Cable harness test

Hi,
I'm trying to build a cable harness tester by using arduino Mega. The cable harness is consist of 26 cores. All cores have its both ends free, except 5 cores (consist of two groups) which are connected to two different devices on the other end.
5 cores is devided as below:
-3 cores are connected to DMX (group1)
-2 cores are connected to DC power supply (group2)
as is shown below

The test method is to pullup all digital inputs and set one digital output to low at a time.
The tester works properly until it comes to above mentioned 5 cores. It gives a message that the cores in group 1 or 2 are connected to each other (short wires). I think the potential difference between arduino digital I/O (0-5V) is trigging the components inside these two devices which make the arduino think that the cores are connected to each other.
But when I conduct the test with multimeter by using the continuity test function, it works properly with no issue.
As I know, the multimeter use about 2-2.5V when it swiths to continuity test with small amount of current. Am I right?

And my questions are:
How to make the arduino goes down to 2.5V? Or how to send an anlog signal by arduino?
Do I need a DAC when I send a PWM signal?
Or are there any way to make the arduino works in the same way as continuity test on multimeter?

Any suggestions would be greatly appreciated !

That's a common way to do it... "Walking-one" or "Walking-zero"

Normally a harness is tested with nothing but the harness connected (wires & connectors).

Yes, usually resistance (or conductivity) is measured with a small constant current and then the voltage is measured. (There is a limit to the voltage and there are "ranges" so you don't over-voltage anything or give yourself a shock when it's trying to "push current" through an a high resistance or open circuit.)

You can make a voltage divider if you know the resistance. Note that the Arduino measures positive voltage relative to its ground and that sometimes limits what you can do. The black lead on a multimeter might be connected to the internal ground, but it's isolated from everything else (until connected).

No. And PWM is NOT true analog.... It's switching on & off rapidly... It can be used to control the speed of a motor or make an LED appear dim, or it can actually dim an incandescent light bulb because the bulb can't respond quickly to the switching. (A regular light dimmer in your house works similarly.)

Thanks a lot for your response

I a gree with you. But in my case I need to test the cable harness when all its cores are connected to either connectors and/or devices (DMX and power supply). In other words I want to test the cable harness at its final stage, to be sure that all cores are connected correctly.

How can we send an analog signal from arduino? is it possible only by using a DAC?

I cant see a reference to those groups on the schematic.

Why would you want to do that? The test signal you are using (0V or 2.5V) isa logic signal - its not "analog" just because it uses a different voltage.

While you COULD use a potential divider on all the channels, perhaps it would be easier (and safer) to use CMOS buffers (in sockets) driven from 2.5V as a level shifter?

I think that your cable harness tester needs to have two separate modes of operation.

One mode for testing the bare cable, and a second mode for testing with the devices attached.

In the second mode you want to be testing the devices connected by sending DMX commands and whatever the power supply needs for testing.

What do you mean by ?

if we take the DMX, it is D4-P
As you can see below the upper 3 wires come from cable harness and the lower 3 ones it go to arduino.

I'm trying to find a way ro make a right reading. When arduino sends a 0 on one of wire it should not read a short wire or continuity on the the other two wires.

Maybe the right thing to do is to read the return signal as an analog to be able to control the reading level of the input voltage/signal.
Or do I'm thinking wrong?

yes, it can be a good ide.
But it will not solve the problem

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