Common pitfalls when installing a Modbus RS485 network on site

30 January 2019

Last week IDX was called to the site at a large commercial residence building in Pretoria, South Africa. Where our client was implementing an IoT solution for remote monitoring and control of various HVAC and power systems in the building. The control system the System Integrator chose in this case was a Modbus-enabled Industrial Micro PC called the Revolution PI.

The client had Modbus sensors connected to boilers, air conditioning systems, ventilation systems and power meters. The Modbus communications between the controller and the sensors were intermittently failing due to various installation and implementation faults:

1. Earthing and Shielding
Within any fieldbus communication installation, one of the requirements to ensure uninterrupted operation is to implement adequate grounding and shielding techniques. Effective grounding and Shielding help to prevent electrostatic and electromagnetic pickup, which can lead to failed communications.

Some of the shielding and grounding requirements for a Modbus RS485 Network:

  • Utilising twisted shielded serial cable (120Ω impedance)
  • Ensuring continuity of the shield across the entire segment
  • Grounding the shield at multiple points (preferably at each device)
  • Ensure devices are properly grounded to an appropriate functional earth within cabinets
  • Running a potential equalisation cable between cabinets where necessary

The shield surrounding the communication wires will transfer any picked-up noise or interference and drain it to the ground. It is important to ground the shield as you enter and exit cabinets and at each device, this provides the shortest path to ground for any interference picked up on the network.

What we found on-site (Figure 1)

  • No continuity of the cable shield
  • No grounding of the shield within the cabinets or devices
  • No functional earthing of devices within the cabinets

Figure 1: No grounding/shield continuity

2. Running Modbus cable close to high voltage cabling
High-voltage cables and machinery can interfere with bus communications, they do so by electrostatic and electromagnetic injection. One way to avoid this interference/injection on the bus is to separate your communication cable from the source of interference.

The adequate separation distances between the communication cable (Category 1) and any high-voltage cables/devices (Category 2,3 & 4) are displayed below in Figure 2:

  • Category 1:  Modbus cable, LAN cables, < 25VAC, < 60VDC
  • Category 2: AC: 25V – 400V, DC: 60V – 400V
  • Category 3: AC & DC: >400V
  • Category 4: Cable at risk from lightning strikes (e.g. cables running between buildings

Figure 2: Cable separation diagram

What we found on-site (Figures 3 and 4)

  • Running close to 400 VAC cables.
  • Severe noise injected on the signal bus
    • Within Figure 4 you can see two scope lines (red and green), this is your B & A lines respectively.

    • RS485 is a balanced system and carries the same signal on both lines A & B, however, the exact inverses of each signal on each line. Thus any noise that is picked up should cancel out once the signal is combined.
    • This scope was taken on a Modbus installation that was silent and indicates poor implementation of RS485 protection mechanisms.
  • Little protection from EMI due to poor earthing and shielding techniques implemented.

Figure 3: Violation of cable separation requirements

Figure 4: EMI picked up on the Modbus cable due to poor shielding and exposure to high-voltage cables

3. Missing end terminators

Reflections on a fieldbus cable are caused by an impedance mismatch. One of the major contributing factors causing reflections is at the end of the fieldbus line where the signal waveform ‘bounces back’ towards to transmitter and disrupts consecutive signals on the bus (Fig.6).

To prevent these reflections, you can absorb the signal waveform at the end of each line within your Modbus network by installing a 120-ohm resistor at the beginning and end of the segment.

As segment lengths are increased, the effect of reflections without effective terminating resistors installed becomes more severe.

Figure 5: Installed 120Ω resistor and implemented grounded shielding

Figure 6: Normal operation (left) vs Missing termination (right) scope image.

<a href="https://blog.idx.co.za/author/kyle/" target="_self">Kyle Roos</a>

Kyle Roos

Site Services Manager

Kyle specialises in industrial data communication protocols and systems. Operating in the space of design, development, integration, technical sales and support, advanced troubleshooting and training.

Show your love!

Recent Posts

Driving Productivity with LineView Training

In light of our partnership with LineView Solutions, our team has taken up the opportunity to equip themselves with insights into the LineView system and its impact on the industry. “LineView is a production line monitoring and recording system that automatically...

Exciting New Launch: Onsite ProfiDay

IDX is excited to announce the launch of our onsite ProfiDay offering. This is available to all industries and companies that utilise PROFIBUS and/or PROFINET within their industrial communications systems. What is ProfiDAY? An exclusive onsite program offering...

Introducing our Partnership with LineView Solutions

We are excited to announce our partnership with LineView Solutions, a leading UK-based company specialising in smart factory solutions for optimising production processes. LineView developed a cutting-edge software suite designed to automatically gather data from the...

Optimise PROFIBUS and PROFINET Network Performance

As a field technician handling PROFIBUS and PROFINET systems, it is crucial to recognise the common errors that can result in costly downtime. From improper network configuration to neglecting regular maintenance, these mistakes can greatly affect system performance...

Delighted Customer Shares Experience

We recently assisted a client in rectifying damage to the Anybus ABC4090 Communicator caused by incorrect power usage onsite. We arranged for the device to be sent to HMS in Sweden for repairs, and they restored it before promptly returning it to us. Here are some...

Price Decrease on PROFIBUS Connectors

PROFIBUS connectors play a crucial role in industrial automation and data exchange systems by facilitating smooth communication between devices and networks. We are pleased to announce a significant price decrease on all our PROFIBUS connectors. Thanks to recent...

PROFIBUS Standards: Functional bonding and shielding

Electromagnetic Interference (EMI) and earthing issues can significantly impact the performance of your network. By following these six proven recommendations, you can ensure that your network is always up and running.

PROFIBUS Standards: Terminations

What is the Volts Direct Current (VDC) required for optimal operation of a PROFIBUS DP termination circuit? Get the answers you need in our latest video on PROFIBUS Standards: Terminations. Don't miss out on this essential knowledge to boost your system's performance!

PROFIBUS Standards: Segment Cable Lengths

Finding the perfect balance between PROFIBUS cable length and network speed is crucial. While longer cable segments might seem tempting, they can lead to signal degradation and slower speeds. Conversely, too short segments might limit your layout options. Striking the...

Sign up for our newsletter

Get exclusive updates and a sneak peek into our world every two months. Stay connected, stay informed.

Related Posts

Our expertise lies in establishing, maintaining, and leveraging plant data for business benefit. Through our innovative solutions, we work alongside you to streamline processes, enhance efficiency, and reduce costly downtime.

CONTACT US

Main: +27 (11) 548 9960
Sales: +27 (11) 548 9970

SOCIAL MEDIA

Copyright @2024 Industrial Data Xchange. All rights reserved.