Am I reading MFM wrong?

I’m beginning to think that I don’t fully understand the signalling coming out of the drive. I understand the concepts behind MFM (I think), but the actual application to this project, I’m not so sure.

As far as I can tell, I’ve got the code done properly on the SX to do what I want it to do. I take this information into the PC app I wrote, and I’m not getting anything intelligible. At the minimum, I should be seeing the Sync pattern (0xAAAA 0xAAAA 0x4489 0x4489) from the drive. The amiga puts this pattern at the beginning of a sector header. And this is raw MFM — so even if my decoding is wrong, I should be seeing this pattern, but I’m not.

I’m reading it wrong in hardware. What I think constitutes a 1 or 0 in terms of a signal, obviously does not. I find plenty of generic references online to MFM, and I have read them — but very few that actually show me a scope image. They have their lame handdrawn pictures but none show a real application.

I’ve bumped my scope from 2us to 5us just so I can look at a little more of the signal. I’m starting to see a pattern where ‘transition to 0, short positive’ or ‘transition to zero, long positive’ might be my two different states, but I dunno.

Time to read about MFM again……

More scope images

About the author

keith

Amateur Electronics Design Engineer and Hacker