Vintage Mashups

Originally published: 07-29-2009

Have an old Commodore PET and need a disk drive? IEEE-488 drives made for use with the PET are getting harder to come by. Using a circuit found on the web I’ve converted the more common Commodore 1541 to be used on the IEEE-488 GP-IB bus.

Commodore PET 4032 with IEEE-488 1541 Drive

After recently rescuing a Commodore PET 4032 from the scrap heap and getting it into functional condition I went on the hunt for a storage method that was faster than the Commodore cassette drive. eBay searches turned up little for a week and when a drive did come up for sale it quickly rose in price well out of range of my collecting budget. Alternatives were to use tape storage accelerators or to build connections to Windows computers. Not having enough room in an already cramped work space to setup yet another machine with a virtual drive I happened upon this:

Commodore Hardware IEEE-488 Projects Directory

At the bottom of the page is vc1541-ieee.tar.gz

Its an internal modification to the 1541 which allows it to speak the same protocol as the PET computer.  It isn’t an overly complicated schematic so I started gathering the required parts to build it.

Of the parts needed the most difficult to find were the 24 pin IEEE-488 connector and the 75160 and 75161 bus drivers. I was fortunate to have a few of the connectors from other tinkering and was able to pull the chips from a HP-IB hard drive with a bad drive (since then I’ve ordered the chips and will replace them in the HP-IB drive, as well as the bad hard drive which is an old but relatively common ST-225).

Construction took from 2-3 hours using point to point soldering. I think it could be done quicker with wire-wrapping if you don’t mind cutting the pins from the wire-wrap sockets to get it to fit into a closed 1541 case.

The completed board:

IEEE-488 1541 Converter Top View

BOTTOM:

IEEE-488 1541 Converter Bottom View

I used 2 machine pin sockets pressed together through the same holes in the perfboard and made my own pins from stiff wire. If I would of had a low profile machine pin wire-wrap socket it would have been a much better (and easier) choice for this purpose. In case It wouldn’t fit into the drive with the case top on I decided to add an additional socket to hold the 6522 VIA and use a ribbon to connect it to the UC3 socket. This wasn’t required and it actually almost made the board to wide for the area. I was able to bend the connectors on the left side (if facing the drive) over to permit the board to fit though.

Since this model (short board) of 1541 used older 2364 ROM chips which are nearly impossible to find these days it required I make an EPROM adapter for 2764 EPROMS. I’ve made almost a dozen of these already for various projects involving Commodore computers. If you need to make one:

Commodore ROM Adapter step-by-step tutorial is a good place to learn how.

Commodore 2364 ROM Adapter

Once the hardware was completed I needed a new ROM. In the .gz package there is a BASIC program that downloads the upper ROM area of the 1541 and patches it. You then need to save it to disk with a ML monitor. Since this is a pain to do I have made it available for others: IEEE-488 1541 Patched ROM

Mounted in the drive:

Installed IEEE-488 1541 Converter Close Up

Installed IEEE-488 1541 Converter Wide View

With the top on the drive there is just enough room to pass the cable over the serial connectors and out through the space above the fuse holder. Not ideal but it works. If I’d had a spare case I would have modified it just for this purpose but I like being able to return equipment to original condition if needed.

IEEE-488 Cable Path

Once assembled and connected to the PET I powered up and issued the DIRECTORY BASIC 4.0 command:

IEEE-488 Test using 1541 Test Demo Disk

Everything appears to work as it should although I have never actually used a IEEE-488 drive on a PET before. If I run into anything that doesn’t work properly I’ll be sure to report it here.