Mysterious Problem?

First off, wiring 2 wire proxies straight to a PLC input card isn't good, the inputs barely takes enough mA to allow the proxie to work.

You didn't prove the input card was good, you proved it was good with something that can provide enough mA.

So I would try using your meter in mA mode and connect the input direct to your inputs supply. So if you have a PNP system you are going from + through your ammeter, then to the input. Check the current drawn on a good input and compare it with the bad input.

You also need to look at the threshold voltages for the input. Use a pot as a voltage divider and find out when the input says it is On.
 
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Hi Steve,

Maybe I missed it, but have you tried the input with a direct 24V signal, and does it works correctly then.

If it is correct, you can have a problem with drawing too less current from the 2-wire proximity switch.
try a pull down resistor on the input to guarantee the minimum current for the proximity switch.

Greetings
John.

*edit* I see BryanG suggests exactly the same...
 
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tighten all terminals and see if the problem goes away.
then stress test the wiring by pulling on it. you may have a connection that is corroded and about to break.

james

Two wire devices need to have some current running through them to power the electronics that make them work. The input cards are normally high impedance, but still allow some current to flow. if the connections aren't tight you will get more resistance, and the device won't work. If you put a resistor from your input card to the neutral, that might allow enough current to flow to get the device to work. You might also check to see what the current flow is when connected to the bad input and the good input and see if there is a difference. Also, measure the voltage on the input when using the bad input and the good input. It may lead you to what part of your system is wonky.
 
Thank you all for your responses so far.

To answer one response directly, the lack of signal was seen both on the input card (no light) and in the PLC program. This led me to think external hardware.

After reading through all the responses, I can now see I wasn't quite as thorough in my testing and troubleshooting as I thought I was.

1) I re-used the existing removable terminal block from the old input card. Could be a terminal block or connection problem there.
2) There is one interconnect between two enclosures where I did not verify continuous wiring. I'm betting this is where I will find my problem.
3) I did not validate all terminations to be tight and clean. We use spring style connectors, so that kinda lulled me some. In any case, the machine is only a few years old and in a fairly equipment-friendly environment. I don't imagine corrosion will be a factor, but who knows?
4) I did not connect 24VDC directly to the input to verify enough current. This is fast and easy and can also quickly isolate where a broken circuit might be should the connection work when all is bypassed. This technique is where I will start my troubleshooting.

Finally, since the final "solution" (switching to an unused input) was not done directly in the PLC enclosure, any of the above could still be true. I will let you all know what I find.
 
Not splitting hairs or anything but you said this in your first post.

I identified that the PLC input, itself, had to be the culprit. I was able to do this by lifting the wire from the "bad" input and touching it to a known "good" input. Doing this, prox works again and a signal is received by the PLC. Putting it back on it's original input, however - nothing.

By that statement it would seem any field wiring or components would already be ruled out as the source of your problem, would it not?

BCS
 
By that statement it would seem any field wiring or components would already be ruled out as the source of your problem, would it not?

BCS

Not necessarily. For example, the specifications for the card is 10V minimum for ON, 5V maximum for OFF. If the signal is 9V, one input could show ON, another could show OFF, but both meet the specs, and both are good inputs.

In this case the description indicates the field device does not work. It could be due to the card, or it could be the wiring, the field device, or the power supply. Since the device in the field is powered by the current/voltage that goes into the card. That means for the device to work, it has to work with less than 24V/3000ohm = 0.008 amps (card has 3K ohm nominal impedance) at 5V or less, => less than .04 watts. It is possible that one input was 2750 ohm, another was 3250. Without knowing an official definition for 'nominal', I'd say 3250 was nominal, but it might be high enough that the field device doesn't have enough power to operate. More test results are needed to determine which components are working to spec and which are not.
 
By that statement it would seem any field wiring or components would already be ruled out as the source of your problem, would it not?
BCS
It should have, yes, but sadly, no. While my statement suggests I lifted the wire and touched "directly" at the input card itself, that unfortunatley was not the case. In fact, the lifting and touching was done in a nearby - but none the less - remote terminal enclosure. As I said in my most recent post, I did not do as thorough of a job troubleshooting, nor it now seems, did I do such a good job describing what I actually did.

I am ashamed. :oops:

I was not able to get over to that machine again today, so the problem remains. Hopefully I will again on Monday if the fires remain low.
 
Customer called and said the plc doesn't work anymore. I get onsite and pull and tug wires. 3 wire prox is sending a signal to the PLC . NO light on that input. So bad input I ASSuME. After changing the PLC (Micrologix 1400) new plc was the same bad input. It was the removable terminal strip. Input 9 had rusted bad the rest looked perfect.
I don't know why terminal 9 was this problem I just know it did.
 
Why now?

Hi, all.
You thought this thread was dead?

I have nearly the identical problem. Card is a 1762 IQ16... on a ML1200.
According to the wiring for the card..... there is a COMMON for the first 8 inputs. T H E N... there is a SECOND common for the NEXT 8 inputs.

My card does not have the SECOND common landed. According to original schematics.... it NEVER was. But somehow, Inputs 12, 14, and 15 worked in the past.... ANYWAY.:confused::confused::confused:

BIG QUESTION.... why does input 12 need that 2nd common.... NOW???
 
Maybe the commons are internally connected, perhaps AB knew one day, the card will have an issue with the commons from 0-7 and 8-15 not working together, so they added a 2nd common at the bottom to use?
 
My card does not have the SECOND common landed. According to original schematics.... it NEVER was. But somehow, Inputs 12, 14, and 15 worked in the past.... ANYWAY.:confused::confused::confused:

BIG QUESTION.... why does input 12 need that 2nd common.... NOW???

Were inputs 12, 14, and 15 in the original schematics? My guess is the internal resistance of the card and power requirements of the input devices was just low enough for it to work initially, and now, the internal resistance of the card across commons drifted too high for it to work.
 
No LED on the PROX
2 wire prox need a load to work the input of a plc will not load it enough to power it.
install a resister or a lamp in plc input / prox output
I have seen this many times that's why I don't like 2 wire devices there load sanative
 
No LED on the PROX
2 wire prox need a load to work the input of a plc will not load it enough to power it.
install a resister or a lamp in plc input / prox output
I have seen this many times that's why I don't like 2 wire devices there load sanative


That's why I NEVER/EVER use 2 wire sensors! HATE THEM!🔨
 

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