to the message. The re-

ceiving device recalculates a CRC during receipt of the message, and compares the calculated value to
the actual value it received in the CRC field. If the two values are not equal, an error results.
The CRC calculation is started by first pre-loading a 16–bit register to all 1’s (0xFFFF). Then a process

begins of applying successive bytes of the message to the current contents of the register.
During generation of the CRC, each 8bit character is ExOR with the register contents. Then the result

is shifted in the direction of the least significant bit (LSB), with a zero filled into the most significant bit

(MSB) position. The LSB is extracted and examined. If the LSB was a 1, the register is then ExOR with
the fixed value

0xA001. If the LSB was a 0, no ExOR takes place.

This process is repeated until eight shifts have been performed. After the last (eight) shift, the next

8bit char is ExOR with the register’s current value, and the process repeats for eight more shifts as

described above. The final content of the register, after all the bytes of the message have been ap-

plied, is the CRC value.
When the CRC is appended to the message, the low-order byte is appended first, followed by the
high-order byte.

Example for CRC calculating for the frame: "0x02 0x07"

MSB

LSB

Start CRC

1111