Preventing an input being pressed more then once during a cycle

Hi there,
I have created a ladder that simulates a traffic light system with a pedestrain request button.
My sequence is as follows:
A "reset" button is initally pressed to set the pedestrain light to red and the traffic light to green.
When the pedestrain button is pressed, the sequence of the traffic light begins and runs through until its back at its inital starting state.

However the problem with my ladder is that when the pedestrain button is already pressed and the sequence is running, if its pressed again during the cycle it messes up the cycle!

I'm not asking for a solution here, but more a push in the right direction. I havent the slightest clue on how I would go about preventing that from happening. I'm sure its probably not majorly differcult?

Any ideas and advice would be gratefully appreciated,
Regards,
Matt

Upon the initial pressing of the pedestrian button, set a bit called "sequence in progress". Use this bit to inhibit any subsequent pressing(s) of the pedestrian button...

Of course, you also need to implement a "sequence complete" bit to drop out the "sequence in progress" bit.

Welcome to control engineering.

Jimmie_Ohio said:

Upon the initial pressing of the pedestrian button, set a bit called "sequence in progress". Use this bit to inhibit any subsequent pressing(s) of the pedestrian button...

Of course, you also need to implement a "sequence complete" bit to drop out the "sequence in progress" bit.

Welcome to control engineering.

Click to expand...

Thanks for the quick reply.
I understand what you are saying with the "bits". But I'm puzzled on how I would go about creating the "in progress bit" that would subsequent the pressing(s) of the pedestrain button.
Many thanks for the help so far,

Matt

Either use a set of latch/unlatch coils, or just seal the circuit in. Basically, the actual input will only be used in that one spot in the program. As was mentioned, the trick is to create a bit that lets you know when the sequence is complete so the in progress bit can be reset. That will come from your ladder, BTW, not the reset button you mentioned in the first post.

-jeff

If it were amotor starter (three wire circuit)......

You might also think of your 'sequence request to start' button as a START button, and the 'sequence restart-holdout' as the actual motor starter coil. The 'sequence complete' (normally closed) would be the STOP function.

Take a look at this. It's a sample of PLC ladder showing what I meant by my earlier post.