An Interrupt Driven Floppy Disk Controller for the S-100 Bus

At the time this written I was married to this girl and attending Georgia Tech as an Electronics Engineering major. She worked for a data processing firm in Atlanta just across the interstate from Tech, Data Systems Corporation, and she insisted that I needed to meet one of her coworkers, a fellow named Chester P. Quinn. Her logic for this was that he was really smart and knew a lot about computers. I have never understood the value a person might have just based on smarts. Finally I gave in and went to meet the guy after class one day. He had an office upstairs from the data center, and I trudged up there trying to be enthusiastic. At the time I had my wonderful book bag, my daily carry, and it had my usual content. So Chester asked me right off if I knew anything about microprocessors. So i responded, “Cat got an ass? Oh yeah brother.” and pulled out some 8080 manuals and a magazine or two. Told him this area was mine. Showed him an article from Radio-Electronics magazine about the MITS Altair S-100 computer and said see this, it is the near future, and this is a kit these folks are peddling. It is happening now.

Then he interrupted me and asked if I smoked pot. Well yes, as a matter of fact… So he says push the door to, and pulled out a bag. Then he asks me if he buys one of these computers would I put it together for him? Chester was legally blind and wore the original Coke Bottle Bottoms glasses. To read the code on a screen he had to put his face on the screen. So sure, Hell yes, I build your computer. At the time of our meeting I had been playing with some UARTS, and I had this little perfboard with one on it and had it rigged to send and receive and I showed it to him as well. Cute with LEDs, kawai. Long story short, we ended up doing a dog and pony show for the owner of the DP company, and we were in business.

Now the thing is that this was in the introductory era of microprocessors, just as they became somewhat useful, and we had decided to build our own machine. Two MegaHertz 8080 Intel processor. Not fast enough to run a floppy disk in polled mode, so we decided to roll our own freak DMA by jamming an Interrupt instruction onto the buss every data byte. In these adventures CVhester supplied half the brainpower and all the software. I supplied the other half of the brainpower and ll the hardware. So this design is a slight mod of the card I designed for our machine, this design being appropriate for an S-100 buss machine.

Another thing about our mnachine was we put the BIOS in an EPROM, and I gave Chester the first card I wired up, and he already had the BIOS in an EPROM! Cool, eh? So I asked him to make me a copy. He said no, here, and gave me a floppy disk. Tell him what was in track #xx and sctor #yy and he would give me an EPROM. Smart ass, I had to enter the data (program) and assemble it, but hey one thing I understood from the get go; microprocessors were code and hardware nuts to butts (US Navy slang, I was already a veteran) so it didn’t take long. You really need to grasp both parts of the machine.

One thing my bride at the time was absolutely correct about, Chester was one smart fellow! Actually brilliant. And I think he may have been a saint. But that’s part of another story.