SLC analog voltage input card

Join Date
Mar 2004
Posts
60
Hello all,
This is my first time using this forum and I am looking forward to it!!
I have a question regarding SLC analog input cards.
I have a SLC analog input card, takes -5 to 5 volts input,
but when you unplug the cabling into the card, making all channels open circuited, take a voltmeter to it,
it reads 0.5 volts, the problem is this,
how would you separate a bad cable, or unplugged cabling with a regular 0.5 volt input reading then?
The impedance of the transmitter is 5Kohm.
The card itself, have 8inputs, I am using two of them, would the problem go away if I were to short out the other channels?
Thanks,
George
 
I have 2 suggestions, first verify that the plc registers "SEE" this .5v ...ie there is a numeric value that would represent .5v.

The second part is try another meter, digital meters especially Fluke, are very sensitive and may offer values that arent "real". Try a simple analog meter if possible.

I dont think I would short anything. If there is a voltage present with nothing connected you will need to determine why.
 
Are you implying that you never short out analog inputs when they are not used?
You simply leave them open? Whether it's current or voltage input?
I see some of our drawings advises us to short out the inputs when they are not used.
George
 
When no meter is attached to that input, the PLC reads a value, once you stick the digital voltmeter probe to it, that PLC reading lowers. Makes sense since there is a small amount of loading effect.
I agree that digital voltmeters can give false readings, and in some cases it's too sensitive, but on the other hand, since analog voltmeter has much lower impedance than the input card, it might give a 0 volt reading as well. Which also can mislead you.
Any thoughts on that?
I read in the AB manual that there is an open circuit status only on analog CURRENT inputs though, they don't apply for voltage inputs.
So this brings us back to the original question/problem.
I'll try to tie all other signals to common as suggested. Meanwhile, can anyone provide more info/suggestion as to
how'd you tell the difference between a bad wire and a 0.5V reading?

Thanks,
George
 
By the way,
The AB manual online does not instruct us to tie input signals to common whereas the paper manual that came with the card has this.
George
 
georgeofthejungle02,

I cannot find an Allen-Bradley analog input module that has an input voltage range of -5.0 to +5.0 volts DC. My Allen Bradley SLC 500 Analog I/O Modules User Manual shows ranges of -10 to +10, 0 to +10, 0 to +5, and +1 to +5. Which catalog Number are you using? I would like to have one of those too.

If it measures 0.5 volts HIGHER over the entire input voltage range, then follow instructions in above mentioned manual, Appendix C, "Optional Analog Input Software Calibration". In other words, introduce a software offset to make it read right in your program.

If it measures 0.5 volts when another of the same type measures 0 volts, throw it away and get a good one.
 
To clarify, the instrument itself send out -5 to 5 volts, and the card itself has configuration of -10 to 10.
These are inclinometers we are using, and apparently they ususally come in voltage output and few are 4-20mA. Hence we've been using voltage input.
 
Hello Lancie1
You sure we are talking about the same analog card?
I can't see anywhere in the SLC 500 NI8 manual where they have
Appendix C, "Optional Analog Input Software Calibration".
You might be refering to PLC 5 analog cards?
George
 
Broken wire protection is exactly why 4-20ma current loop inputs are used instead of voltage. The range of 4-20ma indicates a connected
input, and less than 4 ma indicates broken wire. Never used a
voltage input for a critical function.

Now, I'll get off my soapbox, and offer a useable tip: Try shifting
the analog input bits, so that, you have a little less sensitivity. At 5V your only using 1/2 the A/D anyway. Remember, your meter means
very little, calibrate from the input digital value.
 
I totally agree with your suggestion of using 4-20mA oldgearhead, and these systems were implemented way back because the transmitter itself only offered voltage outputs.
Now I am not exactly sure what you mean by shifting input bits. I'm thinking to see if it's possible to offset the entire reading range, so 0 degree is say 2 volts, by doing this, a broken wire will give 0 volts which is out of operating range. I am not sure if the transmitter can do this though ...but is that what you are referring to?
Thanks,
George
 
georgeofthejungle02,

I was looking in my old copy of the "Allen Bradley SLC 500 Analog I/O Modules User Manual", although I admit this one is 20 years old at least. But then, you are working on old equipment, so maybe we have a match! This manual applies to all the Analog modules, not just the NI8. It has several appendixes with tips and troubleshooting information. Maybe you should check the AB download site and see if a similar one is still available (free).
 
A/B inputs

George,
Yes, you are correct, with A/B you can do a lot of sensitivy and
offset malipulations on the input rung. You may be able to make a
broken wire show up as a detectable negative number if you can
make you true input all positive.
 

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