5. Modbus Protocol
5.1 Communication Procedure
Communication starts with command transmission from the host computer (master), and ends with the
response of the KT4R (slave).
5.1.1 Response with Data
When the master sends the Read command, the slave
responds with the corresponding set value or current
status.
(Fig. 5.1.1-1)
5.1.2 Acknowledgement
When the master sends the Write command, the
slave responds by sending acknowledgement
after processing is terminated.
(Fig. 5.1.2-1)
5.1.3 Negative Acknowledgement
When the master sends a non-existent command or
value out of the setting range, the slave returns
negative acknowledgement.
(Fig. 5.1.3-1)
5.1.4 No Response
The slave will not respond to the master in the following
cases:
• Broadcast address
• Communication error (framing error, parity error)
• LRC discrepancy (Modbus ASCII mode)
• CRC-16 discrepancy (Modbus RTU mode)
(Fig. 5.1.4-1)
Read command
Response data
Master Slave
Write command
Acknowledgement
Master
Slave
Non-existent command or
data out of the setting range
Negative acknowledgement
Master
Slave
Broadcast address,
Communication error
(framing error, parity error)
LRC discrepancy,
CRC-16 discrepancy
No response
Master Slave
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