# Designing a cable tester

I need to check the integrity of 40+ custom cables. Each cable is meant to be the same, and is meant to have 19 ways.

I bought an enclosure, some mating connectors, some LEDS and a bunch of resistors and diodes. My first and second attempts at building tester circuits were unable to catch the three conditions I want to check for:

• short circuits
• open connections
• cross wiring

So now I'm back at the computer. I have searched the internet, and I have found a CAT5 tester circuit here.

I recreated part of this circuit using the awesome tool at Falstad,

Here is the result:

Only when each adjacent LED pair lights when the switch is flipped, can the cable be considered good.

I’m somewhat confident I can extend the circuit so that it can test each way in my cable, but can anyone offer improvements or advice before I start my third implementation? I have a nagging suspicion I am overlooking something, or there might be a really simple way to do this!

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If you want the falstad circuit to play with, I have exported it here: pastebin.com/mU445JVN –  jon May 22 '12 at 11:06
What does "is meant to have 19 ways" mean in english? –  Olin Lathrop May 22 '12 at 11:33
@OlinLathrop I would speculate that he means there are 19 conductors in the cable. –  vicatcu May 22 '12 at 16:44
@OlinLathrop $E$nglish. merriam-webster.com/dictionary/english –  Telaclavo May 22 '12 at 16:45
@vicatcu: 20, more likely; have to have a ground, and it doesn't count as one of the "ways". –  Warren Young May 22 '12 at 17:04

For me, the simplest way would be this one.

For an N-wire cable, get an MCU with at least 2·N available GPIO lines. Connect both ends of your cable to the 2·N lines, like this (where I've used a 3-conductor cable as an example):

The N upper lines would be open-drain outputs. The N lower lines would be inputs. For each one of the N wires of your cable, you would activate (pulling down) only one output (keeping the other N-1 outputs in high impedance), and read all N inputs. This way, you'd be able to detect opens, shorts, and crossings.

You would have an immediate and thorough result, without needing a human to watch LEDs.

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I have done exactly this for custom hdmi cables we make. There are about 14 connections to check, and it does it almost instantly. There's also a 7-seg LED display to tell me which wire has an error. –  Rocketmagnet May 22 '12 at 12:45
For 19 conductors, you need 38 GPIO pins - something like an Atmel ATXMEGA128 would do the trick (there are countless others too I'm sure) –  vicatcu May 22 '12 at 16:51