S7-300 timers and scan cycle time

[TomTom]

Hi all,

I probably have some short questions for you. I'm trying to understand timers and the Siemens operating system in more detail.

Introduction:
We use a Siemens S7-300 PLC. In the program I use timers of various lenght, between 200 and 900ms.
The scan cycle time on avarage is 5ms.
The timing of our timers is important.

Questions:
- I understand from postings in this forum that timers update asynchronously. So if a timer reaches its set value and the program is in its cycle the changing value of the timer will be used in that particular cycle, from that point on, as I understand. Is this correct?
- If the timer reaches its set value and the conditions in the program using this timer have just past, the timer has a delay of maximum 1 scan cycle time. Is this correct? This would mean that my timers have an accuracy of +5ms in this example? (The respons time of actuators is not taken in consideration here)
- Is there a way to avoid this 'inaccuracy'?

Thanks in advance.

Cheers, Tom

 

[cjd1965]

Hi if you require accuracy below 5mS then i would suggest that a PLC isnt the way to go

 

[TomTom]

Well, that's an interesting point of view and I agree.
But then again, I'm still interested in understanding these timing processes inside the PLC better. Just for the sake of understanding and learning.
What do you think of my other questions? Or can you explain a little more about 'the flow of the program' concerning timers?
Thanks in advance.

Ciao, Tom

 

[JesperMP]

You are probably talking about the IEC timers.
The IEC timers are accurate to 1ms.

But your logic that is called by OB1 does not update more often the approx 5 ms. That gives you an "inaccuracy" of the execution of the logic of approximately half of the 5 ms, so 2.5 ms on average.

If you have something that must update faster than 5 ms, then you can consider to use cyclically timed interrupts. Depending on the CPU you can get cyclical updates down to 1 ms. The IEC timers will still be accurate to 1 ms.

 

[cjd1965]

You need to consider the scan time....

Consider SFB4, TON The timer is looked at every scan (provided it is in a called block) so the PLC will process the timers DB accordingly and once the preset has been reached, will turn on the.Q

The .Q will then stay on so if timing is critical then put the critical code immediatley below the timer call in your logic. It will take 1 scan to populate the complete code where yuou have used the .Q

Like wise when the timer enable goes off the .Q will immediatly reset but will take 1 scan to turn off other coils etc where you have used the .Q

Therefore you cannot avoid upto 1 scan time error but you can minimise it by careful coding

 

[TomTom]

Thank you, cjd1965 and JesperMP for your fast replies.
I'll dive into the different options you gave.

Cheers, Tom

 

[userxyz]

Vipa

Vipa is much faster and runs mostly in µs

Regards,
Combo

 

[JesperMP]

Vipa is much faster and runs mostly in µs

Does VPA have a timer with µs resolution ?

 

[L D[AR2P#0.0]]

I understand from postings in this forum that timers update asynchronously.

This means that if you check the state of a timer in one part of your program it may not be the same in another part of the program.

 

[Longbow]

Does VPA have a timer with µs resolution ?

Yes, SFB 7 and SFC 53 provides µs Resolution within the Speed7 based PLCs.

If µs resolution is required for measurement of external events or controlling outputs: VIPA SLIO I/O System provides µs based I/O Control also for SIEMENS PLCs.