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Run any disk drive with your Apple II computer!
Yellowstone is a universal disk controller card for Apple II computers. It supports nearly every type of Apple disk drive ever made, including standard 3.5 inch drives, 5.25 inch drives, smart drives like the Unidisk 3.5 and the BMOW Floppy Emu's smartport hard disk, and even Macintosh 3.5 inch drives. Yellowstone combines the power of an Apple 3.5 Disk Controller Card, a standard 5.25 inch (Disk II) controller card, the Apple Liron controller, and more, all in a single card.


  • Add 3.5 inch drive and smartport hard disk support to your Apple IIe or II/II+
  • Provide more disk connectivity options for your Apple IIgs
  • Bring Macintosh 3.5 and naked Apple 3.5 inch drive mechanisms to the Apple II
  • Drop-in replacement for an Apple Liron controller card (with optional DB-19F adapter)
  • Drop-in replacement for a standard 5.25 inch or Disk II controller card
  • Run two drives of different types on twin independent disk connectors
  • Disk II controller emulation mode for tricky copy-protected disks
  • Works with DOS 3.3, ProDOS, GS/OS, and more
  • User-upgradable firmware for future feature enhancements
  • 20-pin ribbon cable connectors or optional 19-pin D-SUB connectors

Apple Disk Controller Card Comparison

Disk Controller Supports 3.5 inch drives Supports 5.25 inch drives Supports smart hard drives Supports Macintosh drives 20-pin ribbon connector DB-19 connector Number of connectors
Disk II
Controller Card
Disk 5.25
Controller Card
Apple 3.5 Disk Controller Card       1
Apple Liron Card         1
Yellowstone 1 2

[1] optional DB-19F connector

Need to attach a disk drive to your Apple II? Yellowstone has got you covered. Yellowstone is compatible with the Apple IIe, Apple IIgs, Apple II+, Apple II, and most Apple II clones.


Yellowstone is not yet available for sale. General release is expected in March 2022.

Yellowstone Card  Yellowstone Universal Disk Controller
A universal disk controller card for Apple II computers. Combines the power of an Apple 3.5 Disk Controller Card, a standard 5.25 inch Disk II controller card, the Apple Liron controller, and more, all in a single card.
More Info Coming Soon

Yellowstone Disk Controller Everything Bundle  Yellowstone Everything Bundle
The bundle includes the latest version Yellowstone Universal Disk Controller Card and two DB-19 female adapters, all in one convenient package.
More Info Coming Soon

Yellowstone DB-19F Adapter  Yellowstone DB-19F Adapter
For attaching Apple disk drives that have a D-shaped 19-pin connector. Includes a six inch ribbon cable and a detachable mini-board that converts into a DB-19 female shape.
More Info Coming Soon

  Find more items in the BMOW store



Yellowstone Instruction Manual and User Guide: PDF
Yellowstone firmware update version 22-01-08: ysupdate-220108.dsk

Supported Drives

Need to attach a disk drive to your Apple II? Yellowstone has got you covered.

Yellowstone is compatible with these disk drives and drive emulators:

Can be connected directly
Disk II A2M0003 Naked Apple 3.5” 800K, black label
Naked half-height Apple 5.25″ Naked Apple 3.5” 800K, red label
Floppy Emu 5.25 mode Naked Apple 3.5” 1.44MB, auto-inject
Floppy Emu Model C dual 5.25 mode Naked Apple 3.5” 1.44MB, manual-inject
Floppy Emu 3.5 mode wDrive
Floppy Emu Unidisk 3.5 mode [1]  
Floppy Emu Smartport hard disk mode [1]  
  And more…
Requires optional DB-19F connector
Unidisk 5.25 A9M0104 Macintosh 800K External M0131
Disk IIc A2M4050 Apple SuperDrive (Apple FDHD Drive) G7287
Duo Disk 5.25 A9M0108 Unidisk 3.5 A2M2053 [1]
AppleDisk 5.25 A9M01075 Applied Engineering 3.5 [2]
Laser FD100 5.25  
Apple 3.5 Drive A9M0106  
  And more…

[1] smart drive
[2] partial compatibility

A naked drive is a bare drive mechanism without any enclosure, such as an internal drive taken from a Macintosh, Apple IIc, or Apple IIc+.

3.5 inch disks should be 800K type. 1.44 MB disks, 400K disks, and 400K drives are not supported.

Connecting Drives

Yellowstone includes two independent rectangular ribbon drive connectors on the card, named J1 and J2. Drives that use a rectangular ribbon cable can be plugged in directly to these connectors. Examples of such drives include the BMOW Floppy Emu, wDrive, the Disk II, and naked 3.5 inch drives such as internal drives removed from a Macintosh.

Drives that use a D-shaped 19-pin DB-19 connector can be plugged in to Yellowstone with the aid of a DB-19F female adapter cable, an optional item available from BMOW. Examples of such drives include the DuoDisk, AppleDisk 5.25, Unidisk 3.5, and Apple 3.5 Drive. The DB-19F adapter has custom wiring that’s specifically designed for use with Yellowstone. Do not use the adapter with any other equipment.

A maximum of 2 standard disk drives can be connected to Yellowstone simultaneously, or up to 5 drives when mixing smart and standard drives. Pairs of standard drives may be daisy-chained together when the two drives are the same type: two standard 5.25 inch drives or two standard 3.5 inch drives. Daisy chains of mixed-type standard drives are not supported. Smart drives may be daisy-chained together, with up to four smart drives in a chain.

Boot Behavior

Yellowstone boots from Drive 1. If no boot disk is found in Drive 1, then Yellowstone will normally check Drive 2 for a boot disk. If Disk II Mode is enabled, then only Drive 1 is checked for a boot disk.

Older Apple II models like the Apple II+ may not automatically boot from Yellowstone at power-up. To boot Yellowstone on these computers, type PR#n at the BASIC prompt, where n is Yellowstone’s slot number, or enable Yellowstone’s Disk II Mode.

Firmware Updates

BMOW may provide Yellowstone firmware updates to add new features or fix bugs. These firmware updates can be applied to Yellowstone using the Apple II computer itself, without any special equipment. The updater program is distributed on a 5.25 inch disk image. You can boot the computer with this disk, using a drive emulator like the BMOW Floppy Emu, or by using a tool like ADTPro to make a physical 5.25 inch disk from the disk image. You can boot the disk using Yellowstone or another disk controller.

Disk II Mode

When necessary, Yellowstone can enable Disk II Mode, where it behaves exactly like a standard vanilla 5.25 inch Disk II Controller Card, and other types of drives will be ignored. This can be helpful for the small number of 5.25 inch disk programs that won’t boot correctly when using Yellowstone’s normal mode, due to incompatible copy-protection schemes or low-level assumptions about the disk-controller hardware.

Disk II Mode can be enabled temporarily by holding the Control and D keys while booting, or can be enabled permanently using a DIP switch on the Yellowstone card.


Yellowstone was developed by Steve Chamberlin here at Big Mess o’ Wires, with over four years invested in development. Its original goal was to duplicate the Apple Liron disk controller for so-called “smart” drives, but the project later expanded to include all Apple drive types. The Yellowstone hardware is powered by an FPGA – a programmable logic device that replicates the behavior of the IWM chip and various support chips normally found on other disk controller cards. This gives Yellowstone unparalleled flexibility and control over every aspect of disk I/O, and the ability to change its behavior through firmware updates. The concept and prototyping efforts were discussed here in a series of more than 60 technical blog posts during the years of development.


10 Comments so far

  1. Jeremy Moskowitz - January 13th, 2022 4:54 pm

    You are amazing. I just love all the products you produce and the attention to detail. Keep it up and keep crushing it !!

  2. enrico - January 14th, 2022 3:23 am

    Congratulations, Sir!

    At first I thought that it would support 1.44MB floppy format. Why this limitation?

  3. Steve - January 14th, 2022 8:00 am

    Yellowstone supports 800K Apple format 3.5 inch disks, which are the standard for most Apple IIgs software and for other 3.5 inch Apple disks. 1.44MB floppies use a very different standard, with a different low-level encoding and a higher bit rate that would require more hardware and support logic. In the Apple II world, 1.44MB disks were only ever supported by the G7287 drive and a few third-party drives. It was mainly a Macintosh and PC standard.

  4. Michael Keeney - January 14th, 2022 1:44 pm

    Very impressive! I love your Floppy Emu and this is just another great product for the Apple II. I’ll look forward to buying one when they become available.

  5. Joel McWilliams - January 14th, 2022 2:45 pm

    Steve, congratulations! Your talents and perseverance are amazing. I am so glad you finished this project and I can’t wait to buy multiple cards. The quality of your work is top notch and I thank you and the other hardware and software geniuses for creating these new products that are helping to keep our hobby alive.

  6. jason - January 14th, 2022 3:21 pm

    Awesome, congrats!

  7. GaryFDes - January 20th, 2022 5:04 pm

    Yes! Congratulations!

  8. Maurizio - January 22nd, 2022 1:19 am

    Great Steve. But is an Apple IIe Card planned?

  9. Steve - January 22nd, 2022 7:30 am

    Yes, Yellowstone is compatible with the Apple IIe. Basically any Apple II model with expansion card slots.

  10. Steven - January 22nd, 2022 2:48 pm

    Hopefully you’ll be able to take my money for one of these in the near future!

    Nice work.

    The icing on the cake would have been 1.44M MFM mode, but I understand it would have driven the price up to provide a feature that relatively few folks really need.

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