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Offline Changes to a PLC Program

As a PLC programmer, you’ll often be asked to do a change to an existing system. If there’s a significant amount of functionality to be added, you generally get your changes ready “offline” and then do all the changes during a short window of time to minimize disruption to the production schedule.

If you’re using an Allen-Bradley PLC, the procedure is typically this:

  1. Get a copy of the latest program from the PLC (a.k.a. an “upload”)
  2. Make your changes to the offline copy, and write down every change you had to make
  3. Go online with the PLC and apply your changes as online changes

Step 3 is much safer than just taking your modified program and doing a “download”. That’s mainly because when you download, you’re not just downloading the program, but the memory state of the PLC as well. The PLC typically has to track things in memory (like recipe data, part tracking, data collection, sequence numbers, machine counters, etc.). If you do a download, you’re going to overwrite all those values with previous values, and that can cause a lot of problems. The other thing step 3 saves you from is simultaneous changes that were done online while you were busy making offline changes.

The only other option you have is upload-change-download, but you really have to shut the machine down for the duration to make sure that the internal state doesn’t change.

When I did a lot of Allen-Bradley programming, I didn’t question that. It’s just how it was. I remember visiting a plant one time for a service call, and the local maintenance person was a bit suspicious of what I was going to do (after all, I was a young kid who had never seen this machine before). He decided to quiz me a bit, and one of the things he asked was, “when you go online, do you download or upload?” I said “it depends,” but his answer was, “you never download.” I agreed that someone in a maintenance role should never need to do a download unless they’re replacing a CPU, or recovering from a corrupted PLC program.

Now that I mostly do Beckhoff TwinCAT 3 programming, I realized one of the benefits are that offline changes are a breeze. It’s due to the fact that TwinCAT 3 completely separates the program from the memory data. The program is stored in local files on your hard drive and compiled into a TMC file. The persistent data is stored in a different place on your hard drive.

When I want to do offline changes to a TwinCAT 3 project, here’s the procedure:

  1. Get a copy of the latest program
  2. Make your changes to the offline copy
  3. Copy changes back to the machine (keeping a backup, of course), rebuild, and activate configuration

This makes offline changes go a lot more smoothly, of course. I don’t have to copy and paste my changes in while online, so it takes less time and eliminates the possibility of a copy/paste error.

Since we also use Mercurial for version control, getting a copy of the latest program is a matter of pulling the latest from the source control, and copying it to the machine is a matter of pulling the offline changes to the machine. Any changes that were done in parallel can be merged with Mercurial’s built-in diff and merge utilities. (Note: I/O changes can’t be merged nicely, so if someone changed the I/O while you were doing your offline changes, you have to copy those changes in manually, but that’s rare and at least it tells you that it can’t merge them.)

This got me thinking that Allen-Bradley probably has a better way of doing offline changes that most of us just don’t know about. I know that you can do an upload without uploading the memory. However, it seems like it requires you to download both the program and data at the same time. I wonder if anyone out there knows how to do better offline changes to a ControlLogix. If so, I would be interested to know that.

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Upgrading your TwinCAT 3 Version

Beckhoff releases new versions of TwinCAT 3 fairly often, and especially since this is a new platform you probably want to stay on top of their new updates for improved stability and new features. Here are some hard-won lessons I wanted to share with you about how to upgrade your production system to the latest TwinCAT 3 release:

Have a Test System

You should definitely have an offline test system for many good reasons. This test system should have the same operating system version as your production system and should ideally be the same hardware, though I realize that’s not always feasible. It could just be an old desktop PC you have sitting around, but that’s better than nothing. TwinCAT 3 is free for non-production use so you have no excuse for not having an offline test system.

Test your upgrade on the Test System First!

Never try upgrading your production system without running through a dry-run on your test system first. Get a copy of the latest TwinCAT solution from your production machine and get it running on your test system. It doesn’t matter if you don’t have I/O attached because the runtime will work just fine anyway. After your perform the upgrade procedure on your offline test machine, make sure you do a thorough test, including a reboot.

Follow These Steps

  1. Stop the machine and make it safe
  2. Put the runtime into Config mode
  3. Uninstall the old version of TwinCAT 3
  4. Also uninstall the old version of the Beckhoff Real-time Ethernet PnP Drivers
  5. Reboot
  6. Install the new version of TwinCAT 3
  7. Reboot
  8. Open the TwinCAT 3 solution
  9. Re-install any custom libraries, if you have any (optional)
  10. Go to the tool for configuring Real-time Ethernet devices, and install the new driver on your EtherCAT cards
  11. Re-link your EtherCAT master to your EtherCAT adapter under I/O, just in case
  12. Build each PLC project (individually, don’t use Rebuild All because it sometimes ignores errors)
  13. Check that you didn’t lose any I/O mapping
  14. Activate boot project on each PLC project
  15. Check under System that it’s configured to start in Run mode (if that’s what you want)
  16. Activate configuration and restart in run mode
  17. Test by doing a reboot

Upgrading the Real-time Ethernet drivers is critical. We were experiencing cases where the EtherCAT bus would just cut out on us, but only on one machine. All of our machines were upgraded to the same version, so we initially thought it must be a hardware issue. It turned out that we weren’t upgrading the Real-time Ethernet driver when we upgraded TwinCAT 3 versions, so this machine had an old version of the driver loaded, and all the other machines had a newer version. After upgrading the driver, the problem went away, so upgrading the driver is a critical step.

If you find that you did lose your I/O mapping, make sure you’ve built all your PLC projects (which generates a TMC file) and then close TwinCAT 3 XAE down and revert your .tsproj (TwinCAT solution project) file back to the original state. Then start again at the step where you open the TwinCAT 3 solution. Now you should find that your I/O mapping is back. That’s because the inputs and outputs of each PLC project are compiled into the TMC file and TwinCAT 3’s system manager links I/O against that. If the file doesn’t exist (or they changed the format during the upgrade) then it’ll just delete the links. However, the links still exist in the original .tsproj file, so creating the TMC file and then reverting the .tsproj file will put everything back to a happy state. This is also a useful trick when you’re moving the project to a new PC and you didn’t bring the .tmc files along for the ride (because they’re quite large).

Edit: Since writing this article, I’ve added a TwinCAT 3 Tutorial to this site as well.


Those of you who know me remember I’ve been itching to get my hands on a Beckhoff TwinCAT PLC industrial control system for a few years now, but I never found a project or customer willing to take on the risk of trying something new. Like a bunch of penguins on a cliff of ice, nobody wanted to be the first in the water.

I’ve recently had the opportunity to apply a TwinCAT PLC and HMI system to a relatively low-risk application, so I finally have something to share. Those of you who know me also know I don’t pull my punches when I talk about products, so here’s an honest review of the TwinCAT system: the good, the bad, and the ugly.



What draws you to TwinCAT is its combination of low cost with a ton of features. I can’t publish prices, but I encourage you to compare prices from Beckhoff, Allen-Bradley, Omron, and others. Beckhoff offers a similar or better feature-set, with as many or more communication options than it’s competitors, for significantly less money (it’s real-time PC control instead of dedicated hardware). On specs it can do everything, and doesn’t bend you over the table when it comes to the price.


First, Beckhoff TwinCAT PLC takes advantage of a PC CPU and memory, which means you can scale up the performance of your system for a fraction of the cost of scaling up to large PLC and PAC systems.

Secondly, if you use EtherCAT, you get to scale up your I/O sizes without as much impact on performance or price. The individual I/O slices are significantly cheaper than for other bus technologies, and the servo drives are less expensive because the bus speed is fast enough to close the loop from the PLC rather than in the device itself.

Where TwinCAT really shines is when you need to integrate it to the MES layer in your organization. Most other vendors require you to buy expensive OPC or other connection software, but Beckhoff provides a free DLL for Windows applications to communicate with the PLC over a communication protocol called ADS. (They also provide an OPC server as an add-on if you need it.)

What’s the Catch

Switching software vendors is never easy. If you’re used to your existing PLC software like RSLogix, you’ve got a bit of a learning curve ahead of you. Here are the things that down-right annoyed me about TwinCAT when I was coming from the RSLogix/PanelView realm:

  • When you do an online edit, it resets all of your forces. You have to reload your watch list to re-apply them. (Fortunately I always write perfect code the first time, so I never had to do an online change…)
  • The ladder logic editor doesn’t seem to auto-wrap (unless I’m missing a setting somewhere)
  • The HMI editor crashed a couple of times when I was modifying the HMI, and I had to restart the TwinCAT system (not the PC, just the software)
  • The English documention is lacking
  • The TwinCAT System Manager DeviceNET master configuration had a bug where it wouldn’t recognize EDS files where the Class value was stored as a 16 bit, rather than an 8 bit value. The EDS file was valid, according to EZ-EDS, but it wouldn’t read it. Changing the value to 8-bit made it recognize the parameter. This made it hard, but not impossible, to integrate an Allen-Bradley Point I/O DeviceNET node.
  • Even though the HMI will log an alarm history, there’s no alarm history control for you to view it in the HMI itself (it’s saved to a CSV file)

Here are some things that are just really different, but I’ve had no trouble adapting to:

  • You have to define all variables (tags) in a text syntax, instead of in a spreadsheet-like tag editor
  • I/O isn’t mapped directly. You define your hardware I/O and your program I/O and then you map them.
  • You don’t do online changes while you’re online with the PLC. You “log off”, then make your change, and then “log on” and tell it to upload your change. (You can do this while the machine is running.)

What’s Cool

Using a PC based controller opened up some interesting features, particularly in the HMI:

  • You can launch another application from a button click on the HMI (Excel, etc.)
  • You can configure some interesting alarm actions, like launching a program, or sending an email
  • There’s an (optional) add-in to let the PLC communicate in a limited fashion with a SQL Server
  • It runs on commodity hardware. PC hardware is cheap and you can get much larger touch screens for a lot less than the cost of an equivalent PanelView. The “fieldbus” card (EtherCAT) is under $100. Even a Beckhoff EtherCAT to DeviceNET master solution is cheaper than a straight-up DeviceNET card from another manufacturer.
  • TwinSAFE – a Safety Controller sitting right on your fieldbus for less than a stand-alone safety controller from another vendor.


I had nothing but superb support from the local Beckhoff technical support guys during this experience. They respond to emails personally usually within a couple of hours, and they have no problem taking the two hour drive to see us if we have any issues that can’t be resolved over the phone or with email. We’re not a “big” customer in any way.


I’ve just spent about 2 weeks coming up to speed on TwinCAT, having come from an AB/Omron background, with a smattering of other PC based controllers like Phoenix Contact’s Visual Logic Controller (VLC) and MultiProg (formerly PCWorx).

The ease-of-use of TwinCAT doesn’t measure up to RSLogix, but it’s comparable to Omron, and it blows products like PCWorx/MultiProg out of the water (I think I was using version 3 of PCWorx, and I know there’s a version 5 now). Most of my issues with TwinCAT are that its ladder editor just isn’t polished enough compared with RSLogix, but that’s not surprising given its roots in Germany. North America tends to favor ladder logic, where Europe seems to favor instruction list, structured text, and function block diagram. It feels like the TwinCAT ladder logic editor was neglected a bit, but it is certainly workable. Comparing the TwinCAT sequential function chart editor with the RSLogix sequential function chart editor, they were actually reasonably close. I liked that I didn’t have to position individual steps in TwinCAT – it just auto-arranged everything for me.

Price-wise, TwinCAT is significantly less expensive than any other serious player in the industrial automation equipment space, particularly when you start ramping up the amount of I/O and drives you need, and it has similar (and in some cases better) features to offer. It’s even significantly less expensive than the Phoenix Contact solution (from my memory), and given that, there’s no reason I’d ever recommend a PCWorx/MultiProg solution from Phoenix Contact. The Phoenix offering is worse and more expensive.

If you’re in a position where your facility is in the market for a new automation platform, I definitely recommend including the Beckhoff TwinCAT platform in your deliberations. It hits a really unique sweet-spot on price, flexibility, and features that nobody else seems to be able to offer right now. Now that we’re a few weeks in, we have a running machine, it looks like the project will be successful and I have no reservations about saying: if you don’t mind learning something new, give them a try.

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Beckhoff in the Kitchen

I was a little surprised when I read about Industrial PC Technology in High End Kitchens in Beckhoff’s PC Control magazine. The entire kitchen is integrated together using industrial PC technology:

SieMatic’s self-developed software for the selection menu and the control functions forms the heart of the SieMatic Grid. The basis is a
modularly structured Beckhoff Embedded PC equipped with a Windows XP Embedded operating system as well as a .Net framework. Via RS232, the Beckhoff PC controls the T+A components as well as managing all functions that they do not take care of. Above all, these naturally include all the typical PC tasks, such as Internet access, e-mail access and data services (such as weather forecast or newsfeeds), but also entertainment programs, such as slide shows. All [email protected] appliances are also integrated via the Ethernet interface of the Embedded system.

The most important functions, however, run invisibly in the background, because the Beckhoff PC works almost like a mediator between different worlds: for example, a television broadcast can be interrupted for messages from the Miele system – that the dishwasher program has finished, for instance – if the user so desires. The integration of other systems – for example, the picture from an IP monitoring camera – is easy to accomplish using Beckhoff hardware. It was clear from the outset to SieMatic project manager Thorsten Pawelczyk that the SieMatic Grid would be a constantly growing and changing system: “The fact that the Beckhoff automation system hardly sets any limits regarding the integration of other systems comes just at the right time.”

I question the logic of putting industrial technology in a consumer kitchen, but I must admit I’m impressed and envious. I think the real question is, does it have the “hackability” we’ve come to expect from industrial control systems? Can I connect to it with TwinCAT PLC and modify the functions, extend it, or tie in new systems? Somehow I doubt it. The article mentions it runs a .NET application, so in this case they’re probably only using the hardware, not the Beckhoff control software.

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