Analog Input as Multiple Digital Input

69FIREBIRD

Member
Join Date
Oct 2004
Location
New Zealand
Posts
102
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 I’m just trying to get a quick fix with minimal input J

Thanks.
 
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:
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.
 
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?
 
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%.
 
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.
 
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
 
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.
 

Similar Topics

In this sample programming, what does U4 mean? Any assistance would be greatly appreciated.
Replies
8
Views
169
Can someone please explain to me in simple terms, the best practice with multiple 4/20ma pressure/flow transducers on multiple different 24VDC...
Replies
10
Views
339
Hello, I am trying to connect a 0-5v pressure sensor to the Micro 810, but I am not getting a clear input voltage reading. I connected the sensor...
Replies
14
Views
371
Hi all, I’m an electrician with limited knowledge of PLC circuits. Typically I will see simple 2/3/4 wire terminations for transmitters on...
Replies
26
Views
1,158
We have a 4 wire 0-10V analog OPT2012 (Time of flight distance laser) from Wenglor that we're experimenting with. We are able to teach the laser...
Replies
3
Views
494
Back
Top Bottom