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Old July 4th, 2010, 02:34 PM   #1
69FIREBIRD
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Analog Input as Multiple Digital Input

Hi all, I need to add 2 or 3 digital inputs to an existing PLC but have no spares and the cabinet is so tight that I cant add any IO. I have 2 spare analogs and was thinking of switching some sort of resistor combination into it and use the PLC code to view the analog value and switch internal bits to simulate the inputs.

I just need to add a selector switch for 3 different product sizes. Normally I'd have a 3 pos. switch with 2 inputs. LHS input 1/size 1, RHS input 2/size 3, centre pos no input size 2.

Does anyone know how to get a resistor combination to work on this?

PS analog input is configured for 0-10V

I can work this out with a little looking around but Iím just trying to get a quick fix with minimal input J

Thanks.
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Old July 4th, 2010, 02:50 PM   #2
Bob O
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What PLC and I/O? I know AB has high density cards that you could switch out.
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Old July 4th, 2010, 03:17 PM   #3
69FIREBIRD
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Hi Bob, fixed IO - MLX1100 w no space to add on.
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Old July 4th, 2010, 03:59 PM   #4
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From 24DC supply. 4 resistors in series.
R4=10K. R1, R2, R3 = 560. +24 to 10K. Last 560 Ohm to common.
This should give a signal about 1.15V per step. 2 mA current.
10K should be 1 watt. 560's can be half watt.
Series string is:
Common, R1, Tap 1, R2, Tap 2, R3, Tap 3, R4, +24V.

Last edited by keithkyll; July 4th, 2010 at 04:02 PM.
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Old July 4th, 2010, 05:31 PM   #5
69FIREBIRD
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Thanks Keith. So as I see it, it all makes sense - simple V=IR

See attached PDF of how I see it working as per your written description.
Attached Files
File Type: pdf 100705 3 POS SEL SW DETAIL.pdf (22.6 KB, 81 views)
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Old July 4th, 2010, 05:55 PM   #6
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What is the input resistance of the analogue input ?
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Old July 4th, 2010, 06:11 PM   #7
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MLX 1100 Voltage input impedance 210 Kohm. will there be enough current flow through the resistors for it to be detected by the analog input 2mA is what is anticipated so I would expect yes to be the answer?
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Old July 4th, 2010, 08:01 PM   #8
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Schematic is correct. 10K is 5% of 200K. That means there will be a 5% drop in voltage output. 1.15 * .95 = 1.0925. If this becomes a problem, change 10K to 5K. That will double voltages (2.xx - 6.xx) and current (4 mA), and reduce error to 2.5%.
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Old July 5th, 2010, 03:03 PM   #9
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Worked a treat! thanks to all.
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Old July 5th, 2010, 03:46 PM   #10
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I understand this setup perfectly. What I'm upset about is the fact that I never would have thought of it on my own! Just when I'm feeling pretty good about my skills (which are NO WHERE NEAR as good as many others on this forum) I get taught a new trick.

Makes me wonder how much I'd know if the internet didn't exist.
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Old July 5th, 2010, 05:27 PM   #11
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Conceptually it's a potentiometer with fixed stops.
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Old July 5th, 2010, 07:30 PM   #12
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I've done something similar, but in reverse. I needed one more digital output, but only had an analog output available; AutomationDirect DL-05 with an combo analog I/O card. I used the spare analog out to drive the base of an NPN resistor (2N2222, or equivalent) through a 10k resistor; the load (an LED indicator light) was low-side switched.

It worked perfectly. It gave me another digital output without having to replace the PLC with a larger unit or resort to something else of equal overkill, like remote I/O.

I realize this post isn't exactly on topic, but I hope it gives someone else a little help in a pinch.

-rpoet
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Old July 5th, 2010, 08:56 PM   #13
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Necessity is the mother of Invention

At the 11th hour when functionality is added to the original design, we all scramble to make it work with what we have in hand.

One came to mind regarding this issue. Years ago we implemented one of the first generation Micrologix 1000 digital I/O only. The customer added a 2nd product model to the existing machine that needed different machining positions. We added a linear transducer O-10v to measure the axis, but were stuck as no analog expasion was available yet for the ML1000. We found a third party DIN rail module that was a analog-to-freq converter, and fed the frequency into the ML100 Digital Input 0 which was capable of high speed counting.

The axis moved slowly, so we merely interrogated how many counts happened in a given period, and found our anaolg actual position.
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Old July 6th, 2010, 12:13 AM   #14
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only if we can use the digital inputs as analog inputs by some setup.
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Old July 6th, 2010, 04:06 PM   #15
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Quote:
Originally Posted by bernie_carlton View Post
Conceptually it's a potentiometer with fixed stops.
My version used a 10-turn pot with 3-digit dial to retrofit a 16-position product selector on a few dual-pump mixing machines. They had Bailey Loop Command controllers which had all three digital inputs used but one spare analog input.
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