MITS 2SIO serial chip?

From: Peter C. Wallace <pcw_at_mesanet.com>
Date: Fri Dec 14 19:45:44 2001

On Fri, 14 Dec 2001, Richard Erlacher wrote:

> <snip>
> > Dart is Z80 bus, (like an Async only SIO) not Intel. Have to take your
> > word on the TCM78808 - sure it was available in 1981?
> >
> My old TI datasheets are hiding, so I can't verify what was when.
>
> I'm not sure exactly what difference it makes whose bus the serial I/O chip is
> designed for when the whole bunch of devices use essentially the same signals to
> get the job done. The DART doesn't work much differently than any other serial
> I/O device so long as you don't attempt to use some of its unique features. It
> seems to me that it takes about the same quantity of machinations to make the
> 8250, which is also not ideally suited to the ISA bus, work on the ISA bus, as
> it would take to make a Z80 DART or an 8251 or a 2651 do the job. Likewise for
> the 2681/68681. No matter what you need, a small PAL will do the trick. That
> certainly wasn't lost on I/O board makers.


The 8250 is a direct ISA bus interface (no logic other than decode needed)
The DART would be a mess, using the Z80s M1,IORQ and all that. (not that
there's anything wrong with the Z80 way)

> >
> I checked the actual board, and the PLCC part that I designed in to the board I
> was thinking about. It turns out the early version used a few 68-pin PLCC
> sockets, and, in fact, there were no 44-pin PLCC's on that board. The part in
> the PLCC socket, BTW was not a PLCC, but a JEDEC 'C' package. Though there was
> paper for the PLCC, the only parts used on the prototype board in that
> application were in the JEDEC 'C' package. Fortunately, unlike the JEDEC 'A'
> package, (that leadless single-sided ceramic chip carrier in which i80186's and
> i80286's were commonly used) the 'C' package would easily work in a PLCC socket.
> A later version, however, did, indeed have the 8250's in the PLCC-44 on it.

Sure, there are 8250's (and 16450's and 16550's etc etc) in PLCCs, just
not in 1981...

> >
> I really don't think practical considerations such as cost entered into the
> early decision stream in the PC development, once it reached the point at which
> upper management was prepared to pull the plug if at least one milestone wasn't
> met. The way I heard the story from some of the guys who worked at Boca was
> that there wouldn't have been an IBM PC if Intel hadn't presented the guys with
> a board-level prototype of the '188 (not an application of the '188). While
> it's easy enough to believe that the entire project had deteriorated into a
> "Chinese fire drill," I can't believe that Intel would have had the brains to
> present a canned solution to them in time to pull the chestnuts from the fire.
>
> Dick
>
> > > > > ----- Original Message -----
> > > > > From: "Peter C. Wallace" <pcw_at_mesanet.com>
> > > > > To: <classiccmp_at_classiccmp.org>
> > > > > Sent: Friday, December 14, 2001 12:28 PM
> > > > > Subject: Re: MITS 2SIO serial chip?
> > > > >
> > > > >
> > > > > > On Fri, 14 Dec 2001, Gene Buckle wrote:
> > > > > >
> > > > > > > > NS* did use them as did many others. The worst chip was
> > > > > > > > the 8250.
> > > > > > >
> > > > > > > Which makes me wonder what possessed IBM to pick it for the PC.
> > > > > > >
> > > > > > > g.
> > > > > >
> > > > > > The same reason they chose active high edge triggered interrupts on
> the
> > > > > > bus (wrong on both counts)
> > > > > >
> > > > > > The same reason they used 8 bits of an 8255 to read the KB shift
> register
> > > > > > that had a (unused) tri-state
> > > > > >
> > > > > > The PC = A horrible, amateurishly designed kluge
> > > > > >
> > > > > >
> > > > > > Peter Wallace
> > > > > >
> > > > > >
> > > > >
> > > > >
> > > >
> > > > Peter Wallace
> > > > Mesa Electronics
> > > >
> > > >
> > >
> > >
> >
> > Peter Wallace
> > Mesa Electronics
> >
> >
>
>

Peter Wallace
Mesa Electronics
Received on Fri Dec 14 2001 - 19:45:44 GMT