Common pitfalls when installing a Modbus RS485 network on site
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 equilisation cable between cabinets where necessary
|Fig.1 No grounding / shield continuity|
What we found on site (Fig.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
2. Running Modbus cable close to high voltage cabling
High voltage cable 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) is displayed below (Fig.2).
- Category 1
- Category 2
- AC: 25V - 400V
- DC: 60V - 400V
- Category 3
- AC & DC: >400V
- Category 4
- Cable at a risk from lightning strikes (e.g. cables running between buildings)
What we found on site (Fig.3 & .4)
|Fig.3 Violation of cable separation requirements|
- 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 line 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.
|Fig.4 |EMI picked up on the Modbus cable due to poor shielding and|
exposure to high voltage cables
3. Missing end terminators
|Fig.5 Installed 120Ω resistor and implemented grounded shielding|
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.
|Fig.6 Normal operation (left) vs Missing termination (right) scope image|
Written by: Kyle Roos (IDX)
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