The TRS-80 Journey Continues

Mike Katz bitwiz at
Thu May 5 15:50:01 CDT 2022

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.

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.

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 hope this helps.

On 5/5/2022 3:28 PM, Fred Cisin via cctalk wrote:
>>>> Next step...
>>>> Apparently mu original Tandon floppy has bought the farm.
>>>> Does anyone know what the jumper setting are for a
>>>> TEAC FD-55GFR-149 for the TRS-80 4P?
>>>> I tried the usual (like for a DEC RX-33) but that didn't
>>>> fly.  Every sector fails verify after a format.  I am sure
>>>> it's a jumper thing.
>> On 5/5/22 13:34, Mike Loewen via cctalk wrote:
>>>     The FD-55GFR is a 1.2MB drive, double-sided. The Model 4P used 
>>> 360K drives (possibly Teac FD-55A), single-sided as installed at the 
>>> factory. Are you sure that's the model number that was in your 4P?
> On Thu, 5 May 2022, Bill Gunshannon via cctalk wrote:
>> I know it wasn't.  The original was a Tandon 50.  But the TEAC
>> FD-55GFR (in various flavors) was used in a lot of systems and
>> at least one website said they were used in the 4P.  It is an
>> extremely configurable drive that I have used for numerous DEC
>> systems (even the infamous RX50!)  I have several of them and
>> it should work just fine if I can find the magic jumper combination.
> The Teac FD-55GFR is a 96tpi drive.  It can replace a 55G (1.2M), or a 
> 55F (720K) depending on controller, but it still has a narrow head for 
> doing 80 tracks.   It will have to be jumpered to run at 300RPM, not 360.
> The FD-55B is double sided 48tpi ("360K"), and can be used in the 4P.
> I don't know what, if anything needs to be done to use double sided in 
> a 4P (software patches? jumpers on disk controller?), but people DID.
> It's a worthwhile modification to do.
> The 4D, and the later models of the 4P used Teac double sided drives 
> (55B?)
> On the other TRS80 models, the drive select jumpering was a little odd 
> (although nor as odd as PC).  Like IBM, later, Radio Shack did not 
> want to trust their people to be able to do the drive select jumpers.  
> So, they jumpered DS0, DS1, DS2, and used side select as DS3 on model 
> 1!, then used a cable with the other drive selects removed.  So, with 
> the stock cable, the first drive should work as DS0, but jumpers of 
> DS1 and DS2 won't have any effect. 
> Discusses the drive selects and Mentions that you can use 55GFR, but 
> doesn't give the details that you need  (300/360 RPM, double stepping, 
> etc.)
> But, more importantly, is the fault ONLY when format/verify?
> Does it still READ disks that you already have?
> One more possibility to pay attention, (you probably already know 
> this, so sorry if saying it offends you), ...
> You need to use 300 Oerstedt ("360K") diskettes.  If you use 600 
> Oersted (1.2M) disks, they will seem to work, and then immediately, or 
> VERY soon, lose their content.

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