VisiLogic Timers

[Flex727]

I actually like the way Unitronics has distilled down all the possible timer types to 3 basic ones for simplicity. I haven't yet run across something I couldn't do with the 3 basic timers provided, however I have a simple question. What I need is a Delay timer that has an accumulated functionality (i.e. I want the timer to accumulate instead resetting each time it is activated). The issue I have is that when the timer finally does expire, I want it to reset at that point (a TA doesn't do that). A positive transition of the timer bit activating the timer reset function works fine (see below), but the compile always gives me this warning: "To improve performance, use Normally Open/Closed contacts instead of positive/negative transition contacts." I guess I can use a normally open contact since it will only be on for one PLC cycle, but why do I get this warning?

 

  TA1       TA1

--| P |-----( R )

[cantcliff]

You're getting the warning because there is a limited amount of Positive/Negative transitions allowed in the final code.

 

Where it becomes an issue is when the ladder logic only shows that you used, for example, 8 positive transitions but the actual code compiles to 15 when it is converted to STL.  This is quite common and if you use too many it's possible you will exceed the maximum number of P/N transitions allowed when the program compiles without understanding where or why.

 

On smaller ladders it's usually not an issue but on larger ones it may cause unintended logic errors after compiling and uploading.

 

You can reference PDF page 55-56, (Page number 48-49) of the ladder logic programming manual with the maximum number of P/N transitions you can use.

[Flex727]

Thank you, cantcliff.

[Ausman]

Hi Flex,

 

Occasionally I have trouble wrapping my brain around how Unitronics timers work. When I first started years back, the brand I mostly used had many types of timers available, and adjusting to 3 has been "interesting" at times.

Timers have their place, but sometimes I end up using a dedicated counter which often does the job much better and is easier to interpret than some convoluted way of manipulating timers. With the counter you can make it go both ways with ease and manipulate a host of things very simply through multiple compares that adjust, stop, restart, and see where the count actually is.

cheers,

Aus