URGENT - High Speed Counters not working all of a sudden ML1400/Logix 500

Hello,

For a year, I had high speed counters working flawlessly.

Today, they were still working. Now all of a sudden, nothing.

All other I/O analog and discrete is working. Processor is not in fault, I tried power cycle, downloading code... nothing.

I am thorougly confused. Supposed to leave site today, and WHAM.

Any thoughts while I try to track down the problem. I have flow through the meters, did not modify any wiring... totally confused...

Thanks!

I don't know how this could change spontaneously but check the input filtering.

Input Filtering is 5uS

All inputs for flow are energized, regardless of meter moving or not. The 1400 has an LCD and all inputs are exactly 1 for flowmeters...

I also have 12vdc common across all resistors, coinciding with the input being high. The flowmeters are 24vdc pulse with 2.2k ohm resistors

I also have 12vdc common across all resistors, coinciding with the input being high. The flowmeters are 24vdc pulse with 2.2k ohm resistors

Click to expand...

I'm not quite understanding what you are describing. Can you create a wiring drawing and show yoour measuremnt points and the position of the resistors?

I cannot give a diagram at the moment. It worked this afternoon, no program change.

It briefly worked again this evening, then kaput.

I checked all breakers and fuses, checked comms, everything.

I can't believe this happened on the last hour.

at a loss

It does appear to be a voltage issue?

I checked one input, the resistor is fine 2.2k ohm

If I remove the wire from the input and touch the input terminal several times, my flow readings react, and totalizing works as it should.

I am wondering if somehow the entire HSC input bank went south... on two PLC's?

The code is fine. I am reading 12v across the pull up resistors, every input is high, no transitions no matter what the flowmeters are doing.

Damn

this is a LONG shot - but it's a straw to grasp at ...

connect a HIGH VALUE (1 meg or so) resistor between GROUND and the "low side" of the power supply that's powering the "problem" input circuit ... if it works, we'll talk about it ... if it doesn't, at least it shouldn't cost you but a few minutes to try it out ...

Ron Beaufort said:

this is a LONG shot - but it's a straw to grasp at ...

connect a HIGH VALUE (1 meg or so) resistor between GROUND and the "low side" of the power supply that's powering the "problem" input circuit ... if it works, we'll talk about it ... if it doesn't, at least it shouldn't cost you but a few minutes to try it out ...

Click to expand...

Hi Ron,

I had to leave the site. but will be there first thing in the AM.

Are you saying that I should leave everything intact (the 2.2k resistor, etc. and place a 1meg ohm between ground and 24vdc- on the supply?)

Thanks.

yes, everything else stays the same - and you just add a "bleeder resistor" between ground and the negative side of the power supply ...

here's what I'm basing my GUESS on ... I've seen input systems which would work fine for quite a while - but then start intermittently going nuts ... reaching out and touching the "low side" of the 24 VDC power supply with your finger would "settle everything down" and make the circuit work fine again ... disclaimer: I am NOT recommending that procedure from where I sit ... use a resistor instead ... basically the idea is to provide a "bleed off" path for any stray voltage which might build up in the isolated instrumentation loop ...

this goes back to many years ago when I used to teach at a tech school ... the students were told to "ground" the low side of their instrumentation power supplies for their analog input experiments through a high-value resistor to keep the supply voltage from "floating" ... sometimes a student would leave that particular stone unturned - and their input signals would intermittently go nuts ... somehow the description that you're giving of your problem makes me think of those symptoms that I saw years ago ... like I said, it's just a GUESS - but it shouldn't take much to try it out ...

Thank you Ron,

I'm hopeful that this might help, as a clue at least.

All my other I/O work absolutely fine. I'm totally stunned/stumped.

There is no official earth ground that I see, although we do have line (120v) grounds going back to a breaker panel to earth.

I'm going to sleep on your answer tonight. This system worked for 3 months, then was put to rest over the winter. We starterd it up on Tues and the flows were going fine until this (Wed) afternoon.

Thanks again. I will post loop drawings tomorrow if things do not work out.

Regards,

Tim

Without a ground, where do you land your cable shields?

cable shields landed to common ground bus, same bus grounded to breaker panel. Perhaps not the most ideal ground...

Problem is not with analog I/O and not with slow discrete I/O, just high speed counters. Another thing is this stuff worked flawlessly for months.

I do hope it is a ground, and I learn a lesson! I think something may have gone seriously wrong, and possibly, a result of a poor ground?

Wiring diagram attached:

Now we find out it's a NPN with pull-up situation. Note that the specification says above 4.5 volts is a high. Below 1.5 volts is a low. If this setup can't produce both of those situations then recalculate the size of resistor needed to meet the input needs of the controller and the output capabilities of the encoder. If that doesn't work trash the NPN sensor and get PNP sensors or at least get an intervening convertor.

Hi Bernie,

This exact setup works flawlessly on another site. (it worked perfectly here until last evening as well) You actually were a big help with totalizer code and the STI interrupt.

I am going to try to do the 1meg bleed resistor this AM. Bottom line, I have to get things running with the existing equipment.

... and I don't yet have any explanation as to why everything flow meter just stopped cold!