The TRS-80 Journey Continues

Bill Gunshannon bill.gunshannon at
Thu May 5 17:27:51 CDT 2022

On 5/5/22 16:28, 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.

It has a jumper that leave that up to the controller.

> 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?)

I used double sided as far back as my first Model III.  Even had a
full height Tandon 80 track double sided drive.  All I ever needed

> 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, 
> etc.)

The 4P has the missing pins.  I never liked it and usually end out
putting in real cables and using Drive Select like it was intended.
At least until IBM won and the started making 3.5" drives without
any way to do drive select.

> But, more importantly, is the fault ONLY when format/verify?
> Does it still READ disks that you already have?

Haven't really tried that.  Most of the disks I have are over
20 years old.  If it fails to read who's fault is it?  The drive
or the disk.

> 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.

Yeah, I knew about that.  I still have piles of real floppies laying
around.  :-)


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