24 pin cable tester - many cables to be tested

Thread Starter

Bard

Joined Jul 16, 2020
15
Hi there - first time poster and please excuse me if I couldn't find something on my own.
I would like to build a cable tester for 24 pin cables (although the number of pins doesn't really matter for my question I suppose.) My first instinct was just to source a 24 position rotary switch and swtich power to light up leds, but they're expensive and even if though two 12 postion switches would work I'm starting to daydream about something a little more slick. I'm hoping to just plug both ends of the cable into receptacles and instantly have 24 LEDs either light up or not. Of course if I just plugged in power, crossed wires would still light up an led and I wouldn't know I had a problem, so I'm thinking of something more like AND gates that would cycle through and latch an LED on if the pair of wires was good, and not if there was a problem. I'm just hoping for some sort of insight as to what components I would need, or if I should just procure an Arduino or something. I'm looking for an elegant and simple solution. Hoping to just go the way of old school ICs but also hoping to learn a thing or two.
Thanks in advance!
 

SamR

Joined Mar 19, 2019
2,391
Typicaly, one master wire connected to all others as an end cap. Then inject voltage into the master on the other end and light up a single LED as you cycle through the wires. No lit LED = bad wire. Although now it is more common to use Time Domain Reflectometry as it will tell you WHERE the break is. But TDR is not cheap...
 

Thread Starter

Bard

Joined Jul 16, 2020
15
Typicaly, one master wire connected to all others as an end cap. Then inject voltage into the master on the other end and light up a single LED as you cycle through the wires. No lit LED = bad wire. Although now it is more common to use Time Domain Reflectometry as it will tell you WHERE the break is. But TDR is not cheap...
Thanks Sam - that's what I basically pictured if I were to use a 24 position switch, but I was thinking of something a little quicker. The cycling part is what I'd like to speed up using some simple ICs. Just not sure what ICs and if timing by something like a 555 is necessary. I'm out of the loop these days.
 

SamR

Joined Mar 19, 2019
2,391
It would take a bit of doing but a uController could be programmed to cycle through the connections and test for the open... That may be more time than you want to spend unless you have a LOT of cables to test. Might be easier/cheaper to rent a TDR tester. There would still be the problem of building a cable-test connection device/harness.
 

Thread Starter

Bard

Joined Jul 16, 2020
15
It would take a bit of doing but a uController could be programmed to cycle through the connections and test for the open... That may be more time than you want to spend unless you have a LOT of cables to test. Might be easier/cheaper to rent a TDR tester. There would still be the problem of building a cable-test connection device/harness.
Yeah a uController is I guess where I'm heading... although I'd never heard of a uController until you mentioned it and I google it and Arduino etc. came up. Building the device / harness is not a problem. I have 800+ cables to test. I was picturing a latching AND gate IC that would output to one of 24 outputs and a latch an LED on if made, then cycle to the next o/p until done. This may be super old school thinking though compared to what's available now. Thanks again for your time Sam.
 

Externet

Joined Nov 29, 2005
1,527
I would connect a 1KΩ resistor in series to wire 1; a 2KΩ to wire 2; ... a 24KΩ to wire 24 between the receptacles; and an Ohmeter also in series with common. Switching to position '14' as example on the 24 position rotary switch, should read 14(KΩ) Any other non-corresponding reading means a short/open/wrong pinned. Cannot see a simpler and perhaps elegant way.
 

Thread Starter

Bard

Joined Jul 16, 2020
15
I would connect a 1KΩ resistor in series to wire 1; a 2KΩ to wire 2; ... a 24KΩ to wire 24 between the receptacles; and an Ohmeter also in series with common. Switching to position '14' as example on the 24 position rotary switch, should read 14(KΩ) Any other non-corresponding reading means a short/open/wrong pinned. Cannot see a simpler and perhaps elegant way.
Thanks for responding! I like it, although I'm just trying to avoid buying a 24 position switch as they seem to be hard to come by. Also I would think just showing a make or break connection via LEDs would work just as well unless I'm missing something. Of course that would mean 24 LEDs, but your setup would require 24 resistors... but like I said I may be missing something. Cheers!
 

crutschow

Joined Mar 14, 2008
25,247
Typicaly, one master wire connected to all others as an end cap. Then inject voltage into the master on the other end and light up a single LED as you cycle through the wires. No lit LED = bad wire.
But that won't detect a miswire or a short between wires.

A simple way to test for all three would to use two 4-bit binary counter (e.g CD4029) driving two 1 of 16 decoders (e.g. CD4514) in sequence giving 32 possible channels.
Each of the decoder outputs is connected to its respective connector wire.
The other wire ends are connected to one LED for each channel (could use an LED linear array such as this)

The counter is clocked just fast enough that you can see each individual LED in sequence [perhaps 1/2 second per wire (experiment to determine the optimum speed), to give sufficient time to see any shorts (more than one LED simultaneously lit), opens (LED not lit), or cross wires (LEDs lit out of sequence)].
Thus the complete electrical test should take only about 12 seconds for 24 wires, which is likely no more time than it would take to remove one set of cables and connect the next set.

You could automate the observation of the test but that would likely require significantly more circuitry.
 
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SamR

Joined Mar 19, 2019
2,391
trying to avoid buying a 24 position switch as they seem to be hard to come by
And expensive if you can find one. The Arduino UNO has 13 digital I/O pins but the Mega has 53. Programs in a variant of C. Might take a bit to get up to speed on it if in a hurry to do this but would be a good fit. Overall it would probably be cheaper than a 24 position rotary switch if you could find one. Using 24 toggle switches would be doable if you could find someone who wouldn't go crazy flipping 24 switches ON/OFF X ~800 cables.
 

Externet

Joined Nov 29, 2005
1,527
At a certain point, when a wrongly manufactured cable is built and placed in operation; the smokes left to flow; the 'tedious' soldering of 24 resistors may vanish and the need to make things right with the proper 24 position switch will prevail.

As a second way which I cannot foresee as good enough; soldering resistors of varied values to the receptacles as to put all 24 wires in series with resistors and checking for a specific value of the sum -say 244,560 Ohms- from start to end. (No switch) Any different value will correspond to a mistake in the wiring, but will not reveal where.
 
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Thread Starter

Bard

Joined Jul 16, 2020
15
But that won't detect a miswire or a short between wires.

A simple way to test for all three would to use two 4-bit binary counter (e.g CD4029) driving two 1 of 16 decoders (e.g. CD4514) in sequence giving 32 possible channels.
Each of the decoder outputs is connected to its respective connector wire.
The other wire ends are connected to one LED for each channel (could use an LED linear array such as this)

The counter is clocked just fast enough that you can see each individual LED in sequence [perhaps 1/2 second per wire (expeiment to determine the optimum speed), to give sufficient time to see any shorts (more than one LED simultaneously lit), opens (LED not lit), or cross wires (LEDs lit out of sequence)].
Thus the complete test should take only about 12 seconds for 24 wires.
Now we're talking! I would prefer something more foolproof and even quicker, but this is definitely closer to what I was picturing. When I said I had to test these cables, what I should have said was I have to get a couple other unreliable people to test these cables. The test results need to be as simple as possible... and I'm worried they won't be able to watch the LEDs light up in sequence for 800 cables. Nevertheless, thank you SO MUCH for responding and including the links.
 

Thread Starter

Bard

Joined Jul 16, 2020
15
And expensive if you can find one. The Arduino UNO has 13 digital I/O pins but the Mega has 53. Programs in a variant of C. Might take a bit to get up to speed on it if in a hurry to do this but would be a good fit. Overall it would probably be cheaper than a 24 position rotary switch if you could find one. Using 24 toggle switches would be doable if you could find someone who wouldn't go crazy flipping 24 switches ON/OFF X ~800 cables.
My thoughts exactly, on all points.
 

SamR

Joined Mar 19, 2019
2,391
Also, consider the LM3914Dot/Bar Display Driver chip which can be ganged for up to 100 outputs. No programming required. All hardware.
 

Thread Starter

Bard

Joined Jul 16, 2020
15
At a certain point, when a wrongly manufactured cable is built and placed in operation; the smokes left to flow; the 'tedious' soldering of 24 resistors may vanish and the need to make things right with the proper 24 position switch will prevail.

As a second way which I cannot foresee as good enough; soldering resistors of varied values to the receptacles as to put all 24 wires in series with resistors and checking for a specific value of the sum -say 244,560 Ohms- from start to end. Any different value will correspond to a mistake in the wiring, but will not reveal where.
Agreed. And hopefully the worst case scenario is two wires that are switched. Anything more than that can be manually tested, but that situation would hopefully be rare.
 

Thread Starter

Bard

Joined Jul 16, 2020
15
Also, consider the LM3914Dot/Bar Display Driver chip which can be ganged for up to 100 outputs. No programming required. All hardware.
Googling now... although I'm not afraid of a little programming, I'd like to lean on the hardware side in this case. Thanks again Sam!
 

Thread Starter

Bard

Joined Jul 16, 2020
15
What I was really picturing was two corresponding wires hooked up to an AND gate. The ouput of the gate would only be high if both wires were high. One end of the cable would be the cap, so all of those would be high, and the only way the output of the gate would be high is if its corresponding wire came back high. The output would be latched to turn an LED on and keep it on. The inputs of one side of the cable would be cycled through, which would take timing. But the key would be that only one wire is energized at a time.
Caveats: I'm in charge of a bunch of temps that will be producing these cables. Also, I went to school for this 22 years ago, so things like 555 timers and AND gates are still in my vocabulary.
 

crutschow

Joined Mar 14, 2008
25,247
I have a thought on automating the observation for my circuit which would require (in addition to the LED's) two 16-channel multiplexers (e.g. CD4067), a small diode (e.g. 1N4148) for each channel, an analog comparator, and a latch.
If a fault were found it would stop the sequence on the offending channel.

Interested in doing that?
 

Thread Starter

Bard

Joined Jul 16, 2020
15
I have a thought on automating the observation for my circuit which would require (in addition to the LED's) two 16-channel multiplexers (e.g. CD4067), a small diode (e.g. 1N4148) for each channel, an analog comparator, and a latch.
If a fault were found it would stop the sequence on the offending channel.

Interested in doing that?
Yes I'm extremely interested in doing that. May be overkill for what I need, but I like overkill.
 
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