Floppy Disk Emulator

Vintage Events and off topic talk

Floppy Disk Emulator

Postby xprt » Apr Mon 12, 2010 5:22 pm

I've built a floppy disk emulator using an SD card.

It is register compatible with the MITS Altair 88-DCDD floppy disk interface.
This allows it to run Altair Disk Basic and Altair DOS directly without modifying the code.

The hardware is very simple: just one PIC plus a 3.3V regulator and a buffer. About $10 in parts.

You can boot Altair Disk Basic, mount a drive, load programs from disk and save programs to the disk. It can emulate up to 16 drives connected at the same time.

It uses FAT32 file format and the floppy images are stored in the same format as the Altair32 emulator (straight binary images) with one file per floppy. This means you can interchange data with any computer that can read an SD card and you can directly exchange data with the Altair32 emulator.

This plus a dumb terminal is a complete stand alone computer system. No need to connect to another computer through the serial port.

It should also be able to run CP/M, but I don't have a 32K version to try. I may upgrade the RAM to 64k and give that a try as well.
Posts: 23
Joined: Jun Sun 15, 2008 8:05 pm

Re: Floppy Disk Emulator - CP/M

Postby xprt » May Thu 06, 2010 4:06 pm

I increased my RAM to 64k and got CP/M working.

I found that the Altair32 CP/M 2.2 BIOS inexplicably resets the UART after it outputs the welcome message. This clears the baud rate clock settings, number of stop bits settings, etc. and prevents the UART from working. This isn't a problem for the simulator, but causes a real computer to hang. After NOP'ing out the reset instructions, it worked. To patch the disk image I also had to patch up the checksum bytes for those sectors.

So now I have confirmed this will run virtually all the software for the original Altair:
the 3 different versions of Disk BASIC,
Altair DOS (which includes an assembler, editor, debugger),
CP/M 2.2 (and the huge number of programs that run on it).
Posts: 23
Joined: Jun Sun 15, 2008 8:05 pm

Website updated

Postby xprt » Sep Tue 28, 2010 1:41 am

I finally got around to updating the website on this project:
(Ad blockers and popup blockers are recommended.)
It now has pictures, screen shots, schematics and source code.
The disk emulator should be usable on the Altair Micro connected to the expansion port or on an S100 bus system with the addition of a few buffers.
Questions or comments are welcome.
Posts: 23
Joined: Jun Sun 15, 2008 8:05 pm

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