An interesting Disk ][ Interface Card Fault

tl;dr: I recently got into 8-bit Apple retrocomputing due to my work with Adafruit. I had a Disk II setup that couldn't write to any floppy. I fixed it by replacing a 74LS05 IC on the Disk II Interface Card.

First I bought a nicely restored & tested disk ][. With its 20-pin connector it was super convenient to wire into a prototype board for interfacing with a microcontroller.

Soon, I picked up a //e (Also nicely restored) with a 19-pin D-style drive connector, and a later era 5.25" drive with matching cable (not quite as nicely restored as the rest, but ah well).

Things were going along nicely, but I had trouble implementing flux writing with my prototype controller. The reasons turned out to be due to an incomplete understanding of what a flux writing waveform for an Apple floppy disk looks like, but along the way I borrowed my friend Steve's Disk ][ Interface Card. Steve had remarked at the ease of frying these drives due to plugging in the connector backwards or off by a row. In fact, he remarked, all of his 20-pin Disk ][s had the same problem where they could read but not write.

With the new card installed in the //e and the drive connected, I discover that my supposedly-tested Disk ][ has the same fault as all of Steve's drives! hmmmmmm. Is the fault in the drive, or in the interface card?

With the help of a logic probe, I determined that while trying to write to a floppy, the output signal from the interface card called "WR REQ" (WRite REQuest) was never changing. This signal is produced by the 74LS05 chip on the interface card. Indeed, replacing the chip with a new one resolved the problem.

This is not one of the top component replacements to try according to websites and videos that I watched, but in this case it was the faulty component.

It is fervently to be hoped that when I return the card, Steve will discover that (at least some of) his drives work too.

Entry first conceived on 15 March 2022, 21:13 UTC, last modified on 15 March 2022, 22:11 UTC
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