1. Introduction
In this chapter, we assume that you are familiar with the standard Modbus communication protocols. Otherwise, there are some sites to familiarize yourself with the Modbus standard:
- www.modbus.org.
- See the document “Modbus_over_serial_line_V1_01.pdf”
- https://www.lammertbies.nl/comm/info/modbus.html
2. Communication
Modbus has 2 types of communication:
- via RS485 (Modbus RTU)
- via Ethernet (Modbus TCP)
RS485 = a serial interface that can be used to connect devices to for example VPVision or another monitoring and central monitoring systems.
All measurement parameters are available through Modbus in floating-point and integer format. The data will be refreshed every second. The maximum polling interval is 10 ms.
The actual measurement data is placed in holding registers. In order to readout the data, you will need to use the corresponding holding registers. In chapter F1, you can find the VPFlowscope M Modbus register map as an example.
 | The map is also shown below as an example |
VPSensorCartridge connection status.
Available write operations.
3. Modbus settings
The Modbus settings can be changed with VPStudio, VPVision, or with the keypad of the display/ Transmitter when this is available. Below all of the available options are shown:
- Hardware address: 1-247
- Integer multiplier: 1-1000
When the type of your Modbus code is an integer the output will be rounded to an integer. This will sometimes result in a less precise outcome. To get the output more precise VPStudio has an integer multiplier option. With this option, you can multiply the outcome to get a more significant precise output.
Example:
Your output for your temperature in Modbus is 33 degrees. You set your Integer multiplier to 100. The new output of your temperature is 3343. So the actual output is 33.4 degrees and not 33 degrees.
In chapter C5 we show you how to change your Modbus settings.
Data format
Function code 0x03 for reading(Holding register)
Function code 0x06 for writing a single register(Holding register)
Function code 0x10 for writing multiple registers(Holding register)
The data format is in little-endian
4. Extra information
In the following PDF and in the next chapter, some common Modbus issues are discussed in order to help you further.