PLC counter block with configurable edge detection

[dhancock]

Does anyone know of a counter block (either PLC or PAC) that allows configuration of the input signal, such that it can be set to count up/down on either a rising edge or a falling edge?

 

[cardosocea]

Why does counting on a falling edge matter? Particularly if it doesn't need the rising edge being counted?



I'd imagine some encoder cards to be smart enough to do stuff like that... or you can program it.

 

[Steve Bailey]

Easy enough to roll your own as long as the PLC supports falling edge contacts.

 

[Lare]

Easy enough to roll your own as long as the PLC supports falling edge contacts.

Every PLC supports it. You can use old fashioned way without R_trig and F_trig blocks on STL or ladder with temperary saving status of input and conparing with inverted bit one scan before.

 

[cardosocea]

Easy enough to roll your own as long as the PLC supports falling edge contacts.

More importantly, is the scan time faster than the signals coming in...

 

[drbitboy]

Does anyone know of a counter block (either PLC or PAC) that allows configuration of the input signal, such that it can be set to count up/down on either a rising edge or a falling edge?

Here (Rockwell Logix).

In RSLogix 500 a pair of CTU/CTD blocks (here) could be used with the same C5:n counter object.

Also, it is not complicated to roll one's own:

   Inp     Mem
---]/[-----] [----[ADD count 1 => count]------

   Inp     Mem
---] [--+--]/[----[SUB count 1 => count]---+--
        |                                  |
        |  Mem                             |
        +--( )-----------------------------+

Which could be encapsulated in an AOI (or equivalent for whatever vendor is in use).

It may also need supporting rungs:
- to ensure the count is not changed on the first scan at entry to RUN mode;
- to reset the count to an initial value.

 

[dhancock]

Thank all who responded, but I am actually looking for a counter block that will allow the selection of a rising/falling edge as a trigger. Not for programming but for research that I am doing on a legal case. When I have done PLC programming, I would never have thought about such a requirement and would just have inverted the signal if needed. But this is a special instance, so I thought I would ask this community. I have gone through a number of different PLC manuals as well as SCADA software and can't find a block that will allow edge detection configuration.

 

[Steve Bailey]

The only thing I can think of that comes close to what you're asking about would high speed counter functionality. That can often be configured for one or two channel, single ended or quadrature. But in my experience the single ended option has always been rising edge.

 

[plvlce]

Thank all who responded, but I am actually looking for a counter block that will allow the selection of a rising/falling edge as a trigger. Not for programming but for research that I am doing on a legal case. When I have done PLC programming, I would never have thought about such a requirement and would just have inverted the signal if needed. But this is a special instance, so I thought I would ask this community. I have gone through a number of different PLC manuals as well as SCADA software and can't find a block that will allow edge detection configuration.

I've not personally ever encountered a such a counter 'in the wild' as it were.

As others have noted it wouldn't be particularly difficult to make, but no manufacturer I'm aware of provides one as a basic instruction.

 

[Cheeseface]

All counters are rising edge detect.

If you wanted it to be falling edge, you'd just invert the signal. This could be accomplished with a "configurable" input handling object -- something very common. Rockwell's P_DI block for example has an invert option.

 

[drbitboy]

... I am actually looking for a counter block that will allow the selection [configuration]of a rising/falling edge as a trigger.... can't find a block that will allow edge detection configuration.

Ah, clarity .

The closest I can think of in PLCs is the Siemens LOGO!Soft, which has such functionality in its FBD (Function Block Diagram) language; it is not available in it LAD (Ladder Logic) language.

On most discrete inputs in LOGO!Soft FBD, you can invert the signal at the block. See image below.

​

 

[dhancock]

Thank you Brian! This is exactly what I was searching for!