IEC Timer updating & accuracy

Hello guys,

I understand that Siemens IEC timers update asynchronous to the program cycle. While the Time value is stored in milliseconds, I'm wondering how accurate they actually are. As I understand it, the Time value is updated by subtracting the current cpu Time from start time. Can anyone provide information on how often they are truly updated? I originally thought they were updated whenever called, and not updated if not called. I also thought i had remembered reading something somewhere about the cpu clock (at least on the 1200) updating every 10 ms, which wouldn't make them especially accurate (at least in the s7-1200). 300/400 would obviously have to be way more accurately updated than this. But I could be totally off on this, so any input would be appreciated.

Thanks!

The IEC timers are updated when they are scanned. The s5 timers are updated asynchronously.

Thanks for that response L D. I initially thought that's how they worked but when reading over my Hans Berger books I found under the section on Simatic Timers that Simatic timers update asynchronously. The next section was on IEC timers, but there was no mention of how they updated so I assumed it was the same.

Just tested out an IEC pulse timer using PLCSim, calling it in a function with a clock memory bit in front. Exactly right, the timer only updates when called.

What's the deal with the Simatic Timers? Anyone know the answer to my above question with regard to the simatic timers, in other words how often they actually update?

Summary:

S5 timers: Once triggered they will keep going. Even when not scanned.
NOTE: Since they are asynchronous to the scan cycle, it means they can change state within the scan cycle.

IEC timers: These are identical to your general FC/FB's. They need to be scanned to work.

CPU Clock Memory: This is something you configure in the HardwareConfig.
It is a byte of memory, of which every bit in the byte has a different frequency.


S5 timers have a different resolution depending on the time base used.
IEC timers have a 1ms resolution, but the actual accuracy will depend on the scan cycle time.
Clock memory isn't a timer.

Hope this clears things up a bit.