Quapple Clones a Card That Turns an IBM PC XT Into an Apple II Plus Clone
Experience two dominate 1980s computer platforms in one box.
In the early 1980s, the Quadram Corporation created add-in boards for the Apple ][ computer and the IBM PC. One such card, called Quadlink, combined the two computing platforms into one. It allowed IBM PC-XTs to run Apple ][ software! Unfortunately, today these cards are difficult to find. However, thanks to Eric Schlaepfer, there is now a DIY option to owning one of these cards. The Quapple is a near-perfect hardware clone of the original Quadlink.
The original Quadlink card contained 80K of RAM, a floppy disk drive controller, a display circuit, an ISA interface, a 16-pin paddle socket, and of course, a 6502 CPU. In short, it is a clone of the Apple ][ Plus hardware. The only missing piece is the Apple ROM. The PC utility that initializes the card loads compatible instructions in the 16K of the card's 80K RAM, leaving 64K for software.
Like Quadlink, you can install Quapple into an IBM PC (5150) or XT (5160). Then, there are headers to connect a couple of cables for floppy disk compatibility and a pass-through for the monitor.
Schlaepfer created Quapple by first removing components from an original Quadlink card and then creating a high-resolution scanned image. That image helped the painstaking task of re-creating the PCB in Kicad. As the image below shows, Quapple (bottom) is nearly identical to the original Quadlink (top).
Just like the original Apple ][ design, Quapple uses off-the-shelf components. So, it does not contain any application-specific integrated circuits (ASICs). However, several programmable logic devices simplify the IC count.
Since this is a clone of the original Quadlink, it comes with the same limitations the original hardware did. While most Apple ][ (not IIe) software was compatible, the lack of original Apple ROM means some software will fail. Also, the disk controller cannot half-step the IBM floppy drive, so some copy-protected software will not load.
For reference, here is a 1983 InfoWorld article that highlights the limitations of the original Quadlink. And here is a more positive review of the device. Back in 1983, the card sold for $680. (Quapple's build cost is significantly cheaper than that!)
Schlaepfer does not plan to sell a kit. However, all of the information necessary to make one is available. Most ICs are still readily available, though sometimes from surplus sellers. The hardest to find is the 558 timer. (This chip combines four 555 timers into one package.) Schlaepfer warns that counterfeits are very common and that it may be better to build a substitute with 555s or 556s.
The Quapple GitHub repository has the KiCad files and binary images for the EEPROM and PALs. For additional pictures and background, check out this Twitter thread.
(Note, even if you do not plan to build one, check out the PCB files for some funny easter eggs.)