Floppy drive interfacing

Philip Pemberton classiccmp at philpem.me.uk
Mon Apr 9 03:42:35 CDT 2007


Chuck Guzis wrote:
> Actually, about 5 times per second.

Or 5 Hz * 60 seconds = 300RPM.

> Some drives won't output any signals until the onboard controller is 
> satsified that the drive is ready (i.e. that it sees an index pulse 
> every 200 msec).  I don't know if Sony's one of them, but I've seen 
> it.  Similarly, the drive must be satisfied that the disk is seated 
> in the drive.  I've got a Toshiba floppy drive that has a problem 
> seating the diskette firmly and it simply returns--nothing and the 
> FDC thinks that the drive's not ready.

That makes sense.

I think I've found the problem though - good ol' drive select issues. Based on 
the pinout I had, I expected that I'd have to activate /MOTEN_A (pin 10) and 
/DRVSEL_A (pin 14) to get the drive to spin and output data. That assumption 
was wrong - the drive wants pin 16 (/MOTEN_B according to my pinout) low, and 
either /DRVSEL_A or /DRVSEL_B low depending on how the drive is jumpered.

This goes back to the 'swap four lines' action the twist in the cable 
performs. If you want to select drive A, you activate MOTEN_A and DRVSEL_A 
(pins 10 and 14 respectively), and because drive A is after the twist, as far 
as it's concerned, MOTEN_B and DRVSEL_B are active (pins 16 and 12 respectively).

Because I was raising MOTEN_A and DRVSEL_A, had the drive jumpered for DS0 
(drive A), and used a cable with no twist, the drive would get selected 
(green/amber/whatever activity LED on) but the motor wouldn't spin because 
MOTEN_x was inactive.

Now I'm seeing an index signal on the analyser, and a data signal too. Now for 
Phase 2 - the MFM data separator.

This still doesn't change the fact that I've got a Citizen OSD-U that seems to 
have issues (the motor spins when the disc is inserted, DRVSEL turns on the 
LED, but the whole thing goes dead if DRVSEL and MOTEN are enabled with a disc 
in the drive), but my Panasonic JU257 and the Sony MPF520 are both running 
fine. I'll probably put the Sony back where it was (I borrowed it from my 
Acorn BBC Master 128 setup and it's the only drive I've got that can be 
jumpered to DS0) and use the Panasonic for further tests.

Thanks.
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