I have some SCL code written by someone, and I am having problems understanding the timer.

Here is the code

It is performing rising edge detection since when the .Q goes high, Old trigger is still low and the IF executes.  At the end of the IF, .Q and Old trigger match and the IF is no longer executed.

This makes the IF execute only one time when the .Q goes high.

master_blaster

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I do not understand the purpose of checking/having oldTrig

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Although explanation given is very correct, I must confess I don't understand this either.

The timer used is the most probably a TON timer. So its output goes TRUE for only 1 cycle anyway. No need to derive an edge.

Also checking in the IF clause the #StartAveraging is superfluous as this has been already checked at the timer's input.

jacek d

The timer used is the most probably a TON timer. So its output goes TRUE for only 1 cycle anyway. No need to derive an edge.

 that is not correct i think!! and that is why it is not recommended to use the Instanse data block of the IEC timers like in this code (#TriggerTimer.Q). 

and i think that is why he forced to make the OldTrig . 

OpisOm

 

jacek d

The timer used is the most probably a TON timer. So its output goes TRUE for only 1 cycle anyway. No need to derive an edge.

 that is not correct i think!! and that is why it is not recommended to use the Instanse data block of the IEC timers like in this code (#TriggerTimer.Q). 

and i think that is why he forced to make the OldTrig . 

If you tried to run this code on S7-1x00 you'd come to conclusion that the IEC timers does not work because there's no way to capture the state change at .Q by the IF clause. And this OldTrig would not help.

But on S7-x00 it will run like a charm. Also without the OldTrig.