minor success at 2:00am

well I’ve got MFM flowing from the PC drive now.  A couple of things:

Pins 10 and 14, the pins I’ve been using since last night, appear to be correct.

Pin 10 is the motor lead.  Dropping a corresponding SX lead causes the motor to come on.

Pin 14 is the drive select lead.  Although this should technically be an input as far as the drive is concerned, I’m getting a +5v from the drive.  I’m measuring from a ground lead to pin 14, and getting +5 with the SX completely out of the picture.  If I take this lead and short it to ground, then the drive is “selected” and it works.

If I simply cause the SX to drop its lead (pin = 0) and attach it to pin 14, this does NOT work, and the multimeter still shows +5v.

Why doesn’t dropping the SX lead cause that line to go to 0v? (incidentally, measuring the SX by itself, the lead does infact go to 0v, so the SX/programming/pin/etc is ok)

And why am I getting +5v from the drive on an INPUT pin?!@#

The pull-up resistor is amazing on the output READ DATA lead.  It’s screwed up.  With no pull-up whatsoever, looking at the scope shows NOTHING happening.  Put a 1k  (well brown black red, Im too tired to go check, but for records sake) pull-up to 5v, and bam-o I have a perfect signal.  Much faster rise-times on the signal.  It also looks ummmm cleaner than the amiga floppy drive. not sure why yet.

Herb sent me some good links to his site here



Amateur Electronics Design Engineer and Hacker


  • There are 5 ground leads coming out of the drive. The bottom row of the IDC connector on the physical drive, which is all the odd pins 1-33 with the tab being right next to that row, only has 5 actual pins.

    They are 1, 11, 21, 29, and 33.

    I only have pin 1 connected to my ground. Any chance that pin 14, the drive select lead uses a different ground reference? I wonder if I connect up all the grounds if that problem I was seeing goes away?

    I need some automated way to select the drive.


  • well I connected all the ground leads and I’m still seeing the same problem.

    Drive Select is still outputting +5v from the drive to my SX. Even though Drive Select should be an input at the drive.

    I have shorted that pin to ground, although I’m not really sure how safe that is.

    Drive select is directly related to the LED on the front of the unit, if the unit is selected then the LED is on.

    I’m now able to step the drive, but no luck yet in actually reading data. This select issue is causing problems.

    I need to be able to control this, so shorting Drive Select to ground is not a permanent fix.

  • I would think that +5v you get from the drive’s DS signal isn’t actually a +5v source; it is rather ordinary DS just pulled to +5v via some pull-up resistor. It is a common technique with ‘active low’ signals to keep them disengaged (immune to noise etc).

    It is strange your SX can’t pull down the DS line. Does oscilloscope show any change on that line when SX drives it low?

  • I haven’t put a scope on that line, but I can do that.

    I’m going to measure the current between the DS line and ground, and then compare that to the max sink(i think) that my controller can do.

  • Turns out I’ve must of fried the damn pin on my SX. Had the SX raise and lower RB.7 and I didnt see my multimeter move.

    Change it to RB.6 and bam.

    of course I could’ve screwed something up with the programming. I’m going to swap chips and see what happens.

  • Swapped chips and the same code produces changes on RB.7. Yup. fried one pin. I’m surprised I didn’t do it sooner! 🙂

    Ahh well.

    This makes me wonder if this was the reason for my failure with the setup of the amiga drive.

    RB.7 just happened to be a connection to the USB chip before my major rewiring. And this USB chip has been active 24/7 for awhile, because I leave it plugged into my PC (and hence powered up)

    Screwy spike hoses the chip? Hrrmmm.. who knows.

    In any event, the only thing I’ve really lost is time — and at least the project has benefitted from the fact that I’m now using a cheap inexpensive easily available PC floppy.

    Onward and Forward!