# Fun challenge: cat-5 cable tester

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I had this idea, and toyed with it a while (including some computer simulations), but didn't go very far with it. I thought I'd post it here in case anyone wants a brain teaser for the weekend.

Consider this tool: a cat-5 cable tester.

Goal: To make a tester that works entirely from the user's side, without needing batteries at the far end.

Circuit:

+-----+ | | P0/AD0 R1 R11 | |----------\\/\\/\\--< >----\\/\\/\\ ---+ | | P1/AD0 R2 R12 | | |----------\\/\\/\\--< >----\\/\\/\\ ---+ | | P2/AD2 R3 R13 | | |----------\\/\\/\\--< >----\\/\\/\\ ---+ | | P3/AD3 R4 R14 | | |----------\\/\\/\\--< >----\\/\\/\\ ---+ | uP | P0/AD0 R5 R15 | | |----------\\/\\/\\--< >----\\/\\/\\ ---+ | | P1/AD0 R6 R16 | | |----------\\/\\/\\--< >----\\/\\/\\ ---+ | | P2/AD2 R7 R17 | | |----------\\/\\/\\--< >----\\/\\/\\ ---+ | | P3/AD3 R8 R18 | | |----------\\/\\/\\--< >----\\/\\/\\ ---+ | | cable +-----+

The idea is that each GPIO pin can either drive high, drive low, or measure the voltage present. I.e. you're creating a programmable resistor divider network. So you can set up various combinations of resistors being driven from various combinations of voltages, and use the A/D converters to measure the voltage, and compare with what you "think" it should be.

The challenge: Select values of R1..R18 such that you can accurately detect the following cable errors:

• shorts between any N (2..8) conductors. * opens in any one or more conductor. * swapped (or N>2 miswired) conductors.

Bonus points for minimizing the number of A/D bits you need.

My idea was to provide eight red/green or RGB leds on the uP side to indicate the status of each conductor (green=OK, other combinations=various error conditions), with a serial or usb port for diagnostics to a laptop. I have a cable tester now, but the battery is at one end and the lights are at the other.

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Its been a while, but back when i was an electrician, we had these neat little testers that cost about \$2000 Aussie dollars. You plug a little passive device in one end, the instrument in the other. It would then map out on its graphic lcd the actual pinout, the loss, the length of the cable and the position of the break if any. Given some domain/network data it could even show you routes, network statistic, analyse connection issues ect.

I am guessing that now days you could get something similar for a lot less, and with more features. For a simple go/no go test you can get some pretty basic testers for a very reasonable sum. Hardley worth reinventing the wheel.

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there are two parts to the tester - the part with the uP (R1..R8) and the "fob" that connects on the far end of the cable (R11..R18). The two ends don't have to be near each other, but both ends of the cable have something plugged into them. The goal, however, is to have nothing active - no lights or batteries - on the far end, so everything is controlled from the near end. My current tester requires you to go back and forth, since the battery is on one end and the lights are on the other end.

For testing wall jacks, you'd just use a known-good patch cable to connect the tester to the wall jack.

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It's not about reinventing the wheel, it's about challenging yourself to think and to make something with your own two hands. Heck, I already have a cable tester, it's not like I need two.

Some people do this for fun, you know ;-)

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Not to mention the financial advange, especially if you have more time than money.

Luhan

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How is it going to detect split pairs or miswired connectors?

```--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.```
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He is "to smart" to understand. :)

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....

maybe 1n914 diodes in a ring topology at the far end. then it can be done using 1 bit at the UP end. +-----+ | | D0 | |--------+---------< >-+-->|----. | | D1 `-\\/\\/\\-+ `------. | | |--------+-------|-< >-+-->|--' | | | D2 `-\\/\\/\\-+ `------. | | |--------+-------|-< >-+-->|--' | | | D3 `-\\/\\/\\-+ `------. | | |--------+-------|-< >-+-->|--' | | uP | D4 `-\\/\\/\\-+ `------. | | |--------+-------|-< >-+-->|--' | | | D5 `-\\/\\/\\-+ `------. | | |--------+-------|-< >-+-->|--' | | | D6 `-\\/\\/\\-+ `------. | | |--------+-------|-< >-+-->|--' | | | D7 `-\\/\\/\\-+ `------. | | |--------+-------|-< >-+-->|--' | | | `-\\/\\/\\-+ cable `-----|

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Yeah, the fact that the diodes are all the same does that. With resistors, you can make them all different values.

I thought of that, with diodes to leech power from whatever pins were high or low, running something like asynchronous i2c on each of the eight conductors.

I worried that shorts in the cable would damage the micros.

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It thats the case, then a 4017, 8 bicolour LED's + a handful of resistors and capcitors.

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My existing tester does something like this, but the LEDs are on the far end of the cable. The challenge is to figure out how to do it with a resistor network on the far end of the cable, so the LEDs can be on the same end as the uP.

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It's not. most cheap testers only do continuity, and that catches over 99% of wiring errors. pairing errors that otherwise test good reqire that the same mistake be made at both ends.

if the run is long enough (I have no idea how to quantify long enough) it may be possible to detect the differences pairs by the driveing them in differential mode and measuring the response time somehow.

Bye. Jasen

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TDR.

```--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.```
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A diode on the far end of each pair.

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Most professionals do it for more than fun, since the shop/PCB jerks never see the advantage of doing it without batteries and music.

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No, I'm pretty sure I said resistor network.

I'm rather disappointed in the response I've gotten from this. I've seen a lot of people tell my why it's a bad design, or all the things it can't do, or alternate ways to build cable testers, and yet NOBODY has addressed the original challenge.

GIVEN THE SCHEMATIC I POSTED, what resistor values let you detect bad cables (as far as the design allows) with the least ADC bits?

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Well the least you can do is one A/D resolution cell per fault type with exactly one fault, then exactly two, then three, etc up to the limit with the maximum number of faults possible. Maybe a 20-bit job will do.

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Except that you have eight A/Ds and you can reconfigure the rest of the pins to drive high or low. I've gotten it down to about 12-13 bits so far, with random fiddling.

Part of the challenge is to come up with the techniques for setting up the tests - whether to drive H or L, or tri-state, what to measure and for what, etc.

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Sort of semi-OT:

I have a misty distant memeory that the CAT-5 designation actually referred to plenum wiring flame-proof and smoke-proof qualities, and had nothing to do with the number of conductors, their twistedness, or the type of connectors. Am I thinking of something else?

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