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