Floppy controller Q - VCO Inhibit / VCO Sync, and IBM format

Chuck Guzis cclist at sydex.com
Tue Jul 29 16:49:23 CDT 2008

On 29 Jul 2008 at 20:55, Philip Pemberton wrote:

>    At the moment it's driven permanently high; this seems to work OK insofar
> as the sync detector (which is why the datasep is there -- to extract a clock
> signal for the MFM data stream) will pick up SYNC-A1 signals and the data
> seems to be valid. What I don't know is if this is how things are supposed to 
> be done...

ISTR (and it's been a long time since I read the document) that the 
later versions of the 765 (765A?) imposes something like a 500 usec. 
VCO inhibit after the leading edge of INDEX/.  Earlier versions (765, 
8272) imposed something like a 1000 usec. VCO inhibit.  This can be a 
real problem if the original disk was not formatted with an IAM, 
leading to failure to recognize the IDAM for the first sector on the 

Even so, the 500 usec. "blind spot" exhibited by the 8272A/765A can 
be a problem for diskettes formatted on other systems where a too-
short gap occurs before the first sector header.  This leads to all 
sorts of dodges in reading them on PCs, such as taping over the index 
hole or tweaking the drive spindle speed down a bit.

WD controllers of the 17xx family did not do this; you could start a 
sector very close to the index pulse and still be fine.

My advice would be to fuggedaboudit, particularly if you want to read 
diskettes produced on other systems than PCs.


>    The uPD765 and 827x datasheets are predictably rather sketchy on this 
> front... All they really say is that the VCO line inhibits the VCO in the PLL, 
> which would have the effect of allowing the PLL's loop filter to discharge, 
> and reset it to a predetermined state. What they don't say is under what 
> conditions the FDC will do that...
> So I guess the million dollar question is what I should do with said VCO line. 
> Wire it to /INDEX via an inverter to reset the PLL on every rotation? Or just 
> wire it to VCC (VCO enabled) and leave it?
> Thanks,
> -- 
> Phil.
> classiccmp at philpem.me.uk
> http://www.philpem.me.uk/

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