15 Nov
2007
15 Nov
'07
5:18 a.m.
On Wednesday 14 November 2007 23:44, Fred Cisin wrote:
Ok. I'm glad I asked. :-)
> 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. 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