PETdisk and Commodore CBM 8032-SK / 8096-SK / 8296


A couple of days ago I received the PETdisk  from the smart guy Petfixer aka Gubbish aka Mike. This is a cheap and powerful device that acts as a disk drive for your Commodore PET  series (2001/30xx/40xx/8xxx). For those who don’t know anything about it, this little device is made of two boards: a smaller one, to be connected to the CBM tape port and a bigger one, to be connected to the CBM IEEE-488 port. With the smaller one, you can load and save tape through an audio device (i.e. pc audio card, mp3 player/recorder, etc.); with the bigger one you can load software on your PET from a MicroSD card, and save your programs into it. Actually the device (firmware 1.21) can only load and save PRG files. It doesn’t support neither folders (programs must to be loaded on the root of the MicroSD card), nor D64/D80/etc. disk image format: just the LOAD”…”,8 (or DLOAD from BASIC 4) and SAVE “…”,8 (or DSAVE from BASIC 4). The firmware should be a work in progress and it’s open source, so hopefully these features will be added in the future.

The PETdisk has a tipical edge connector that fits the IEEE port on most Commodore PETs.

Edge connector – Source: Internet


However, some PET series use a different connector. I’m talking about the “Separate Keyboard” series: 8032-SK, 8096-SK, 8296 and 8296D.


8096-SK IEEE port – Source: internet

That’s the IEEE connector you can find on these kind of machines. Obviously, you need an adapter! Since I love homebrew solutions and (more important) the local electronic store here is so little that often you can’t even find common stuffs, I started to think about how to solve quickly my problem. Of course I can buy almost everything I need on eBay, but it usually take at least a month to come here at home, due to Brazilian customs delays). 


Centronics connector – Source: internet

Soon I realized the IEEE connector is very similar to the old, Centronics connector on the old printers connected to the LPT port of your PC. And of course I have some old LPT -> printer cables here. But the Centronics connector is larger: it has 36 pins (18 per side), while the  IEEE has just 24 pins (12 per side). This is still good, the inverse condition would be harder to solve! We only need to cut it, this way:


Be sure to cut it at 13th pin and on the edge of the 18th pin (counting from right to left), as showed by the red lines. Throw away the part in the middle and glue the remaining parts together. You should have something like this:


Before to glue, be sure it won’t be too large to fit the IEEE port. Usually some adjusts are needed to reach a good result. Be sure the connector pins meet the IEEE port pins.

Hopefully, your connector will be better than this my first experiment! However mine works fine, and this is all I need. Now, time to build the adapter.


You need to make a small PCB: this is quite cheap and can be easily made at home, since the traces we must to produce are quite thick. In your local electronics parts store, buy some ferric chloride and a small piece of phenolite board for printed circuit, copper on both sides. Cut it to exactly fit the PETdisk edge connector. Now with a permanent marker draw the 24 (12 per side) traces, reproducing the IEEE port you can find inside your PET:  the IEEE tiny port shown above is connected to the main board with a flat cable and an edge connector like the PETdisk one. So what you need to reproduce (except for the two cuts and the green part) is more or less this kind of connector:


Made it longer than the one shown in this picture, because you will need some extra space where the wires will be soldered.

On one side, traces must to fit the edge connector, so they will be thicker; on the other side, they must to fit the smaller custom IEEE connector, from the soldering side, so traces here will be thinner. When you finished to draw the traces, simply drop it into the ferric chloride, keep shaking very gently for some minutes, until the acid has corroded all the copper not covered by the marker ink (use Google if you have some doubt about this procedure).


Ok, now what you really need to know is: the connection between the two connectors is fully crossed, as shown in the picture below.



So you can’t draw traces from pin 1 to pin 1, from pin 2 to pin 2, etc. as I stupidly did in my board! In fact I was sure it was a straight connection: so sure that I did a wrong PCB and it took half of an hour to understand why the PETdisk worked fine on my 3032 but refused to work on my 8032-SK!


Luckily the upper side and the lower side aren’t crossed: so the result you need to achieve is this one, or better of course!


Not a masterpiece, indeed; but it works fine.


You can make a solid, small adapter like this, or make a cable. The cable is good because you will have the PETdisk close to you and not plugged behind the computer: easier to insert the SD card and without the risk to crash it on the wall behind the PET. But I *hate* cables and wires! And, honestly, a 1 gigabyte Micro-SD is enough to store all the software even made for the whole PET series many times!


The PETdisk with the adapter plugged in, both sides.


A little, trivial problem: the PETdisk gets the needed +5 volts by the USB port. In the “normal” PET models you can plug the USB cable in the smaller, PETdisk tape adapter. But this one can't be plugged to the 8032-SK tape connector, since the hole in the PET chassis is too small. So you need an external USB charger or your PC USB port to power the PET disk.