Cable Tester circuit

[JMACgyver]

Howdy,

I'm building a circuit for work, to test the cables we're custom shortening.

These are from normal computer-type DB-9/RS232 male/female and male/male patch and extension data cables. What I'm doing is chopping them in half, and soldering on new male DB9 connectors. However, we only need to resolder the TX, RX, Ground and Shield wires. I've narrowed down the range of wire colors these could be (depending on cable manufacturer) and have the pinouts needed based on whether I'm making a new M/M or M/F cable.

I've come up with a circuit that will light up LEDs when each correct wire is connected to the appropriate pins on the new conn. However, the circuit does not discriminate which of the four wires is connected to which pin. I've attached a quick-and-dirty sketch of the current circuit.

I am using a single 12Vdc center positive power supply and need a way to independently isolate the pins from each other. I'd like to stay away from ICs and uCs, since I'm not a programmer. Right now, the circuit doesnt care which pin is connected to which pin. I need it to care that let's say, pin 3 of the molded end of the cable is connected to pin 3 of the new connector, and pin 2 is connected to pin 4, etc.

(This is basically to help idiot-proof and speed up my cable building; Time is Money, they tell me...LOL)

Thanks for any help; schemata would be most helpful.
--David--
JMA USA

[admin]

One option is to use 4 separate and isolated 12V supplies with separate +ve and Common for each LED branch. Another option would be as shown in attached circuit diagram. Use 4 double pole switches. But you have to remember to turn on the switch for which you are checking connection and ensure other switches are turned off.

[JMACgyver]

Hi,

Thanks for the reply.

The switch idea would probably work, but is a little more interactivity than I was looking for. Was hoping to get by with something passive. The separate DC supplies is not a workable solution at this time. But, is there a way to *simulate* four different supplies from one? Say, using either diodes or capacitors between each branch? Or if not, is the a logic IC that could do that? the simpler the better of course.

Thanks for your time,
--David--

[pebe]

It would be possible to make a dynamic tester that ensured you were connecting each wire to its corresponding socket pin.

It would require a 555 timer, two 4017 decade counters, four transistors and some resistors, diodes and leds. Would that be too complex?

[JMACgyver]

Hi,

As long as it can be wire-wrapped, I'm interested.

Thanks,
--David--

>>It would be possible to make a dynamic tester that ensured you were >>connecting each wire to its corresponding socket pin.

>>It would require a 555 timer, two 4017 decade counters, four >>transistors and some resistors, diodes and leds. Would that be too complex?

[pebe]

[quote="JMACgyver"]Hi,

As long as it can be wire-wrapped, I'm interested.

Thanks,
--David--
[/quote]

If you mean wire wrapped - as distinct from soldered - then No, it can't

[JMACgyver]

[quote="pebe"][quote="JMACgyver"]Hi,

As long as it can be wire-wrapped, I'm interested.

Thanks,
--David--
[/quote]

If you mean wire wrapped - as distinct from soldered - then No, it can't[/quote]

Well, could you send me the schem anyway? If i could see the circuit, I could see how much trouble laying down traces for soldering would be. (Personally, I was really hoping there's be an easier way; but perhaps there isn't)

--David--

[pebe]

OK, I'll do you a circuit.

You don't have to make a PCB - it could easily be made up on stripboard or perfboard.

[admin]

A simpler approach to my idea with the switches above would be to use a rotary two pole switch so you cannot make a mistake by turning on multiple switches.

[pebe]

Here is the circuit as promised. It tests the cable connections sequentially and continuously by energising each one in turn from the outputs of a decade counter.

The 555 timer IC1 clocks the two 4017 decade counters, IC2 and IC3, at about 250Hz. Only one Q output of each counter can be high at any time, so they step a high from Q0 up to Q3 while the remaining outputs stay low. Each time IC3 steps on to Q4, the high output there resets both counters so they count together through Q0, Q1, Q2, Q3, and repeat Q0…etc. The reset time is increased with R5 and C1 to ensure IC2 resets. R1, R2, and R3 are the same values at each Q position. The components shown inside the red outline connected to D need to be duplicated for the other 3 lines, A, B, and C.

To understand the circuit operation, look at the components between the Q3 pin of IC1 and Q3 of IC2. When any of the other outputs is high, then the two Q3s will be low and there will be no drive to TR and the LED will be off.

When the 555 clocks both Q3s high, the test circuit for that line will be enabled. The diode at D will be reverse biased because its cathode is high, but the voltage on D will stay low if no cable connection is present because R3 will pull it down to 0V, so TR will stay off. But if the cable is correctly wired, point D will be on potential divider comprising R1, R2 and R3, and will be at about 10V. That will be enough to turn on TR and the LED. Only a correctly connected cable wire will light the LED.

You can leave the circuit running as you work on the cable connections, because the resistors R1 and R2 protect the ICs from static. An added bonus is that a short circuit between adjacent pins will cause the LEDs for those wires to decrease in brightness.

Best of luck and I hope you find it useful.

(20/05/11 Edited penultimate para re. LED brightness )

[JMACgyver]

Pebe,

Thanks, man. I look forward to trying it out. Might be this weekend before I can get the parts, but will breadboard it and let you know. I appreciate your help.

Thanks again,
--David--

[JMACgyver]

Hi,

Was thinking today about your explaination of how your circuit works, Pepe, and was inspired to draw this up. Does this seem like it would work? I would think only a correctly wired cable, or a totally messed up (beyond my capability of error) cable will light the LED. Any reasons why it might not work? Plz let me know.

Regards,
--David--

[pebe]

Hi David,
Yes, that will work OK for that wire, but what about testing the other wires?

[JMACgyver]

[quote="pebe"]Hi David,
Yes, that will work OK for that wire, but what about testing the other wires?[/quote]

Actually, that circuit tests them all at once. It is basically just one big single circuit. With each of the pins to be soldered leading into the next one, only when they are all done correctly does the LED light up.
However, that is it's flaw. You only know when you're done soldering, if it is done correctly or not. And a mistake could lie anywhere along the path.

So, i finally went with the rotary switch method. It's actually kinda fun to flip through it, and see the LEDs light up in sequence.

Thanks for all your help! I figured I should let you know how things turned out.

peace,
--Electro--
aka David M.
~~~~~~~~~~~~~
"In Theory, there is no difference between practice and theory. In Practice, there is."
~~~~~~~~~~~~~