The TRS-80 Journey Continues

Fred Cisin cisin at
Thu May 5 15:28:35 CDT 2022

>>> 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 

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.[19]
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, 

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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