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I need to configure a PLC with 8 digital 24VAC inputs and there doesn't seem to be much available.  Additionally, and more common, I need at least four digital 24VDC inputs. For outputs, I need 12 relay outputs (which will switch 24VDC).

We've used several Vision700's with great success in the past, but have never needed AC inputs before, but now we do. Any suggestions?

Thanks!

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I'm not sure how you can have a 24VAC digital input - makes no sense to me. However, can you just rectify the inputs to obtain 24VDC? Or use relays?

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Flex is technically correct re 24AC as digital, but if you are needing to know whether a 24AC switched item is in operation, then you have a variety of options.  One of the simplest is if it is already switched by relays, use the second pole set on the relay if it has them, or change the relays to give another pole.  You then use the other pole to switch the DC signal.  This does have minor drawbacks in that it is only the "called" action being monitored.  If you really want to be precise, you should be monitoring the 24AC at the driven endpoint.  There are many ways to do this ranging in complexity and cost.  If the action is not hugely repetitive/cyclic/quick, then slimline relays/couplers will do the job.  The AC turns them on, and they switch the DC signal.

cheers, Aus

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Sorry, I said "digital" to make it clear I wasn't sensing a variable analog signal. They are on/off 24VAC signals from some European machinery. There are not accessible relays, rather when the machine is powered up, it sends a 24VAC signal. I need to detect when this happens so the PLC can take some coordinating action. There are PLC's with input modules that will detect 24VAC and that seems the simplest method. But I'd prefer to use a Vision PLC if possible, however I didn't see a module that would do it.  But it's a good  suggestion to just have the AC switch  a relay that has a DC voltage going through it.

Or I suppose I could use a cheap Click PLC and 24VAC module (they have one),  and have it output a  DC signal  to the Vision.  I appreciate the comments;  this is only my 3rd project , so I'm far from experienced...

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You could hook something to the Vision using Modbus.  Plenty of (cheap) modules around that would let you do this.

Cheers, Aus

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On 10/12/2019 at 9:08 AM, Ausman said:

If the action is not hugely repetitive/cyclic/quick, then slimline relays/couplers will do the job. 

I did!

But Joe, you would not believe the huge price difference in Aus on those things.  Greed coupled with hopeless service can be a nasty/frustrating thing to deal with.  I often whinge that I can get the same components from a quicker responding, more diligent overseas supplier at way less price than I can get locally, including the high shipping cost from o/s, and they often arrive quicker as well. 

"Use our online form to do an order"  Ring to check on no response a day later.  "Ohhh, that doesn't work properly, send me your stuff to my direct email" is what I hear a lot locally.  It can take a week just to arrive at a final invoice.  All under the guise of "our prices are higher because we have so many overheads".  Bollocks.  More like "money for nothing", and useless at that.

I know I've gone a bit off topic.  End of rant.

cheers, Aus

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I try to compare  V200-18-E46B 24VDC I/O module and a Siemens 24VAC input I/O module.
It is interesting that the input
electrical schematic diagram of both modules are actually the same since the PS2705 optical converter is used.
In V200-18-E46B PS2705 it is used for switching PNP or NPN logic.
It seems to me that the difference is only in the response time - 10ms for Unitronics 24VDC I/O  and 25ms for 24VAC siemens I/O.

So I think that to work with a 24VAC one diode (or 4diode bridge) and a small (0.1-0.47mF) capacitor are enough.
However, keep in mind that AC experiments should be done using a separate group of inputs.

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kratmel, I had thought of something like this, but didn't mention it because I was concerned about inherent pickup/dropout delays and inrush loads in using a cap possibly causing issues.  It would be good to know through someone trying if it actually works well, and the best tweaked values.  From your comparisons it looks like it would.  It can't be a coincidence that the response time on the Siemens is 25ms running on 50Hz.

cheers, Aus

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