When Apple introduced the Apple III at the National Computer Conference in May of 1980 in Anaheim, California, it was touted as supporting up to 512K of RAM (as opposed to the Apple II and II Plus, which commonly shipped with up to 64K). Thus, I was somewhat surprised when I opened this Apple III and took a look at the installed On Three 512K memory board.
On Three's 512K Board
I don’t know specifically what I was expecting probably a similar set up to what you see in Apple IIIs with 128K and 256K boards. And that’s sort of what I got, with the exceptions of the additional wires and cables coming from the memory board and attaching to various spots on the motherboard. The person from whom I acquired the Apple III didn’t have any documentation about On Three’s expansion, so I was confused. Why would a 512K board need such additional electronics if the III natively supported that much memory?
On Three's board includes several modifications
One of the unfortunate side effects of the III’s short lifespan is that Apple never developed more than a handful of expansion cards for the machine. While cards such as the ProFile controller and the UPIC were produced in fairly plentiful numbers and are easy to find today, Apple never got around to releasing memory boards with more than 256K RAM, or much of anything else for that matter. This left it up to small third party developers to manufacture and support peripherals for the die hard users who refused to migrate to Macintosh or other Apple products.
One of these developers, a user group called On Three, holds a special place in Apple III history. While most UGs of the day held weekly or monthly meetings and offered other services such as shareware sales and newsletters to members, On Three took it much further. In addition to printing a professional monthly Apple III-specific magazine, they also published numerous commercial-quality software titles and the 512K memory expansion. The latter is doubly impressive, as Apple never released much in the way of low-level technical documentation for the machine, so it represents an excellent hardware hack in the truest sense of the word.
Back to the issue at hand. I had this neat add-on, and no documentation for it. Fortunately for me, when Dave Ottalini decided it was time to get rid of his Apple III collection, it ended up in the hands of Bill Degnan. Degnan photographed and commented on much of the collection, including the 512K board. It was his website that turned up in the Googles when I was hunting for more information. After a brief email exchange, Bill provided me a copy of the scan he made of the user’s guide. (You can get a copy here, if you’re interested.)
A quick perusal of the PDF gave me the answer to my question: the Apple III doesn’t actually support 512K. SOS does but not the hardware itself, at least not without some modifications. The manual doesn’t really go into much other technical detail, but there it is. In the mean time, enjoy some pictures of the board while my search for the specific function of On Three’s hacks continues.
Several hardware modifications are necessary to allow the Apple III to access 512K of RAM
On Three's new board includes addressing ROMs as well new address lines
In the end, the rather high $949 introductory price (On Three later dropped it to $449) and fact that it was introduced after the III was discontinued to an already-shrinking market, helped to limit the number of boards that made it into the hands of users.
Update (1/19): I added a gallery of high-resolution photos of the On Three board. Check it out in the Picasa galleries.