In these days, a proper API would have been a .sys driver loaded in config.sys, or a TSR (terminate but stay resident) program leaving some routines in memory, hooking a software interrupt as the entry point.
The (even then) ancient laboratory equipment I used in the 2000s, and often moved to more modern machines, never did this. It was all monolithic applications directly talking to parallel ports (port read/write to 0x378) or to memory mapped on the ISA bus. On the other hand, most stuff came with schematics, and from this most of the logic (short of a few PALs with blindingly obvious functionality) could be deduced. It all was quite simple stuff.
But as long as hardware is still available (as are the Compaq computers in the article) investing the time isn't worth it. Would I have to do this today, I'd probably run the applications in DOSbox, add a emulation of the old hardware to the emulator, and pass the commands via USB or Network to a FTDI USB to something chip. Or to a BeagleBone/Raspberry/... with its GPIOs connected to the real hardware (mind the 5V levels).
Unrelated to software running on old PCs, here's a guy who replaces the old CPU in the lab equipment with an embedded board running an emulated version of the original software:
The (even then) ancient laboratory equipment I used in the 2000s, and often moved to more modern machines, never did this. It was all monolithic applications directly talking to parallel ports (port read/write to 0x378) or to memory mapped on the ISA bus. On the other hand, most stuff came with schematics, and from this most of the logic (short of a few PALs with blindingly obvious functionality) could be deduced. It all was quite simple stuff.
But as long as hardware is still available (as are the Compaq computers in the article) investing the time isn't worth it. Would I have to do this today, I'd probably run the applications in DOSbox, add a emulation of the old hardware to the emulator, and pass the commands via USB or Network to a FTDI USB to something chip. Or to a BeagleBone/Raspberry/... with its GPIOs connected to the real hardware (mind the 5V levels).
Unrelated to software running on old PCs, here's a guy who replaces the old CPU in the lab equipment with an embedded board running an emulated version of the original software:
http://www.jks.com/5370/5370.html