Instruction manual

6. MODBUS COMMUNICATION PROTOCOL
IMR01H02-E3
47
(3) No response
The slave ignores the query message and does not respond when:
• The slave address in the query message does not coincide with any slave address settings.
• The CRC code of the master does not coincide with that of the slave.
• Transmission error such as overrun, framing, parity and etc., is found in the query message.
• Data time interval in the query message from the master exceeds 24 bit’s time.
6.5 Calculating CRC-16
The Cyclic Redundancy Check (CRC) is a 2 byte (16-bit) error check code. After constructing the
data message, not including start, stop, or parity bit, the master calculates a CRC code and appends
this to the end of the message. The slave will calculate a CRC code from the received message, and
compare it with the CRC code from the master. If they do not coincide, a communication error has
occurred and the slave does not respond.
The CRC code is formed in the following sequence:
1. Load a 16-bit CRC register with FFFFH.
2. Exclusive OR (⊕) the first byte (8 bits) of the message with the CRC register. Return the result to
the CRC register.
3. Shift the CRC register 1 bit to the right.
4. If the carry flag is 1, exclusive OR the CRC register with A001 hexadecimal and return the result
to the CRC register. If the carry flag is 0, repeat step 3.
5. Repeat step 3 and 4 until there have been 8 shifts.
6. Exclusive OR the next byte (8 bits) of the message with the CRC register.
7. Repeat step 3 through 6 for all bytes of the message (except the CRC).
8. The CRC register contains the 2 byte CRC error code. When they are appended to the message,
the low-order byte is appended first, followed by the high-order byte.