The TRS-80 Journey Continues

Bill Gunshannon bill.gunshannon at
Thu May 5 17:41:38 CDT 2022

On 5/5/22 16:50, Mike Katz via cctalk wrote:
> If you use a 80 track (96 TPI) 5 1/4" drive to read 40 track (48 PTI) 
> floppies that will work but if you write onto them they will not be 
> readable in a 40 track drive any more.   The reason for this is that the 
> had is half the width and when the 80 track drives writes to a 40 track 
> written disk it only overwrites half of the track.  And when the 40 
> track drive goes to read the data it sees nothing but noise because the 
> two halves of the track are different.

Well known and documented back when we first started using 80 track

> If you want to write floppies on an 80 track drive and read them on a 40 
> track drive.  Magnetically erase the disks with a demagnetizer and then 
> format the disk in the 80 track drive but double stepping the format.

I always erase floppies with my tape degauser before formatting.  Even
new disks usually have some pattern written on them.

> Double stepping means writing track zero on physical track zero and then 
> writing track 1 on physical track 2 and track 2 on physical track 4, etc.
> This avoids the problem of tracks with different data on each half of 
> the track.  The 40 track drive will only see the half track data.
> Not all operating systems nor drivers can handle this.
> Gimix Flex for the 6809 could handle this as a special configuration.
> I'm sure you could get the greaseweasel to support this also.

I just got a GreaseWeazle.  Not impressed so far.

> I hope this helps.

All information helps.


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