Quick Fix Allen-Bradley PLC Communication Errors

As a controls engineer, you will need to connect to a PLC quite often. You connect to a PLC to download a project to the PLC, upload a project from the PLC, or to monitor and edit a PLC project while it is executing in the PLC.

With Allen Bradley PLC systems, you use RSLinx Classic or FactoryTalk Linx to communicate with a PLC. These products let you configure communication drivers that enable data exchange between a computer and a PLC.

Most of the time, connecting to a PLC is easy but sometimes things go wrong and that can be very frustrating. In this video, I’ll go through some common troubleshooting steps that you can use when communication does not go smoothly.

The most common issues that people face when trying to connect to a PLC is that they cannot find the PLC or cannot communicate with the PLC over an EtherNet/IP driver.

Before moving on, I want to point out that I’ll be using FactoryTalk Linx in this video but most steps will be applicable to RSLinx Classic too.

Autobrowse

When you open the Who Active view of FactoryTalk Linx through Studio 5000 Logix Designer, you see a list of communication drivers.

When you expand a communication driver, you can see a list of devices that can be reached through that driver.

Occasionally, you may click a driver and notice that nothing happens.

The most common reason for this is that the Auto browse feature is not enabled so FactoryTalk Linx is not actively looking for devices on the network.

To fix that, click on the Settings icon and then check the Make Discovery Continuous (Auto browse) option and click OK.

Now FactoryTalk Linx will send discovery broadcasts over the network to find all EtherNet/IP enabled devices that can be reached through a driver.

You can see that broadcast messages are being sent because the driver’s icon animates when you click on it.

Note that a driver has to be selected to enable Auto Browse. If you click on a device under the driver, then FactoryTalk Linx will stop sending broadcast messages and the list of devices that are reachable will not update.

IP Address

If your settings are configured correctly and you still cannot see your devices, the issue may be that your computer is not in the same subnet as the devices on the network.

The EtherNet/IP driver only discovers devices on a single subnet to prevent flooding a network with broadcast traffic.

The best thing to do is to check the IP address of your laptop and confirm that it has an IP address in the correct subnet.

To do that, open the Start menu and type CMD.

In the command prompt window, run the IP config command. The command returns the configuration of all of your network interfaces.

One of these interfaces should be in the same subnet as the devices you are trying to communicate with.

If not, you will have to update the IP address of your computer. You can do that by following the steps in our video How to Set the IP Address of your Computer.

Network driver

If the IP address of your computer is correct, the issue may be that the driver is using the wrong network adapter to send broadcast messages.

By default, FactoryTalk Linx uses the Windows default adapter which can be the Ethernet or WiFi card.

To specify a specific card, right-click on the driver and select driver configuration.

In the advanced tab, you can specify which network interface the driver should use.

Device icons

By now, you should be able to find devices on the network. All of the devices on the subnet are listed in FactoryTalk Linx.

Some devices may be shown with error or warning icons.

A Red X indicates that a device was previously reachable but can no longer be found. This might happen if the network cable to the device is removed or the IP address of the device has changed.

You can remove these devices by right-clicking on the driver and selecting remove offline devices.

This feature is very useful because it lets you understand if your network has changed since you last browsed it.

Devices may also be shown with a yellow question mark. This indicates that FactoryTalk Linx has found a device but doesn’t know how to communicate with it. This often happens with third-party devices.

FactoryTalk Linx uses a file called an EDS or electronic data sheet to define how it communicates with a device.

EDS files for devices can be found on the manufacturer’s website and very often you can upload the EDS directly from a device by right-clicking and selecting upload EDS. Once the EDS for a device is installed, the yellow question mark will go away.

For devices that don’t support uploading the EDS file like this ifm IO-Link Master, you have to manually install the EDS file. We cover how to find and manually install EDS files for third-party devices in our upcoming course on industrial sensors for Allen-Bradley IIoT Systems which will be available to all RealPars members.

Conclusion

In this post, we went through a step-by-step guide to troubleshooting communication issues with Allen Bradley PLCs.

Specifically, we saw how to configure a driver to Auto browse a network, how to check the network adapter assigned to a driver, how to check your computer’s IP address, and how to interpret device statuses in FactoryTalk Linx.

With this knowledge, you should be able to connect to Logix 5000 PLCs to download projects to them, upload projects from them, and monitor and modify projects running on a controller.

If you’d like to learn more about programming Logix 5000 PLCs, check our Learn Logix courses.

These courses teach you everything you need to know to be a competent and confident PLC programmer specializing in Allen Bradley PLCs.

Want to reduce downtime by training your engineers and technicians? Visit realpars.com/business and fill out the form.

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