Rising and Falling Edge-SoMachine Basic

Hi All,

Does anyone of you know what is the function of rising and falling edges? I essentially want to trigger a timer once the state of a coil is changed (turns off after being on). I am using Somachine Basic..

Any help will be really appreciated.

Thanks guys

These are also called On Shots in other PLC's. Basically they are instructions that will remain true for one scan of the PLC when the bit that is referenced in the instruction goes from off to on (Rising Edge) or if the bit goes from on to off (Falling Edge).

The bit that is referenced can be a Digital Input or any Boolean memory location. There are many uses for these instructions. I use these a lot to move data when a day changes. I only want to move the data once and only when the day changes. So I will have some logic that looks at today's date and compare it to the PLC clock date. As soon as they are not equal I will trigger a rising edge that will move my data to the next register. Since the rising edge is only true for one scan the next scan it will no longer be true so there is no data move.

You can use a RE or FE to start a timer it can get complicated because a timer will only count as long as the rung is true. If you use a RE or FE it is only true for one scan and the timer will not complete the cycle. You dont really need to use a RE or FE to start a timer. There is a much simpler way to make a timer start when a bit goes from on to off. You can use a NC contact in the rung so that when the input is off you start the timer. I will attach a snip of a rung that may do what you want.

I hope this makes sense. If you need more info post back.

I haven't really understood it unfortunately.. I essentially want to run M1 for 9 seconds once M0 goes to LOW, from HIGH..

Falling or rising edge is just a one scan pulse so you cannot use it to run your timer directly.

The way you want to do it at the moment will not work. You need one more line of code to make it working.

QasimFSH said:

I haven't really understood it unfortunately.. I essentially want to run M1 for 9 seconds once M0 goes to LOW, from HIGH..

Click to expand...

Just use a Pulse timer. It works with falling edges

Hi,

Check the snapshot for a possible solution.

Thanks.

Qasim > I essentially want to trigger a timer once the state of a coil is changed (turns off after being on).

>>>

Sounds to me . . .
Like you need to use the falling edge from the coil change to latch in a latch relay. Then
have the latch relay start (and keep running) the timer . . which then unlatches the
latch relay when it finishes?

Poet.

I have used this logic, and looks like it is working: