G'day A-List,

I'm currently working on a large baggage handling system that has 44 S7 PLCs. Our customer is concerned about making sure the PLCs all keep their time synchronised.

My project manager believes that PLCs clock may drift by up to an hour a day. I just don't believe this at the moment.

So does anyone have any experiences they can share regarding how accurate or otherwise S7 PLCs keep? Additionally how have other people made sure a whole cluster of PLCs keep their time in synchronisation?

Thanks in advance,

Kelvin

 

Apparently the time error is consistent and there is a clock correction factor which is set up in the CPU hardware configuration.

Regards
Donald P

 

Hello Kelvin;
I've worked with Siemens' PLCs for a few years now, and have not experienced such drift of the Real-Time clocks as your project manager seems to fear.
When we need to synchronize the clokcs, we first set one CPU as clock master (in the CPU properties of the Hardware Config editor), then use SFC48 ("SNC_RTCB")on every CPU we want to synchronize (we call it in OB1). We then manually update the master's clock from the programming computer as part of regular maintenance (once a week is very adequate). There are probably more elegant ways to obtain the same results, but this works well for us.
Hope this helps,
Daniel Chartier

 

Hello Kelvin,
Wenn your customer is corcerned, you have to synchronize one PLC with a DCF-clock and synchronize the other 43 PLC's on this "Master". The synchronizing might be on MPI/Profibus or Ethernet. When you have Ethernet, then the clock on the CP443-1 might get the time from a Time-Server of the Internet.

Wenn you put the clock with his correction on, it is only changing when the Temperature is changing in a big way. So it is important that you synchronize the PLC's on one that is in a room with a stable temperatur when you don't use a DCF-clock or a time-Server.

Best regards

 

Hi Kelvin,

It's really amazing how often people believe in things instead of checking them. And even more interesting that such a person became a project manager. Let's get back to business. Siemens guaranties that accuracy of the RTC of
their S7 CPUs is no worse than +/- 9s per day. If it's not enough it's possible to synchronise the CPUs via network.
At Siemens web site you can find enough info regarding this subject.

Regards,

Jacek Dobrowolski, M. Sc. E.E.
Software Eng.

 

I can see how a PM might think this since typical PCs (not PLCs) can drift off by a lot. My PC at home that used to run W98 was often 30 minutes off or more, but I think this was a windows issue, since rebooting it would reset the correct time. My guess is when it rebooted it read the RTC and reset the system time, but that afterwards W98 kept track of time. Once I upgraded to WinXP, it does not have to be rebooted all that much (in fact usually only when installing or removing software) and keeps much better time. I must admit I did add 256M of memory, so possibly that may have been of some help as well.

 

Unrelated to the real answer below - I received once a complaint about one of my products that had a real time clock - that it drifted by one full hour. Further investigation revealed that the product was OK - it was observed when country switched in or out of daylight saving time - so the observer was in error by one hour.

Meir