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5.2 Communication Timing of the RS-485
Master side (Take note while programming)
When the master starts transmission through the RS-485 communication line, the master is arranged so
as to provide an idle status (mark status) transmission period of 1 character or more before sending the
command to ensure synchronization on the receiving side.
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 collision of transmissions between the master and the slave, send the next command after
carefully checking that the master has received the response.
If a response to the command is not returned due to communication errors, set the Retry Processing to
send the command again. (It is recommended to execute Retry twice or more.)
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 ms or more
to ensure synchronization on the receiving side.
The slave is arranged so as to disconnect the transmitter from the communication line within 1 character
transmission period, after sending the response.
(*) Can be set in [Response delay time (p.5)] within a range of 0 to 1000 ms.
5.3 Transmission Mode
There are 2 transmission modes (ASCII and RTU) in Modbus protocol, which have the following structure.
5.3.1 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 command is transmitted as ASCII characters.
Data format
Error detection: LRC (Longitudinal Redundancy Check)
5.3.2 RTU Mode
8-bit binary data in command is transmitted as it is.
Data format
Error detection: CRC-16 (Cyclic Redundancy Check)
5.4 Data Communication Interval
5.4.1 ASCII Mode
No communication interval limit between characters
5.4.2 RTU Mode
1.5 character transmission times or less
(Communication speed 9600 bps, 19200 bps: 1.5 character transmission times,
Communication speed 38400 bps: 750 s)
To transmit continuously, an interval between characters which consist of one message, must be
within 1.5 character transmission times.
If an interval lasts longer than 1.5 character transmission times, the KT4R assumes that transmission
from the master is finished, which results in a communication error, and will not return a response.
Phone: 800.894.0412 - Fax: 888.723.4773 - Web: www.clrwtr.com - Email: info@clrwtr.com
Start bit: 1 bit
Data bit: 7 bits (8 bits) Selectable
Parity:
Even (Odd, No parity) Selectable
Stop bit: 1 bit (2 bits) Selectable
Start bit: 1 bit
Data bit: 8 bits
Parity:
No parity (Even, Odd) Selectable
Stop bit: 1 bit (2 bits) Selectable
before sending a response
(*)
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