Modbus Rtu; Settings; Function Codes - OnRobot VG10 User Manual

MODBUS RTU

The VG10 supports MODBUS RTU. MODBUS RTU is a well proven and easy to use
fieldbus, which is supported by a vast amount of industrial equipment. MODBUS RTU
uses RS485 as the physical layer. For further details on the protocol, please refer to
modbus.org's MODBUS over Serial Line Specification and Implementation Guide
MODBUS Application Protocol Specification

Settings

Both 24V I/O data line and MODBUS RTU line is enabled at power-up. If the VG10
detects a command on the 24V I/O data line before receiving a MODBUS RTU frame,
it will disable MODBUS RTU and use the shared MODBUS RTU line (RS485) for analog
outputs (Vacuum %).
Upon reception of the first valid MODBUS frame, the VG10 will lock to this interface
and disables the 24V I/O data line.
The table below states the settings to be used when communicating with the VG10
over MODBUS RTU. See Electrical Characteristics for physical connection details.
Setting
Baudrate [bit/sec]
Start bits
Data bits
Parity
Stop bits
Slave address
CRC Check
CRC Polynomial

Function codes

The VG10 currently supports the function codes listed below. The VG10 will respond
with an appropriate exception code, if the function is not executed correctly. Please
refer to MODBUS Application Protocol for detailed description of the different
exception codes. Note that the VG10 will provide no response, if the serial settings,
slave address or CRC code is not correct.
3 / 0x03, Read Holding Registers:
Use this function code to read out one or multiple consecutive registers. Please refer
to MODBUS Application Protocol for frame and response details.
6 / 0x06, Write Single Register:
Use this function code to set the value of a single register. Please refer to MODBUS
Application Protocol for frame and response details.
1
http://www.modbus.org/docs/Modbus_over_serial_line_V1_02.pdf
2
http://www.modbus.org/docs/Modbus_Application_Protocol_V1_1b.pdf
2
Value
115200
1
8
Even
1
65 (0x41)
16 bit (MODBUS default)
0xA001 (MODBUS default)
1
and
Page 17 of 40