Programmable Logic Controllers (PLCs) are designed and manufactured to sustain constant operations in harsh industrial environments, making them rugged computing devices for a whole host of applications. From control systems on the International Space Station to safety devices on naval ships.

PLCs allow us to control and automate low-level operations by sending rudimentary signals as needed to activate sensors, read data, or simply record data in databases. This ensures reliable and consistent control output.

No rugged machine lasts forever, though; machines will tend to fail over time. A clock’s cogs are starting to rust, worn-out gaskets in the air compressor, and electronics are failing due to electromagnetic interference.

Why Should You Care?

PLCs are primarily deployed in mission-critical, predictable control systems.

Factories generate profits from producing goods on a factory line; stopping that line could mean the difference between a loss of thousands to millions of rands.

On mission-critical systems, such as nuclear power plants that were commissioned in the 90’s in the US, PLCs enable predictable control of safety systems. A PLC failure in a nuclear power plant could mean overheating of the cooling system, a lack of pressure regulation in the reactor core, or a failure of an emergency shutdown to trigger. Consequences that could level a city and cause mass fatalities.

In most software disciplines, a slip-up could just mean the user isn’t happy with a graphical glitch. In PLCs, a failure could mean a loss of human life, thousands of rands of profit down the drain.

Signs of Failure

Immediate Signs: LEDs

PLC manufacturers flash their hardware to show red lights on CPUs, communication cards, fuse blocks, and I/O cards. If you see a red light on any of these hardware components, you could be facing general issues with the hardware itself.

It is also advisable to inspect Ethernet ports and communication interfaces for link and activity lights. A lack of communication lights could indicate damaged cabling, failed communication cards, switch failures, or incorrect network configuration.

Some PLCs may also display fault indicators directly on the CPU itself. Intermittent fault lights or faults that temporarily disappear after cycling power can potentially indicate degrading hardware, unstable power supplies, or communication issues.

Burning smells, excessive heat inside the cabinet, or visible discoloration around terminals and connectors can also indicate failing electrical components or damaged hardware.

• Fuse blocks: Check if the fuse was blown and retry the setup. It was just the fuse, and the machine should start up again.
• I/O cards: Inputs changing state without outputs reacting correctly can indicate faults with output cards, field wiring, or PLC logic.
• Analog modules: Incorrect analog readings that do not match field measurements can indicate sensor faults, scaling issues, wiring faults, or analog input card failures.

It is important to remember that the PLC itself is not always the source of the issue. In industrial environments, field devices, sensors, wiring, actuators, and communication equipment are often more exposed to environmental conditions and can fail more frequently than the PLC CPU itself.

Network Issues

PLCs and the devices connected in networks can fail over time, either due to faults, misconfiguration, changes in the network, or failed components.

Often in PLC configurations inside panels, components can fail due to static discharges or electromagnetic interference. You’d need to slowly identify which router, PLC, switch, computer, or cabling is affecting the network.

It can often be traced back to devices not picking up any data, not being able to make any API requests, losing communication with HMIs or remote I/O devices, or being unable to access the network altogether.

Repeated communication dropouts, intermittent Ethernet connectivity, or devices randomly disconnecting from the network can also indicate degrading communication hardware or damaged Ethernet cabling.

In some situations, outputs may remain active when they should not be, or equipment may fail to respond even though sensors appear to be operating correctly. This can indicate communication faults, failed I/O modules, or logic-related issues.

Need assistance troubleshooting Ethernet communication on PLC systems? Check out our troubleshooting Ethernet on PLCs guide.

The post Signs of a Failing PLC appeared first on Control Hub.

Some applications require a PLC to communicate with different modules or devices via Ethernet.
This makes it easier to transfer data or send commands to different machines/devices.

Due to a lack of maintenance or incorrect configuration, Ethernet connections can fail. You’d need to troubleshoot at the correct source to figure out how to resolve the specific issue. Always identify what the issue is before attempting to solve it; you can’t solve a problem you haven’t defined.

Process of Elimination

The best course of action is to systematically check where the issue is:

1. Check Ethernet Interface: Check the physical lights on the Ethernet interface. If no lights appear, the interface could be faulty.
2. Check Ethernet Cable: Take a known working device and see if the cable works in another device.
3. Check PLC Communication: Check if the PLC can communicate with another device, and attempt to ping the device.

Check Ethernet Interface

Ethernet interfaces have built-in LEDs to indicate link status and network activity. If no lights appear on the physical interface, you could potentially have a network issue stemming from broken Ethernet cabling, powered-down devices, broken Ethernet interfaces, or in some cases, incorrect network configuration.

Look for physical damage on the board. Is there any burnt residue near or on the port? If not, move on; it could mean other factors.

If physical damage is present, you can either replace the communication module or attempt a repair by physically desoldering and resoldering a replacement Ethernet port to the PCB. In the case of a PLC Ethernet interface, it is possible to repair, but no guarantees can be made that it will work due to manufacturer-specific hardware and firmware implementations on the PLC. You will be repairing at your own risk.

After replacing or fixing the Ethernet interface on either the I/O or the PLC, or both, reconnect the system and see if the system is operational again.

Check Ethernet Cable

Ethernet cables can break over time due to excessive bending of the cabling, mishandling of equipment, or potential machine breakdowns.

Replace with appropriately rated Ethernet cabling if the cabling has been damaged. Reconnect the cabling to the relevant interfaces and see if communication is restored. If not, you could have a misconfiguration issue.

Check PLC Communication

For the PLC and the I/O device to be able to communicate over Ethernet, a physical connection through an Ethernet cable has to be made. Both of these devices then get assigned an IP address.

Both devices have to have an IP address on the same network to communicate with each other. On already commissioned PLC installations, there should be an IP address for the PLC near the installation inside a cabinet. You can connect a laptop and simply attempt to ping that IP address. If that does come through, then you know the cabling and the PLC’s IP configuration are correct. If not, the PLC could be incorrectly set up.

This goes for the I/O device too; attempt to ping the I/O device with a laptop Ethernet connection. If a ping does come through, communication is established. If not, the I/O device could be misconfigured.

Also verify that subnet masks are configured correctly and that no duplicate IP addresses exist on the network.

Finally, connect both the PLC and the I/O device and see if communication is restored. If communication is restored, the issue has likely been resolved. Otherwise, IP configuration could still be a problem. Go over both IP configurations and try again.

Summary

Essentially, check the Ethernet cable, check if the PLC can communicate with another device, and check the physical interfaces on both the PLC and I/O. You should be able to go from there and replace the cabling or interface if you know how to solder.

If any of the mentioned equipment, Ethernet cabling, and interfaces still have a warranty, it is advisable to use them. Do not attempt to repair equipment under warranty, as you have a chance to void the warranty.

The post Troubleshooting Ethernet on PLCs appeared first on Control Hub.