Something Odd for a Monday Morning

PLC Pie Guy

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SO I retrofitted a 4 headed filler machine a while back. It had a SLC 4 and an old off brand HMI screen that didn't work. I upgraded to a SLC5/05, used roughly the existing code with some modifications to work with my existing filler set up and away we went. I also added a new PV800 as its a cheap solution to talk to the SLC5/05 and connected it through a small 4 port unmanaged Ethernet switch.
One of the screens I made is simply an I/O screen that gives visibility to a green box if the I/O listed next to it is on. These visibility tags look directly at the Input with no buffering or conditioning whatsoever. Input on, green box is visible. This all works well for some time now.
We wanted to push the machine a little so in order to get an idea of how it was consuming air I added a small PSI transducer to the header. It goes to a 4-20MA input on CH0 of the 1 slot on my rack. None of the other channels are used. I display the PSI value on the HMI with a tag called Machine PSI that is declared to look at F8:1 as a REAL tag. F8:1 is the OUTPUT for an SCP instruction scaling the transducer input. The device is accurate and the HMI display of the value works just fine.
Here is where it gets odd.......

When this transducer is connected, the visibility controlled green boxes on my HMI screen that are associated with the discrete BOOLEAN inputs I.E. I:2.0/8 , I:2.0/9 and so on,,, are randomly changing. Even with all other I/O wiring disconnected. Also, its not lighting up the input card or turning on the inputs in the PLC, just displaying my visibility associated boxes at random. 3 of them might come on together and 1 might go off, then they might all go off and 5 of them come back on. I can also make them change by simply moving valves on the machine to make the PSI reading fluctuate. Its almost like my visibility tags are associated with the transducer value some how... but they are not!

Any thoughts on this??? Has anybody else experienced weirdness from the PV800 in such a manner???

Thanks
 
I don't have an exact solution to your problem, but it has been my experience that PanelView HMI's do not have access to the direct hardware address ie: I:00/01 etc.

In all my years of programing those with an RS500 compatible processor I have always needed to make a Ladder which simply examines an actual digital input and writes a coil of a B3 type tag.
 
Makes sense, and easy enough to try. However, it works just fine looking at the address when the analog channel is not active.
 
There is a strong chance that something that is messing with your 0V voltage level and you are getting false readings from the inputs.

Yesterday I wired three PLCs together on my test bench to work on some software. One of the PLCs gives out a steady PWM signal simulating an encoder, however the receiving PLCs lights weren't steady, they were galloping. I was scratching my head until I realised I hadn't bonded the 0Vs together and the 0V on the receiving PLC was being pulled up or down, I should have checked which. After bonding together the system worked as expected.
 
There is a strong chance that something that is messing with your 0V voltage level and you are getting false readings from the inputs.

Yesterday I wired three PLCs together on my test bench to work on some software. One of the PLCs gives out a steady PWM signal simulating an encoder, however the receiving PLCs lights weren't steady, they were galloping. I was scratching my head until I realised I hadn't bonded the 0Vs together and the 0V on the receiving PLC was being pulled up or down, I should have checked which. After bonding together the system worked as expected.

I would be with you on this, except that.... I have disconnected the input and output terminal connection arms completely from the cards. All except for the analog input one. Nothing in the program is indicating that any of these inputs are coming on. especially since the communication speed to the HMI always makes it "Normally" update much later than the PLC.
 
DISCLAIMER: this is probably a classic chase of the wild goose ... but ... I offer the thought as a straw at which to grasp ...

especially since the communication speed to the HMI always makes it "Normally" update much later than the PLC.

if ALL else fails - then I suggest that you read up on the SVC (Service Communications) instruction ...

I bring this subject up since you mentioned that your transducer is connected to the Channel 0 port on your processor ...

with that in mind, you might consider "slowing down" the baud rate through that channel ... (I know that sounds counter-intuitive - but if nothing else pans out, sometimes you've just got to try "something" - even if it's wrong ...

as I said - this is all just "grasping at straw" type stuff ... good luck with your problem - and please let us know how it turns out ...
 
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PV800, as in old Panelview that's programmed with Panelbuilder 32? Is the comm via ethernet? I seem to remember that it can read I: and O: directly.

In any case, as part of the troubleshooting, have you tried to read the discrete I/O as entire integer number and see what the number is on the PV?
 
If the input card LEDs aren't coming on, then I would assume the issue is inside the CPU. With some PLCs, this would be an addressing issue. Memory addresses overlapping, so the analog bits are writing to your digital locations.
Try different addresses for the analog. Try setting it to integer instead of float just to test.
If it's hardware, maybe a bug in the CPU memory addressing, or analog card addressing.
 
If the input card LEDs aren't coming on, then I would assume the issue is inside the CPU. With some PLCs, this would be an addressing issue. Memory addresses overlapping, so the analog bits are writing to your digital locations.
Try different addresses for the analog. Try setting it to integer instead of float just to test.
If it's hardware, maybe a bug in the CPU memory addressing, or analog card addressing.

Lets say the analog bits were writing to my digital locations, wouldn't that show up on the DATA FILE window for the Digital Input words when online with the PLC ?????

I do plan on changing this data type to INTEGER instead of FLOAT when I get back to the machine in question, just to try it. I'm actually thinking I might try a new program that simply has the /O configuration, minus the logic, just to see if that helps anything.



Thanks
 
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maybe inhibit/disable the analog card - and manually enter a non-changing value into its register ...

maybe do the same for the digital cards ...
 
If the input card LEDs are not showing correctly, I would think the problem must be in wiring, power source, etc. Have you put a meter on an input and watched for varying voltages? Do the digital inputs and 4-20ma loop share a power supply?
 
OK so if it isn't an electrical problem, then it is a programming issue. I don't know the PLCs you are using but can you mix the data coming from the analogue input with the Digital input data. Analogue data is stored in a series of bits, if you alter analogue values then the bits go off and on.
 
Lets say the analog bits were writing to my digital locations, wouldn't that show up on the DATA FILE window for the Digital Input words when online with the PLC ?????
Yes.
With some other brands, you can assign single word addresses to double word variables, and they will write all over each other.
I've done a little editing work with RS Logix 500. With memory locations pre-assigned, it looks impossible to make this kind of mistake with AB.
It does appear that's what's happening, but how?
Tim Ryder (post #2) may be on to something. Using his method would allow you to monitor bits in RS Logix.
Confident you will get to the problem fast with Ron Beaufort's help.
 

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