YASKAWA GPD 515/G5 Modbus RTU, Техническое руководство

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The Message Format 6-13
No Response Message
The slave disregards the command message and does not return the respond message in the
following cases:
1. In simultaneous broadcasting of data (slave address field is 0), all slaves execute but
do not respond.
2. When a communication error (overrun, framing, parity, or CRC-16) is detected in the
command message.
3. When the slave address in the command message does not coincide with the address
set in the slave.
4. When the command message data length is not proper.
CRC-16
At the end of the message, the data for CRC error checking is sent in order to detect errors in
signal transmission. In Modbus RTU, the error check is conducted in the form of a CRC-16
(Cyclical Redundancy Check). The CRC field checks the contents of the entire message. It is
applied regardless of any parity check method used for the individual characters of the message.
The CRC field is two bytes, containing 16-bit binary value. The CRC value is calculated by the
transmitting device, which appends the CRC to the message. The receiving 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 is started by first preloading a 16-bit register to all 1ís. Then a process begins of
applying successive 8-bit bytes of the message to the current contents of the register. Only the
eight bits of data in each character are used for generating the CRC. Start and stop bits, and the
parity bit (if one is used) do not apply to the CRC.
During generation of the CRC, each 8-bit character is exclusive ëORíed 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 is a 1, the
register is then exclusive ëORíed with a preset, fixed value (A001h). If the LSB is a 0, no
exclusive OR takes place.
This process is repeated until eight shifts have been performed. After the last (eighth) shift, the
next 8-bit byte is exclusive ëORíed with the registerís current value, and the process repeats for
eight more shifts as described above. The final contents of the register, after all the bytes of the
message have been applied, is the CRC value.
For applications using a host computer, a detailed example of a CRC generation using Quick
Basic is shown on the following page.