PLC communication troubleshooting tips: polygon diagnostics, cable testing, and node isolation

1. Description of the problem and scope

This is a guide for troubleshooting PLC system communication failures in industrial automation systems, including Profinet, EtherNet/IP, and Modbus polygons. These disturbances can occur in various types of equipment such as control systems, sensors, actuators and modular controllers. Depending on the severity of the violation, they can be classified as critical, medium or less significant.

2. Preventive measures

Use personal protective equipment (PPE): appropriate gloves, goggles, helmets and other protective equipment.
Lockout and Attach Power Sources: Before performing any diagnostic activities, perform a Lockout and Attach (LOTO) procedure to eliminate the risk of electrical disturbances.
Ensure No Stored Energy: Check the system for stored energy that could cause diagnostics to fail.
When testing cables: use insulating materials to avoid electrical disturbances or injury.

3. Necessary diagnostic tools

4. Check before starting diagnostics

5. Systematic flow of diagnostics

1. Disrupted PLC communication:
   1. Check for power supply:
      1. Measure the voltage:
         1. Use a multimeter to measure the voltage on the input and output pins.
         2. Permissible voltage: 230 V ±10% (380 V for three-phase systems).
   2. Check the cabling:
      1. Measure the resistance:
         1. Use a multimeter to measure the resistance of the cable lines.
         2. Permissible resistance: < 100 Ω (for cables with 0.5 mm² insulation).
   3. Check for connection:
      1. Use the cable tester:
         1. Use the cable tester to test the cable lines.
         2. Permissible distance: 100 m (for Category 6 Ethernet cables).
   4. Check configuration:
      1. Check communication settings:
         1. Check IP addresses, ports, protocols and other settings.
         2. Valid IP address: 192.168.x.x (for local network).

6. Matrix of causes of defects

7. Analysis of the root causes of each defect

7.1 Disruption of power supply

Cause: The power supply does not provide stable voltage or current, causing the PLC to lose communication.

Confirmation: Measure the voltage on the input and output contacts. If the value is outside the permissible limits, this indicates a violation of the power supply.

Impact: A power failure can lead to a loss of PLC functionality, affecting the entire automation system.

7.2 Violation of cable lines

Reason: Broken cable lines cause electrical faults, resulting in loss of PLC communication.

Confirmation: Measure the resistance of the cable lines. If the value is outside the permissible limits, this indicates a violation of the cable lines.

Impact: Disruption of cable lines can cause loss of PLC communication, affecting the entire automation system.

7.3 Violation of configuration

Reason: A communication configuration violation causes the PLC to lose communication.

Confirmation: Check the communication settings, including IP addresses, ports, protocols and other settings.

Impact: A misconfiguration can cause the PLC to lose communication, affecting the entire automation system.

8. Step-by-step solution procedures

8.1 Resolving power failure

1. Check the power supply:
   1. Use a multimeter to measure the voltage on the input and output pins.
   2. If the value is outside the acceptable limits, check the power supply and restore it.
2. Check the electrical conductivity:
   1. Use a multimeter to measure the electrical conductivity resistance.
   2. If the value is outside the permissible limits, repair or replace the electrical conductivity.

8.2 Solving the violation of cable lines

1. Check the cable connection:
   1. Use the cable tester to check the cable lines.
   2. If the value is outside the permissible limits, repair or replace the cable lines.
2. Check the insulation:
   1. Use a multimeter to measure the insulation.
   2. If the value is outside the permissible limits, repair or replace the cable lines.

8.3 Resolution of configuration violation

1. Check communication settings:
   1. Check IP addresses, ports, protocols and other settings.
   2. If the value is outside the acceptable limits, adjust or replace the settings.
2. Check the configuration:
   1. Check the configuration of the PLC and other components.
   2. If the value is outside the acceptable limits, adjust or replace the configuration.

9. Preventive measures

10. Spare parts and components

Please refer to our catalog: https://www.unitecd.com/e-catalog/

11. Links

• ISO 13849-1:2015 - Automation security
• EN 50170:2000 – Technical requirements for automation systems
• Manufacturer's documentation for PLC systems
• Additional UNITEC-D maintenance manuals