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  3. C5. RS485 (Modbus RTU) Settings

C5. RS485 (Modbus RTU) Settings

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Modbus in-depth

Modbus extra wiring

1. Introduction

RS485 and Modbus are specialized protocols!
Familiarize yourself first! Check www.modbus.org

This chapter is kept short. Since RS485 and Modbus are specialized protocols, we refer to Chapter G9 for more information on the communication protocol, definitions, and other information.

2. RS485. (Modbus) overview

RS485 is a serial interface that can be used to connect your VPFlowScope to e.g.:

  • VPVision.
  • Remote monitoring software.
  • Building Management System (BMS).

The VPFlowScope’s have an RS485 output for communication via Modbus.
Modbus sensors and devices can be daisy-chained in a network, for instance:

  • VPFlowScope M: Modbus RTU & Modbus TCP
  • VPFlowScope DP / In-line: Modbus RTU

Modbus RTU is the communication protocol which is commonly used in serial communication, note that every junction needs a Modbus junction box

Modbus TCP is a communication protocol where Modbus is transferred over Ethernet networks. Check chapter C7 for Ethernet settings and chapter E4 for Ethernet wiring.

The holding register table to readout Modbus is identical for Modbus RTU and TCP within the same flow meter but different in between different flow meters.

Always check the register table for your device! Please go to the corresponding manual.

The holding register table to readout Modbus is identical for Modbus RTU and TCP within the same flow meter but different in between different flow meters.

The RS485 interface is standardized according to ANSI/TIA/EIA-485-A-98.

All measurement parameters are available through Modbus in floating-point and integer formats. The data will be refreshed every second. The maximum polling interval is 10 ms.

Data format:

0x03 for reading (holding register)XX
0x06 for writing a single register (holding register)X
0x10 for writing registers (holding register)XX
32-bit Floating point Little-endian [CDAB]X
32-bit Signed integer Little-endian [CDAB]X

3. Settings

Factory default:

  • Baud rate: 38400
  • Stop bits: 1
  • Parity: None
  • Modbus address: 09
  • Integer multiplier: 10
By default, VPInstruments flow meters are set with these same settings. There is possibly a need for adjustment, for instance when connecting to devices from 3rd party suppliers.
Change via
Baud rate9600








Stop bits1





Modbuss Address1 to 247VPStudio

Transmitter/ Display

Modbus (register 1201)

Integer multiplier1 to 1000VPStudio

Modbus (register 1201)


 All devices in the same network need to communicate using the same settings!

Except for Modbus address which needs to be unique per sensor.

3.2 Steps: 

  1. Assign per device a unique Modbus address.
  2. Choose the communication settings and program all the devices with the same settings.
  3. In case of Modbus TCP: Match the Ethernet including the IP address range.

4. Manually via the Transmitter/display

Change the Modbus address of the flow meter.

In the Transmitter/display go to :
Menu >> Settings >> Modbus address.

Use the up and down keys to change the number of the Modbus address. Press Enter to save your settings.

Keep in mind that the VPFlowScope needs a power cycle after chancing these settings before they go into effect

5. Change settings by VPStudio

In VPStudio unfold the menu settings of the flow meter and select “Settings”.

5.1 For Baudrate, Stop Bits, and Parity:

Go to the tab ‘Installation’:

VPStudio Modbus settings

  1. Adjust the baud rate, stop bits and parity accordingly.
  2. Click “Save”.VPStudio save button

5.2 For Modbus address and Integer multiplier:

Go to the tab “Modbus”:

VPStudio modbus settings 2

  1. Adjust the Modbus address (Hardware address here) and Integer multiplier accordingly.
  2. Click “Save”.


6. Next Step 

Learn more about the next communication protocol: C6. Analog settings.

Or go directly to the next step in installing your flow meter.

In the case that you have a data logger, it is time to configure the right settings:C9. Data logger settings.
Using Modbus communication will also provide you with the option to set an alarm. The alarm settings can be found in chapter C8. Alarm settings
Or go straight-away to the mechanical installation: D1. Select the right installation point.
Updated on January 25, 2021

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