Siemens PLC troubles


we have a factory with a Siemens S7-300 PLC connected to Ignition 7.5.13. They work well all the time and they communicate each other properly, except when we have to connect one of our PCs to the PLC (through Siemens Step 7). Since that moment, the communication between PLC and Ignition becomes very very slow (tags sync every some seconds and randomly), and some of our scripts start to fail generating stops and troubles in the factory production process.

We have tried to increase the PLC communication time in its scan cycle, but the problems continue. We don’t know if this is common, and we’d like to know if anyone else has the same kind of troubles in other factories.

Thank you very much
Best regards

It sounds as if you’re using up all the available sockets.

What else is connected to the PLC?

As an example, I have a line that uses an AB SLC 5/05. The rule of thumb is to have no more than 3 HMI’s attached to the processor. But what we really have is:

5 HMI’s
3 Cognex cameras
3 3-axis robots
1 six axis Robot

All 12 devices wanting to talk to the PLC. The SLC only has 8 socket connections available. So, someone has to wait-- or timeout.


first of all, thanks for your reply. In our case, that wouldn’t be a problem, because our CPU is Siemens 317-2 PN/DP with 32 socket connections available. Currently, we have connected the following devices:

  • 4 HMIs (through PROFIBUS)
  • Ignition gateway (through Ethernet)

The problem appears when we connect another device through Ethernet (our PC with Siemens Step 7 to maintenance and programming tasks).

In our CPU configuration, we have 6 reserved channels for HMI connections, and 4 free additional channels for programming. In fact, we have 12 reserved channels for Ignition Gateway (we have tried testing with more and less channels in order to get a solution, but problems continue).

Thanks a lot
Best regards

When you connect Step 7 to the device you can go into the Ignition gateway under Configure>OPC UA>Devices and click on the diagnostics for the device. What is the average request duration at? Also, how fast is your scan class. It sounds like your device is not able to handle the number of requests you are hitting it with.

Thanks for your reply,

this is the information of the diagnostic for the PLC when we are connected to the PLC too (through Siemens Step 7, when we have problems):

Request Cycle Request Throughput 43.6/sec Min Request Duration 6.0ms Max Request Duration 94.0ms Avg Request Duration 24.8ms Last Request Duration 33.0ms

As for our scan classes, we mainly have four:

  • Default: 1000 ms. (228 tags)
  • Default_historical: 10000 ms. (19 tags)
  • SlowScan: 5000 ms. (865 tags)
  • Triggers: 500 ms. (65 tags)

How do you think about this data?

Best regards

Ok, I need the diagnostics information from the entire page. This will give us a clear indication as to the request duration for each scan class. I do think you may have answered your own question by saying this only happens when you have Step 7 connected to the device. I think the problem is most likely being brought on by the device being overloaded with requests. You should be able to confirm this by comparing the average request duration of the scan classes with Step 7 connected and disconnected. With Step 7 connected you should see the average request duration for the scan classes taking longer that the rate of the scan class.