Tarbell helped make me insane

Roy J. Tellason rtellason at verizon.net
Wed Nov 14 23:18:44 CST 2007


On Wednesday 14 November 2007 23:44, Fred Cisin wrote:
> > > Then they switched to selling a sandwich board that added a 179x for
> > > double density, while still retaining the 1771, for the oddball DAMs
> > > that it could provide (needed for TRS80)
>
> On Wed, 14 Nov 2007, Roy J. Tellason wrote:
> > What was oddball about DRAM (Is that what you meant?) for those machines?
>
> Well, I'm sure that there ARE some oddball DRAMs, but that ISN'T what I
> meant.
>
> DAM is Data Address Mark.

Ok.  I'm glad I asked.  :-)

> Oversimplifying just enough to offend the experts, . . .
> it is a slightly "out of spec" byte (such as missing a clock pulse) that
> can be used to provide an out-of-band signal to the disk controller,
> to mark the start of a block of data.
> The 1771 could provide a bunch of different ones.
> The 179x could provide some, but not all of the ones that the 1771 could
> handle.

This is interesting,  I had no idea about those differences between those two 
parts.

> Unfortunately, Model I TRS-DOS (1771) used some that the 179x could not
> create.  Therefore, there wasn't any practical way to create a disk
> using a 179x that UNMODIFIED Model I TRS-DOS could handle.

Makes sense.

> There are unconfirmed stories that the reason that TRS-DOS used "oddball"
> ones was due to a misprint on a spec sheet.

Heaven only knows,  at this point.

I don't recall having those 17xx parts in the datasheets yet, I guess I'll 
have to dig some up and add them in there.  I also remember one that I think 
I ran into in some of the Osborne stuff -- MB8877 sounds about right -- that 
seemed to be equivalent to one of those.  And there was some common data 
separator part I used to run into all the time, too -- 9316 sound at all 
familiar?  I forget if that's it or not but at one point in time I found 
myself with three different machines and all three of them had the same FDC 
and data separator chips in them.

-- 
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space,  a critter that can
be killed but can't be tamed.  --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James 
M Dakin




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