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5. Communication procedure
Communication starts with command transmission from the host computer (hereafter Master) and ends
with the response of the KT2 (hereafter Slave).
Master
Command
Data
Command
Acknowledgement
Command
Negative
acknowledgement
Command
No response
(Fig. 5-1)
Communication timing of the RS-485
Slave side
When the slave starts transmission through the RS-485 communication line, the slave is arranged
so as to provide an idle status (mark status) transmission period of 1 or more characters before
sending the response to ensure the synchronization on the receiving side.
The slave is arranged so as to disconnect the transmitter from the communication line within a
1 character transmission period after sending the response.
Master side (Notice on programming)
Set the program so that the master can disconnect the transmitter from the communication line
within a 1 character transmission period after sending the command in preparation for reception
of the response from the slave.
To avoid the collision of transmissions between the master and the slave, send the next command
after carefully checking that the master received the response.
6. Modbus protocol
6.1 Modbus protocol
Modbus protocol is a communication protocol for the PLC developed by Modicon Inc.
6.2 Transmission mode
There are 2 transmission modes (ASCII and RTU) in Modbus protocol.
6.3 ASCII mode
Hexadecimal (0 to 9，A to F), which is divided into high order (4-bit) and low order (4-bit) out of
8-bit binary data in the command is transmitted as ASCII characters.
Data format
(1) Message configuration
ASCII mode message is configured to start by [: (colon)(3AH)] and end by [CR (carriage return)
(0DH) + LF (Line feed)(0AH)]. (See Fig. 6.3-1)
Header
Slave
(:)
address
• Response with data
Slave
When the master sends the reading command, the slave
responds with the corresponding set value or current
status.
• Acknowledgement
When the master sends the setting command, the slave
responds by sending an acknowledgement after the
processing is terminated.
• Negative acknowledgement
When the master sends a non-existent command or a value
outside the setting range, the slave returns a negative
acknowledgement.
• No response
The slave will not respond to the master when broadcast
address is set, or when there is a communication error
(framing error or parity error), or when LRC or CRC
discrepancy is detected.
Start bit
: 1 bit
Data bit
: 7 bits
Parity
: Even
Stop bit
: 1 bit
Error detection: LRC (Longitudinal Redundancy Check)
Data interval : 1 second or less
Function
code
(Fig. 6.3-1)
Error check
Data
LRC
4
Delimiter
Delimiter
(CR)
(LF)
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