Interfacing an Apple II to a General Processor GPS-4 8 inch floppy disk drive box

macnoyd wrote:

Nice work !  I've wanted to do that for quite a while but could never find the time.  Also not sure I have the talent to complete the job as you have done.  I'm sure there is more to this than what is indicated in your video.

Speaking of, how long this project take you?  Also, would you share the interface card schematic you used?  Looks like something hardware fo

Yes there is more work than that I've shown but I couldn't make a 2hr long video :) , that was a summary of all the steps involved.

Regarding how long the project was here is an approximate list:

Sourcing electronic parts from China, delivery 30 days

Stripping down the unit, cleaning all parts, lubing drives, 2 days

Painting the box and the fan half a day

Designing and 3d printing the PSU adapters 1 day

Rebuilding the box  electrical wiring 1 day

Reassembly of the whole unit, including mechanical work like rebuilding threads, new screws and bolts 1.5 days

Checking the interface card , chip by chip with a chip tester , replacing the faulty ones , half a day.

Understanding how to setup basf drives jumpers to match the eprom interface card firmware was the hardes, it took around 20 days.

I had first to disassemble the eprom code, comment it all, but, it turned out, the 1771 controller chip registers are not accessed as an Apple II memory location as one would expect but through a latch where a command directed to the 1771 registers is mixed with the one of the four possible interface SELECT (drive/side) signals and , later, the hardware on the interface card splits them again, so disassembly of the code itself was a puzzle because the commands sent to the interface card seemed randomly cryptic. I had to hook up the card to the logical analyzer and rebuild a schematic of the command register part to understand what was going on with the 1771 register commands and how the drives were indexed.

Actually, it's  a simple thing once I understood it, the guy that wrote software for the interface card, used the 4 SELECT hardware signals to index the 4 floppy disk sides that you have in a dual disk box, so that, from the DOS 3.3 point of view, there are 4 logical/phisical drives (D1->D4) in slot 7, the only slot the card can be installed in (this is also clear if you read the dissassembly code) . The Basf 6104 jumpers must be set accordingly for the HW/SW combination to work correctly. They probably used a latch and a rather weird way of mixing the register commands with the SELECT signals to , probably, cut on circuit complexity.

I have no complete schematic of the interface card because I didn't want to risk to ruin it by completely disassembling it so I don't have a complete one to provide.

One more thing that might not be clear watching my video is that a single Basf 6104 drive can hold 500kbytes of formatted data per floppy side, there are two sides so 1000kb per disk but this is the high density mode, the Apple II interface card I have has one 1771 controller chip that is an FM only encoder chip, in other words single density mode. So when I write 25Kg / 1 MB it means my Apple II can index 4(floppy disk sides) X 256 (single density only) Kbytes of data. A more interesting solution would be to find a schematic of an high density controller for the Apple II :) so I could write 25Kg / 2 MB :))

Luca

PTB wrote:

Luca,

Another incredible project. What a great restoration and such an unusual piece.

Amazing effort.Cheers

Dave

 

Thank You very much!!!!

Luca