BMOW title
Floppy Emu banner

Floppy Emu on the Apple III

image6

The poor sad Apple III never gets much love. It wasn’t popular in its time, and had a short lifespan, but today it’s a sought after collector’s item. My Floppy Emu disk emulator for vintage Apple computers supports just about every machine Apple ever made except the Apple III – or does it? The enterprising Patrick Longinotti reports his success using the Floppy Emu to boot and run his Apple III system, using a custom cable and the stock Apple II firmware on the Emu.

How is this possible? The Apple III uses a 26-pin rectangular disk connector that’s physically incompatible with the 20-pin rectangular connector of other vintage Apple computers. But it turns out that Apple didn’t innovate much in their Disk III design, and the leftmost 20 pins on the Apple III disk connector are the same as the 20 pins of the standard Disk II connector (the remaining 6 pins are used for daisy chaining and auto-sense). All that’s necessary is an appropriate 20-to-26 pin adapter cable, and this guy on Tindie will make it for you!

disk3-adapter

Patrick reports that he’s been successful patching in the Floppy Emu as the Apple III’s internal drive, as well as using it as the sole external drive, and also daisy chaining it behind another Disk III external drive. It can be set up as drive 1 and boot the Apple III without any real floppy media, or set up as a higher numbered drive, and used after booting the Apple III from a real floppy in drive 1. According to Patrick, there occasionally will be errors, because the Disk III drives were either slower or faster than Disk II, but for the most part it works with very little issue. (Can anyone confirm this? I thought the drives were the same speed.)

Lots of Apple III software is available at apple3.org.

The only tricky part is getting the correct gender of connector on your adapter. I’ll admit that I’m confused by the Tindie seller’s description of the gender on his adapters, because they seem backwards to me. Maybe he’s referring to the gender of the connector shroud, which is the opposite of the actual pins? To be clear, this is what I mean by a female connector:

20-pin-female

And this is a male connector:

20-pin-male

The 26-pin connector on the Apple III logic board is male, as are the 26-pin daisy chain connectors on the Disk III drives. The Floppy Emu has a 20-pin male connector on its PCB. With a 26-pin female to 20-pin female adapter cable, you’ll be all set. The Tindie seller has these.

The female-to-female adapter cable isn’t necessarily the most convenient solution, however. Reaching the 26-pin male connector on the Apple III logic board is a major pain, because it involves taking apart the case, and removing the entire internal drive just to get access to the port. A simpler alternative is to disconnect the existing 26-pin cable from the internal Disk III, and then attach that cable to a 26-pin male to 20-pin female adapter cable to reach the Floppy Emu. The Tindie seller has these, but they’re only a couple of inches long.

A third alternative is a very short 26-pin male to 20-pin male adapter, combined with the existing 26-pin cable from the internal Disk III, and with the 20-pin cable that ships with the Floppy Emu. But nobody sells these.

In the end, you might need to make your own cable adapters. It’s not difficult, but if you experiment with homemade adapters, please be careful not to release the magic smoke from your valuable electronics!

Read 1 comment and join the conversation 

1 Comment so far

  1. Eric Smith April 7th, 2018 8:01 pm

    You are correct that the Apple /// drives (both internal, and the Disk /// external) run at exactly the same speed as the Disk II. The motor speed is 300 RPM, and the controller on the Apple /// motherboard uses the same clock frequency, so there should be no difference at all.

    The 26-pin header has three signals beyond what a normal Apple Disk II 20-pin cable has. Two of these are the additional select lines for the daisy-chained drives, and each drive’s analog board shifts those select lines one position. The third extra signal indicates whether the Apple /// is in Apple II emulation mode.

    The reason for the emulation mode signal is that the Disk /// has an extra circuit to sense disk change, which the Apple /// SOS operating system uses to avoid accidentally writing to the wrong disk if the user happens to swap disks at an inopportune time. This works by detecting transitions from the write protect sensor, and setting a latch that causes the read data from the drive to be suppressed. In Apple II emulation mode the disk change detect circuit is disabled, since Apple II operating systems don’t know about it.

    I recently discovered that the strange DB-25 connector uses on the Apple DuoDisk 5.25 dual drive matches the pinout of the Apple ///+ floppy connector (also DB-25). I don’t have either an Apple ///+ or a DuoDisk 5.25, but @jrobj_ on Twitter confirmed that the DuoDisk connected to an Apple ///+ passes the diagnostic test for the disk change detection.

Leave a reply. Comments may not be monitored regularly. For product support questions, visit the Contact page.