Modbus RTU is available on Rheonics process density meters and viscometers over a 2-wire RS-485 interface. The device does not have internal biasing resistors and termination resistors and proper setup of the RS485 network has to be ensured. 

Modbus RTU is a widely utilized communication protocol in industrial automation, known for its simplicity, efficiency, and reliability. Modbus RTU operating over RS-485, is inherently a half-duplex communication protocol. The half-duplex nature of RS-485 means that data transmission can occur in both directions, but not simultaneously. In other words, devices on the network can either send or receive data at any given time.

A common RS-485 communication interface is the RS-232 port, which is used to convert RS-485 communication to a serial port. However, many modern computers and devices have USB ports. By converting RS-485 signals to USB signals, an RS-485 to USB converter bridges this gap, enabling communication with USB-equipped devices.


TABLE OF CONTENTS

1. Rheonics RS-485 specifications

All SME devices implement a MODBUS RTU compliant RTU slave. Connection is by RS485-2W where the interface is not electrical isolated. electrically

Figure 1. Rheonics SME-TRD(RS-485 port)

Termination: Termination should be only at the end and beginning of the bus to avoid reflections. Requirements on termination differ not only on the baud-rate used but also on the slew-rate of device's internal drivers. As a general rule, if there are devices on the RS485 bus that allow fast transmission speeds, and cable lengths exceed a few meters, then termination is required. Commonly the termination is done with a 120Ω resistance.

Biasing: Biasing of the RS485 BUS is required. If biasing is omitted, communication may work unreliably or not at all. See Figure 1 for Pull-Up/Pull-Down resistors. 

Connections: Connection stubs on the RS485 bus should be kept to a minimum.

Common: RS485 does allow common mode voltages of +12V / -7V. It is therefore recommended to use a common wire to ensure common mode voltage is within safe limits for all receivers. In case of problems, it might be necessary to introduce RS485 isolators into the network. The common terminal for the SME device is the 0V terminal (GND) for the power supply.

Default configuration:

The SME device allows the configuration of the Modbus address using the RCP software. Please refer to the RCP manual for instructions.


Factory default settings of Modbus parameters:

address= 1

baudrate=38400

parity=odd

The system is capable of supporting: 

  • parity settings of odd, even, and none

  • baudrates of 9600, 19200 and 38400 

 

Figure 2. RS-485 Modbus configuration with Slave and Master devices.

2. What is an  RS485 to USB converter?


An RS-485 to USB converter connects an RS-485 network to a USB port on a computer or USB-enabled device. Computers can now communicate with devices on RS-485 networks using Modbus RTU.

RS-485 to USB converters translate data between the RS-485 protocol used by devices on the network and the USB protocol understood by computers. Data sent from the computer is formatted appropriately for the RS-485 network and vice versa.

When a computer lacks native RS-485 support but still needs to communicate with devices on an RS-485 network, such as programmable logic controllers (PLCs), sensors, or other industrial equipment that uses the Modbus RTU protocol, this type of converter is crucial. Below we can find a proposed wiring diagram to be used with the Rheonics SME and a Modbus RS485 to USB converter.Figure 3. RS-485 to USB converter wiring diagram.

Figure 4 shows a photo of how the wiring can be done with a Modbus RS485 to USB converter.


Figure 4. SME-TRD RS485 to USB converter.


When doing integration with Modbuspoll, data will be updated based on the configuration used. It is important to use the correct Slave address. For more information about modbuspoll integration visit this link: Quick Modbus RTU connection verification with ModbusPoll master software 

Figure 5. Modbuspoll integration.

3. Recommended RS485 to USB converters

The list provided is not intended to recommend a particular category or type of product.

Brand: FTDI

Web: USB to RS485 Converter Cable 

Figure 6. FTDI Chip USB to RS485 Converter Cable

Brand: Moxa

Web: https://www.moxa.com/en/products/industrial-edge-connectivity/usb-to-serial-converters-usb-hubs/usb-to-serial-converters/uport-1100-series#overview

Figure 7. Moxa RS485 to USB converter.

Brand: Lin Engineering 

Web: USB485 

Figure 8. Lin Engineering RS485 to USB converter

Brand: Advantech

Web: BB-485USBTB-2W-A 

Figure 9. Advantech RS485 to USB converter.

Brand: Brainboxes

Web: US-324 1 Port RS422/485 USB to Serial Adapter - Brainboxes 

Figure 10. Brainboxes RS485 to USB converter.

Brand: Siig

Web: USB to RS-422/485 Converter 

 Figure 11. Siig RS485 to USB converter.

Brand: Waveshare

Web: Industrial USB to RS485 Bidirectional Converter 

Figure 12. Waveshare RS485 to USB converter.

4. References

  1. The Modbus Organization 

  2. Modbus Master Simulator 

  3. Modbus RTU (RS-485) 

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