1771 floppy controller questions

From: Tothwolf <tothwolf_at_concentric.net>
Date: Tue Nov 20 03:46:58 2001

On Tue, 20 Nov 2001, Richard Erlacher wrote:
> From: "Tothwolf" <tothwolf_at_concentric.net>
> > On Mon, 19 Nov 2001, Richard Erlacher wrote:
> >
> > > You can get one for cheap if you go down to the thrift store and buy an old
> > > TRS-80. That will cost less than a new WD1771, I'd bet.
> >
> > That conflicts with my ideals and goals on collecting/repairing these
> > machines. The expansion interface for the model I also seems to be
> > somewhat hard to find, as I haven't seen one in a thrift store in quite
> > some time. The model 3 and 4 have a different 1771, which I seem to
> > remember is not an exact replacement.
>
> The model 3 has a 1793, which is very similar, but not identical.
> The 1771 has an inverting data bus, and the MFM-capable version with
> an inverting bus is the 1791. I do believe I saw a message that
> indicated that BG Micro has the WD1771 for $4.95. I doubt there's a
> version that wouldn't work in the Model 1.

I figured the model 3 used the same controller as the model 1, but it
makes sense that they changed that part of the design. I own both a model
3 and 4, neither of which currently work. Any ideas on where a service
manual can be found for either of these? I also have a model 2 that worked
the last time I pulled it out, tho I'd like to find a manual for it too,
since I imagine I will have to service it at some point in the future.

> You're right, in that the model 1 expansion box is not a really common
> item. I've not seen one in quite a while. They've probably gone to
> the great beyond by now.

Kinda sad too, I used to see this kind of gear in the resale shops all the
time. Over the last 4-5 years, very little of it seems to be showing up in
the places I used to see it in.

> The 1771 was quite popular, and had several second-sources, so perhaps
> the NS part is worth a try. In the Model 1's single-density
> application, almost any 1771 should work fine, though working fine
> doesn't really mean working reliably.

I've heard a few stories about reliability problems with the expansion
interface's floppy controller. I sure hope not all of them were true ;P

> What I'd suggest is that you synchronize the data from the drive with
> the 1771 by interposing a pair of 74HCT74's between the FD data in
> (after the schmidt trigger) and the 1771, clocked with the fastest
> harmonic of the FD clock that you can find on board. That forms a
> dual-rank register and synchronizes the data in a way that has a good
> chance of improving the system performance. You might confer with Tony
> Duell, as he's apparently persuaded the M1 FDC to work fairly well.
> An upgraded data separator was quite a popular enhancement.

The original owner of this model I did add an external data separator
board between the expansion interface board and the 1771. Would that board
have provided a similar circuit?

-Toth
Received on Tue Nov 20 2001 - 03:46:58 GMT