Technology

Writing 800kB Mac disks with FluxEngine

I’ve just merged in a change to FluxEngine, my open-source and easy to build
USB floppy drive interface, which will let you use it to write Macintosh 800kB
GCR disks using a normal PC drive.

For context: the FluxEngine is a off-the-shelf, unmodified Cypress PSoC5
development board, costing $10-$15 (plus Cypress’ exorbitant shipping, sadly).
A single component is soldered to it, which is the floppy drive connector. (You
can get away with a row of header pins if you don’t have one.)

When programmed with my software, it becomes a USB flux-level floppy drive
interface, capable of reading and writing raw pulsetrains from and to a normal
PC floppy disk drive. The client software will then decode the flux information
into disk images, or encode disk images into pulsetrains for writing back to
the disk.

Currently it supports a wide variety of disk formats, including the ubiquitous
FM and MFM IBM scheme PC-compatible disks, but also oddities like the FB-100
(with two sectors per track) or the Brother GCR word processing format (240kB
per disk, with two different weird GCR encoding schemes). Because all the
decoding happens in software, it’s not constrained to a limited selection of
formats by the floppy drive controller logic — it’s theoretically capable of
reading anything physically compatible with the drive. It can even handle CLV
formats, where the drive changes speed: the PC drive always spins at 300rpm,
but instead the pulse frequency changes. The software decoder sees no problem
with this.

However, it’s also possible to encode disk images back into pulsetrains, and
write them back to the floppy disk. I’ve been using this successfully on
Brother word processor disks for a while (this was originally why I built it).
What I’ve just done is add support for 800kB Macintosh GCR disks, and tested
them on my new PowerBook 150.

What this means is that you can now make your own Mac Plus or Mac SE boot
disks, from DiskCopy 4.2 images downloaded off the internet, without needing to
either buy them off eBay, exchange them in a shady pub, or buy a more recent
Mac with a SuperDrive.

You do still need to build a FluxEngine, though…

It’s had limited testing, but the written disks work fine on my PowerBook 150,
and I’ve had a report from someone who’s successfully booted a Mac Plus using a
FluxEngine-written disk. I’d appreciate any user reports — please file a
github issue via the link on the main page.

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