I spotted a useful cable at a used computer store recently. I wasn’t 100% certain what it was, but I had the ability to rewire it into what I needed if it turned out I was wrong. Here’s how I went about identifying the cable with my multimeter.
First, find pinouts
In this case, I believed I had found an RGBI cable for a Commodore 1084 monitor. This was the cable Commodore intended for you to use with a Commodore 128 or Commodore PC10 or Commodore Colt computer.
The cable will also work with any other PC equipped with a CGA card. It will also work with a Tandy 1000.
So the first thing I did was to find pinouts for the CGA connector on a PC and the eight pin DIN connector on the back of the Commodore 1084.
One thing to keep in mind is the pinouts you find online maybe the cable side or they may be the device side. So when you are probing around with your multimeter, you may need to try the opposite side of what your pinout diagram says.
When it comes to old fashioned TTL RGB, I had two aces in the hole. Pins one and two on the PC side are both ground. So I could just check the two pins on either end of the cable side to got an instant clue of the plug’s orientation. Pins one and two are connected together, but pins four and five are not.
I also noticed that the pin for the red signal is on pin 3 on the PC side, which is in the center of the top row. And on the monitor side, red was on pin 2, which is also in the center. So I checked to see if those two pins were connected, and they were.
But I’m a little bit ahead of myself. Let’s go over the multimeter a bit.
Using a multimeter on a cable
Multimeters can measure resistance, and they can also test for continuity. The continuity test is a little bit more useful for this case because it emits an audible tone. The continuity setting on your dial probably has a speaker symbol on it. Turn your dial to that setting, and then touch the two probes together. You should hear a tone and see very low resistance on the screen. And in case you’re wondering, a cheap Harbor Freight coupon special multimeter is good enough for this purpose.
Now you can go to your cable, put each probe on a pin you expect to be connected, and then listen for a tone.
I find it helpful to either print or draw a diagram of the pin out I am checking, and then mark off each pin after I check it.
Getting the probes onto a pin on a cable can be tricky. I keep some assorted small diameter K&S tube around because I find it helpful. You can get K&S tube at hobby shops and at very good hardware stores. Not the big box stores. Look for a True Value or an Ace or an independent, ideally one with some family name on it.
I slip a short length of 3/32″ tube on the pin that I want to check on each side of the cable, and then touch a probe to each piece of tube. It makes the process go much faster.
David Farquhar is a computer security professional, entrepreneur, and author. He started his career as a part-time computer technician in 1994, worked his way up to system administrator by 1997, and has specialized in vulnerability management since 2013. He invests in real estate on the side and his hobbies include O gauge trains, baseball cards, and retro computers and video games. A University of Missouri graduate, he holds CISSP and Security+ certifications. He lives in St. Louis with his family.