Modbus Data Format; Standard Frame; Bit Sequence; Modbus Message Rtu Framing - Mitsubishi Electric AE630-SW Instruction Manual

5. Modbus Data Format

For details, please download and refer to MODBUS over Serial Line Specification & Implementation guide from the following URL;
http://www.ModBus.org/

5.1 Standard frame

The standard communications frame consists of:
Slave Function
address code
Slave address
Function code
DATA
CRC
<NOTE>
A procedure for generating a CRC is:
1. Load a 16–bit register with FFFF hex. Call this the CRC register.
2. Exclusive OR the first 8–bit byte of the message with the low–order byte of the 16–bit CRC register, putting the result in
the CRC register.
3. Shift the CRC register one bit to the right (toward the LSB), zero–filling the MSB. Extract and examine the LSB.
4. (If the LSB was 0): Repeat Step 3 (another shift).
(If the LSB was 1): Exclusive OR the CRC register with the polynomial value 0xA001 (1010 0000 0000 0001).
5. Repeat Steps 3 and 4 until 8 shifts have been performed. When this is done, a complete 8–bit byte will have been
processed.
6. Repeat Steps 2 through 5 for the next 8–bit byte of the message. Continue doing this until all bytes have been processed.
7. The final content of the CRC register is the CRC value.
8. When the CRC is placed into the message, its upper and lower bytes must be swapped as described below.

5.2 Bit sequence

With RTU character framing, the bit sequence is:
＜Example＞With Parity Checking
Start 1
LSB
＜Example＞Without Parity Checking
Start 1
LSB

5.3 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.
The entire message frame must be transmitted as a continuous 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 should be discarded by the receiver.
more than 3.5 char
Response Slave
address
data
：01～7FH
*When selecting slave address 0, a message is sent to all the instruments present on the network.
When the slave receives it, the slave does not make a response.
：03H ...... Read Holding Registers ( maximum 250 bytes)
：08H ...... Diagnostics
：10H ...... Preset Multiple Registers
：8 bit HEX data
：The Cyclical Redundancy Check (CRC) field is two bytes, containing a 16–bit binary value.
2 3 4
2 3 4
less than 1.5 char
Function
code
E-18
5 6 7
5 6 7
less than 1.5 char more than 3.5 char
CRC(Lo) CRC(Hi)
CRC
8 Parity Stop
MSB
8 Stop Stop
MSB
Response