TG43 signal and 8" drives on commodity hardware (was Re: WTD: 8-inch floppy drive)

Chris Elmquist chrise at
Thu Mar 3 20:15:46 CST 2011

On Thursday (03/03/2011 at 09:22AM -0800), Chuck Guzis wrote:
> On 3 Mar 2011 at 10:58, Chris Elmquist wrote:
> > What is the theory of operation behind a hard-sector filter?  I
> > believe I have seen this done with just a one-shot but I'm not sure I
> > understand how that eliminates only the sector marks, leaving just
> > index?  It seems like with just a one-shot, it would sync to any hole
> > and call that index.
> One of the things that makes this easy for 8" drives is that, but for 
> a few very early types, all 8" HS floppies are 33-hole (32 sectors + 
> index).  
> The index on an 8" HS floppy is located midway between two sector 
> holes.  So, for example, if you use a one-shot whose output width is 
> set to, say, 2/3rds of the time between index holes, any hole 
> occurring under that "umbrella" must be an index--so you generate an 
> "index" pulse.   Very simple, actually.

Ah... I see now.  The one-shot is used as a gate and if another index
pulse comes in while the gate is "open", then you let that pulse pass.

Thanks. Makes sense.

This should apply to 5.25" hard-sector too.  Same concept.

Chris Elmquist

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