One 16 byte register value is 0x12AB; register is transmitted in below sequence:
High-order byte = 0x12
Low-order byte = 0xAB
z
Error Check Field
In Modbus RTU mode, the 16-bit Cyclic Redundancy Check (CRC-16) is used.
The sending device calculates a 16-bit value, based on the information stored in
the address, function and data fields using the CRC-16 algorithm and appends
it to the end of the packet. The receiving device performs the same calculation
upon the reception of a packet. If the result does not match the checksum
stored in the packet, transmission errors have occurred and the packet will be
ignored by the receiving device.
For detail of CRC16 parity arithmetic, please refer to Section 4 .
8.2.4. Abnormal Responses
If a Modbus master device sends a non-effective command to a SPM33 or
attempts to read a non-effective holding register, an exception response will be
generated. The exception response consists of the slave address, function code,
error code, and error check field. The high order bit of the function code is set to
1 to indicate that the packet is an exception response.
31
