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2. COMMUNICATION INTERFACE
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Communication protocol conforms to MODBUS and the protocol can be selected from RTU and
ASCII.
Refer to MODBUS standard specification and commercial reference manuals for details on
MODBUS.
2.1: Data transmission system
Transmission system: half-duplex asynchronous data transfer (polling/selecting)
Transmission distance: Max 500m (results may vary depending on environment)
Connection method: Multi-drop system (max 31 units)
Start bit: 1 bit
Transmission code: ASCII (ASCII mode)
Binary (RTU mode)
Error check: LRC (ASCII mode)
CRC-16 (RTU mode)
2.2: Frame configuration
In RTU mode, frame is composed of message portion only.
In ASCII mode, frame consists of start character ": (colon, 3AH)", message and end character
"CR (carriage return, ODH) + LF (line feed, 0AH)".
RTU mode: message
ASCII mode: : | message | CR | LF
2.3: Message configuration
MODBUS messages are composed of the following in both RTU and ASCII modes.
Slave address
Function code
Data
Error check
2.4: Slave address
Slave address can be set from 1-99. Master normally communicates with one slave device. All
slave devices receive any message sent from the master in common, however, only the device
with slave address corresponding to the directive message responds.
