Tony Duell ard at
Thu Dec 4 12:54:37 CST 2008

> Chris M wrote:
> > what I had been thinking was shouldn't it be somewhat *easy* to make an IDE
> > drive into an MFM. Just strip off what you don't need!
> ISTR there's still quite a bit of logic on a ST506/412 drive (I think the 
> manual's kicking around on Bitsavers and contains schematics) - and I'd assume 

There's a fair amount of circuitry, but it's quite simple in fuction, and 
splits into several separate blocks. It's basically similar to a floppy 
dirve in that you have : 

Spindle motor (get the disk turning at the right speed -- unlike a 
5.25"/3.5"/3" floppy drive, a hard disk turns all the timem, there's no 
motor_on/ signal no the interface connector)

Head positon. On older/simpler drives this is a stepper motor, later ones 
ahve a voice coil postioner and have conisderably more complex circuitry. 
Often, even in the stepper motor positioned drives this is based round a 

Index pulse. On the original ST412, this was a proximity sensor that 
detected a magetic 'flag' on the spndle motor, this 'flag' being balanced 
by a non-magetic one oposite it.

Write : The normal differential write driver driven from a toggle 
flip-flop clocked by the wtite data pin. Proaly some kind of write 
current control (either driven from the RWC/ pin on the connector or 
automatically from the positioner control circuit)

Head switch. Again similar to a floppy, a diode-based switch

Read : Normal differential read amplifiers followed by a comparator and 
the nn a pulse generator. 

If you compare the circitry to that of an old-ish floppy drive (TM100 or 
similar), you'll see that although the component values change (the hard 
drive works at a muuch higher data rate), the circuiry is very similar in 

The one dead IDE drive I pulled apart used a voice coil positioner. There 
was no separate servo head AFIAK, presumably this thing used an embedded 
servo system. Which means you can't write just where you like on the 
platter, you mustn't overwrite theservo bursts. Obviously the internal 
'IDE' controller enofrces this, but you couldn't simply feed an ST412 
type of bitstream to the heads.

As I said, the electronics was built round a few ASICs. It would be very 
difficult to modify.

As a more prcatical issue, I thought the idea of this project was to be 
able to use a more modern drive with machines that expected an ST412 kind 
of interface. If you have to modify the IDE drive, then presumaly it 
would only work with that particualr model of IDE drive (and maybe only 
one version of it). This hardly seems a Good Thing, I susepct finding the 
'right' type of IDE drive (== the one that the original desiger hacked 
about iowth to make it have an ST412 interface) would be harder than 
finding an ST412.


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