The Ultimate Test Loopback Dongles

If you’ve spent any time diagnosing faulty Amiga machines, you’ll have undoubtedly come across two indispensable bits of software – Keir Fraser’s Amiga Test Kit and John “Chucky” Hertell’s DiagROM. The former is a bootable floppy which can run a variety of tests on partially working Amigas (i.e. it has to be able to boot from floppy, output to a display and use either a keyboard or a mouse) and the latter is a replacement for Kickstart which takes over the machine and hits the hardware directly to run a few more low-level tests, even when a machine isn’t booting. Thanks to the ability to be controlled and display results over a serial connection, it doesn’t even need a keyboard or display.

One thing they both have in common is that they can test serial and parallel ports, by use of a “loopback” connector. This shorts various pins together in order that the test can output on a set of pins, and listen back on others which, if successful, should mean that the ports are functioning properly. To use this feature you need to make the plugs, or buy some ready made from eBay, but handily both Amiga Test Kit and DiagROM use the same connections. Both DiagROM and Amiga Test Kit tell you what these connections are in the software itself.

As well as providing the loopback functionality, they have LEDs which light if the 5V, 12V and -12V rails are working. But this got me thinking…. how well are those rails working? Could a low voltage still light the LED enough to fool you into thinking everything is OK? It’s a bit of pain to always have to get the multimeter out to check the voltages. What if those dongles had voltmeters built in?

In theory, by replacing the LED and resistor with a voltmeter you could get an exact reading of how healthy those voltages are without guessing from the brightness of an LED. The trick is to get something small enough, and Amazon turned up these tiny little three digit voltmeters in a pack of four for around £10. They’re self-powered if you feed them with between 3 and 30 volts so perfect for our application.

They’re only designed to read positive voltages, but by wiring one backwards it’s more than happy to give you the positive version of a negative voltage so in your head you just need to add a minus sign when you read it. It really is that simple – replace the LED and resistor in each dongle with a voltmeter so the parallel port (which only has 5V) gets one, and you need to squeeze two into the serial port dongle for 12V and -12V. I bought some standard DB25 connectors – one male, one female – and the shells to go with them, and set about them with my dremel.

My advice is cut a small hole and then gradually open it up and test the fitment of the displays. If they are a firm “friction fit” then it makes it easier to glue them into place later, if they’re loose in a hole that’s too big then it’ll cause you a headache. You’ll see on the serial port one with two displays, there’s a thin bit of foam between them – this is because the PCB on the bottom is slightly wider and it caused the displays to angle towards each other. The foam keeps them aligned and both pointing upright. You’ll also need to cut the “ears” off the displays – just chop them with the dremel, it’s a blank bit of PCB that you don’t need.

With the displays in the shells (don’t glue them in yet!) you can then solder the pins of the DB25 connectors together as per the instructions in AmigaTestKit or DiagROM. These are reproduced at the top of this page to make it easier. Many of these are just side by side pins so you can use a short piece of old component leg, and only resort to wires where you need to jump over other pins.

Solder the positive legs of the 5V and 12V displays of the voltmeters where it indicates 5V and 12V are on the socket. For the -12V display, solder the positive lead to a ground point (I cut it short and attached it to the 12V display ground alongside it) and the negative lead to the -12V connection. This will have the effect of creating the correct voltage across the display, so it knows no difference.

Save space by cutting the wires short and solder it all up. Test it before you start gluing things down – one of my displays was reading a bit low, claiming my 5V was only 4.1V so I checked with a multimeter and turned the small adjustment screw on the back until it was aligned with what my trustworthy multimeter was telling me.

Hot glue them all into place (this is why a snug fit to start with was important) and screw the shells together. You could even then inject more hot glue into the whole thing from the hole in the back so it’s one solid piece and has no chance of the displays pushing inside, but mine seem fine without that.

There you have it! Job done and they can go into your trusty test kit.

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