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6. Modbus Protocol
6.1 Transmission Mode
There are 2 transmission modes (ASCII and RTU) in Modbus protocol.
6.1.1 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
Start bit: 1 bit
Data bit: 7 bits (8 bits) (Selectable)
Parity:
Even (No parity, Odd) (Selectable)
Stop bit: 1 bit (2 bits) (Selectable)
Error detection : LRC (Longitudinal Redundancy Check)
6.1.2 RTU Mode
8-bit binary data in command is transmitted as it is.
Data format Start bit: 1 bit
Data bit: 8 bits
Parity:
No parity (Even, Odd) (Selectable)
Stop bit: 1 bit (2 bits) (Selectable)
Error detection: CRC-16 (Cyclic Redundancy Check)
6.2 Data Communication Interval
6.2.1 ASCII Mode
No communication interval limit between characters
6.2.2 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 BCx2 assumes that transmission
from the master is finished, which results in a communication error, and will not return a response.
6.3 Message Configuration
6.3.1 ASCII Mode
ASCII mode message is configured to start by Header [: (colon) (3AH)] and end by Delimiter
[CR (carriage return) (0DH) + LF (Line feed) (0AH)].
Data section: Max. 2 x 252 characters
Header
(:)
Slave
address
Function
code
Data
Error check
LRC
Delimiter
(CR)
Delimiter
(LF)
6.3.2 RTU Mode
RTU mode is configured to start after idle time is processed for more than 3.5 character
transmissions, and end after idle time is processed for more than 3.5 character transmissions.
(Communication speed 9600 bps, 19200 bps: 3.5 character transmission times,
Communication speed 38400 bps: 1.75 ms)
Data section: Max. 252 bytes
3.5 idle
characters
Slave
address
Function
code
Data
Error check
CRC-16
3.5 idle
characters