Replacing an 8" disk drive with a newer/smaller drive on an old 8-bit computer?

Question

I used to have an out-phased Danish 8-bit computer as a hobby machine back in the 90's. It had two external 8" floppy drives, and I have kept the old floppies. Now I'd like to salvage my old projects, and therefore have gotten access to a similar machine at the local computer museum.

Unfortunately the B: drive is not working well so I investigated if another A: could be made into a B:. Findings:

• The unit is basically a YD-174 8" drive with a power supply - https://datamuseum.dk/bits/30001084

• The cable is a straight mapping to the drive connector, and optionally on to the next drive. To my untrained eye it is a straight pin mapping on the connector on the machine to the drive connector.

• The selection of the drive is done by shortening one of four pairs of pins in the upper right corner of the print at the bottom of the drive (they are upside down on the photo), and only that (I might be wrong on this). The A drives had pair 1 shortened, the B drive had pair 2 shortened.

Unfortunately the spare A: drive did not work well when converted to B:. I forgot to see if it worked as it should before tinkering, so I do not know if I missed something or the drive was broken all along.

Now I am considering options. My guess is that any drive that works the same as this drive (same connector) could be a replacement. Does such drives still exist? Any other thoughts?

Answer 1

It's probably doable!

The floppy interfaces and signals were all quite similar [at least until the cost-optimized all-in-one USB peripheral drives] although some of the pin usages did evolve a bit over time. The 8" drive would likely use the "Shugart" style interface rather than the later "IBM/PC" one in terms of the two tables at https://old.pinouts.ru/HD/InternalDisk_pinout.shtml.

You may find the thread at http://www.vcfed.org/forum/showthread.php?16765-Any-way-to-image-8-quot-CP-M-disks-or-convert-to-5-25-quot helpful although it is focused in the reverse goal of connecting an old 8" drive to a newer PC rather than vice versa (and some of its links have rotted and will need to be run through the Wayback Machine, e.g. https://web.archive.org/web/20190622115010/https://majzel.blogspot.com/2009/04/converting-from-8-to-35-inch-floppy.html).

IIRC the particular RPM and number of tracks make the 3.5" inch drives actually more similar to the 8" formats even though the 5.25" feel more similar at first glance.

Depending on your preference, it may either eliminate (or introduce) complication to use a floppy "emulator" instead, e.g. installing a firmware like FlashFloppy on a Gotek-style device. There's a forum thread at https://torlus.com/floppy/forum/viewtopic.php?t=2708 where you might find some additional tips in using a Gotek as an 8" drive replacement.

Answer 2

Your link has a description of the pinout of the cable on page 32:

Interface J1
(Rest is "Return")

2 Alternate I/O
4 Alternate I/O
6 Alternate I/O
8 Alternate I/O
10 Alternate I/O
12 Alternate I/O
14 Side Select
16 Alternate I/O
18 Alternate I/O
20 Index
22 Ready'
24 Low current
26 Drive Select 1
28 Drive Select 2
30 Drive Select 3
32 Drive Select 4
34 Direction
36 Step
38 Write Data
40 Write Gate
42 Tack 00
44 Write Protect
46 Read Data
48 Reserved
50 Reserved

Note that this confirms your theory about drive selection.

It looks sufficiently close to other floppies that you could try using them. Here is the standard Shugart pinout used in many non-PC 8" and 5.25" floppy drives and, slightly modified, for PC 5.25" and 3.5" floppy drives. And PC floppy drives should still be easy to find somehow.

This assumes that the "Alternate I/O" pins are not used for anything (please verify by looking at the board), and also that the "Low current" pin is either not used or can somehow be emulated.

Also, it won't be as easy as just plugging in another drive, you'll probably need to spend a bit of time (with a scope etc.) and figure out more details about the signals (polarity, voltage level, etc.). With access to a working drive that shouldn't be hard. Somebody at the computer museum should have a scope somewhere.