DB25 #3 — yes, and …
It turns out that even improvisation has an orthodoxy. In comedy improv, some preach “yes, and …” as a set form to return serve and volley. Some allow “no, but …” as well but I don’t really care for either. In my circles, we prefer “tristate, XOR …” to keep the laughs rolling along on the bus.
Computer designers have an improv language all their own. Our first installment in the series was the was the DB25 connector used on the IBM Printer Adapter. A funny thing about that adapter is that the printers used a completely different connector — the micro ribbon 36-pin “Centronics” connector. Why the switch?
I think the main reason is that the Centronics connector barely fit on the card edge in the PC case. As laptops grew up in the mid-90s, the 8086 Toshibas with floppy drives and monochrome CGA displays gave way to 386 machines with hard drives and color VGA displays, eventually to machines powerful enough to be workstations in their own right rather than companions. As they did, new classes of machine sprang up to fill the companion role. At the tail end of the 80s came the first notebooks, then further miniaturized versions of the original PC laptop marketed as palmtops, then a stable island coalesced around the subnotebook.
Where the Centronics port was too bulky for the original PC, the DB25 port became almost too big for the subnotebook. Hard drives had gone from full-height 5.25” 10 MB drives in the original PC XT to 2.5” drives in the new subnotebook. Hard drives suddenly took up less space than floppy drives. Out went the floppies. Apple took a lot of grief for excluding a floppy drive from the original iMac in 1998 but Apple had already ditched the floppy half a decade earlier with the PowerBook Duo series of sub notebooks. Floppies might not fit with the program anymore but they were still necessary for exchanging information. Most subnotebook machines, therefore, had some story for plugging in an external floppy drive. Alas, there was no standard connector for an external PC floppy. The answer turned out to be surprisingly simple — Yes, and …
That original PC XT might have had a IBM Printer Adapter, an IBM Asynchronous Communications Adapter, an IBM Game Control Adapter, an IBM 5.25 Diskette Drive Adapter, and an IBM Hard Drive Adapter taking up 5 of its eight slots. Ten years later, the hundred or so chips spread across those boards had been condensed into a single ‘Multi-IO chip‘ made by any one of a dozen Taiwanese firms. WinBond was among the most prolific.
Because the multi-IO chip implemented the functions of almost all of the legacy peripherals, it was straightforward for it to also multiplex the floppy interface signals onto the parallel port. An unexpected benefit of letting WinBond choose the pin mapping from floppy to DB25 is that the proprietary external floppy drives for this generation of subnotebooks are more interchangeable than you might otherwise suspect. It also means that there were potentially millions of other PCs that were prepared to boot from a thing pretending to be a floppy drive connected to their parallel port — perhaps even an evil printer. Access is improvisational.