S7 Timer

Hi Folks,

I am using S7, S_ODT Timer for my project. But i note that, the timer count are slightly slow then the real time ( where i refer to my watch ). is about 1 Sec (S_ODT)= 1.2 sec ( Watch ).

Any body can give me advise on that?

Thanks

What type of S7-??? and what is program scanning-time ?

Dear seppoalanen ,

S7-300 CPU 316-2DP, the scan time is 98ms maximum and current is 63ms.

I face same problem in other project which my scan time is over-shut to around 180ms so, i change the default scanning time from 150ms to 200ms.

Thanks

hello!
Did you measure the time from a output or from the screen ?
If you watch the screen you will get a delay..
I think you must put your timer in a interupted block if you require
exact times..

Timer 'time has done bit' could be from 0 to 2 times max scanning time late, depends where it has asked. Sorry, read also Niklas advice above. Kiitos Niklas!

Gosh - scan times higher than 100ms !

It could account for some of the difference between timer value and output ON time.
But you should also take into account the propagation time for an output to change from ON to OFF. If the output drives a 24V coil that is protected by a freewheeling diode, then it delays the ON to OFF transition significantly. Protection via a zener barrier makes the ON to OFF time shorter.

Is it normal with such high scan time for S7-300 ?
I use S7-400 and have no problem keeping the scan time down to 3-4ms.
For a similar program in an AB SLC500 or PLC5 I get 20-30ms.

I would look into revising the program in order to squeze the scan time down.
- Change code from cyclical to event-based execution where possible.
- Check for unnecessarry code, especially in sub-routines that are called often.
- Change the update rate from every scan to a lower rate where possible (analog i/o seldomly needs to update every scan).
- Task-switch noncritical code (f.ex: execute subroutine A+B+C in one scan, execute subroutine D+E+F in the next, and so forth).
- Use interrupts for critical i/o that should update as fast as possible (but that won't help if there are timer-dependencies !)

Hello !
It is no problem to get the scan cycle to > 100 ms in a 316-2dp,
but usually it depands on the programmer/program if you have lot of eg. indexing and blocks that do internal cycles(loop-until) then you will get the cycle time up.
If you make the program so that you only do the requeired operations when needed then you can get the cycle time to < 10 ms.
316 is not a "number-braker" PLC, the 400 series are much faster.