Some Apple II computers can daisy-chain disk drives: connect multiple disks, one after another in a single chain. I’m exploring options for the BMOW Floppy Emu disk emulator to support daisy-chaining more flexibly than it does now. I’d love your thoughts on whether you’d find this personally useful – please read below and leave a note in the comments section.
Daisy Chain Basics
Daisy-chaining makes it possible to connect many different disk drives to the same computer, without needing a separate disk controller card for each drive. Among early Apple computers, only the Apple II family has daisy-chaining capability – the Macintosh and Lisa don’t support it. Even among Apple IIs, daisy-chaining is only very useful on the Apple IIgs, the rare Apple IIc+, and to a lesser extent the Apple IIc. Other Apple II models lack the necessary hardware or the ability to control multiple types of drives, so this discussion will focus on the IIgs.
The Apple IIgs supports three different types of disk drives:
- “dumb” 3.5 inch drives, like the Apple 3.5 Drive (A9M0106)
- “smart” drives, like Floppy Emu’s Smartport HD mode, or the Apple Unidisk 3.5 (A2M2053)
- 5.25 inch drives
At most two drives of each type can be placed in the chain, six drives in total.
Drive Ordering and Boot Limitations
A critical requirement is the ordering of drives in the daisy chain: the dumb 3.5 inch drives must appear before the smart drives, which must appear before the 5.25 inch drives. This requirement stems from the way the disk port’s control signals are multi-purposed to serve several different types of drives, where some types didn’t yet exist when the older types were designed. You can try physically connecting the drives in a different order, but some of them won’t work and may even be damaged.
The ordering requirements have the side-effect of constraining the computer’s options for boot disks. With complex daisy chains, these constraints can become inconvenient and annoying.
Dumb 3.5 inch drives and smart drives are placed in a single logical group that’s mapped to slot 5. Only the first drive in that group can be used as a slot 5 boot drive. If two dumb 3.5 inch drives are connected, only the first can be used to boot the computer. And if both a dumb 3.5 inch drive and a smart drive are connected, only the 3.5 inch drive can be used to boot the computer. The smart drive can never serve as a boot drive, in this case.
With the Floppy Emu
The Floppy Emu was originally designed as a Macintosh device, and it lacks a daisy chain output connector. That means it can work happily in an Apple II daisy chain with other drives, but only as the last drive in the chain. Often this isn’t an issue, but for some combinations of emulation modes and disk drives, it causes difficulties. Here are some Apple IIgs setups that aren’t currently supported, due to Floppy Emu’s absence of a daisy chain output:
- Floppy Emu in Smartport HD emulation mode, combined with one or two 5.25 inch drives.
- Floppy Emu in 3.5 inch emulation mode, combined with one or two 5.25 inch drives.
- Floppy Emu in 3.5 inch emulation mode, combined with another 3.5 inch drive, where Floppy Emu serves at the first (boot) drive.
- Floppy Emu in 5.25 inch emulation mode, combined with another 5.25 inch drive, where Floppy Emu serves at the first (boot) drive.
Giving a daisy chain output to the Floppy Emu would enable all of the above setups. Unfortunately it would also add more complexity to the device, because the daisy chain output isn’t simply another physical connector with its data signals bussed from the daisy chain in. It’s a complex logic device whose behavior must change depending on what type of drive is being emulated, whether that drive is currently enabled by the computer, and what type of drive (if any) appears next in the daisy chain. Depending on the circumstances it must alternately cross-over, gate, or otherwise modify the disk control signals. This would require a new programmable logic device, likely a small CPLD similar to the one Floppy Emu uses for disk emulation. But the added cost and complexity would be little benefit to anyone except Apple IIgs owners.
I’ve been exploring a different route – a stand-alone daisy chain adapter rather than a modification to the Floppy Emu. This adapter would function like a T splitter, taking Floppy Emu’s place in the daisy chain, with a connection for the Floppy Emu as well as a standard daisy chain output for additional drives. This would provide a daisy chaining option for Floppy Emu owners who want one, without negatively impacting anything else.
Note there’s still one Apple IIgs setup that can’t be supported: Floppy Emu in Smartport HD emulation mode, combined with another 3.5 inch drive, where Floppy Emu serves as the boot drive. The Apple II drive ordering and boot requirements make this combination impossible, as described earlier (except for some very awkward work-arounds). This has nothing to do with the Floppy Emu or its lack of a daisy chain output – the same limitation exists with other 3.5 inch and smart drives.
My question to you, Apple II reader, is whether a stand-alone daisy chain adapter would interest you for enabling the Apple IIgs daisy chain setups listed above. It would be clearly a niche item for a niche product, so I’m unsure if it makes sense.
Aside from the cost of manufacturing and possible low demand for this adapter, the biggest hurdle would be finding a supply of female DB-19 connectors for the daisy chain output. This could be a major challenge. When the global supply of male DB-19 connectors was exhausted a few years ago, I had to commission the manufacturing of 10,000 new ones to continue with Floppy Emu production. That was very expensive, and I definitely can’t afford to do something like that here. Hopefully enough DB-19F’s are still in a surplus parts warehouse somewhere!