MITS 2SIO serial chip?

From: Peter C. Wallace <pcw_at_mesanet.com>
Date: Sat Dec 15 09:38:42 2001

On Fri, 14 Dec 2001, Richard Erlacher wrote:

> I don't think the DART would have been such a mess. If you AND (/IOR and /IOW)
> you get a useable IORQ, not that you really need it, since it's only used in
> conjunction with M1 to signal the mode-2 interrupt acknowledge, which wouldn't
> occur in this case. If the device is selected I'm not at all sure it cares one
> iota whether IORQ is active.

Not true, the DART like the SIO has no write signal...

>
> see below, plz.
>
> Dick
> ----- Original Message -----
> From: "Peter C. Wallace" <pcw_at_mesanet.com>
> To: <classiccmp_at_classiccmp.org>
> Sent: Friday, December 14, 2001 6:45 PM
> Subject: Re: MITS 2SIO serial chip?
>
>
> > 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)
> >
> Not exactly direct. you do have to invert the ALE to form the DataStrobe or
> whatever that signal was. I always liked the 8250 because it was a 1-part
> solution to a problem otherwise using two or more parts. It is a convenient
> part for the ISA, but since the ISA presents all the other signals, /IOR, /IOW,
> etc, from which you can derive the required signals in a 16L8 anyway, which is
> what most of them used for decoding the addresses, you could make whatever
> signals you needed.

Yes exactly direct! There is only decode and direct connection from IOW to
input data strobe and IOR to ouput data strobe. Take a look at the Asyc
card schematic in the XT tech ref. ALE is not needed for I/O on the ISA
bus, only for latching the LA bus (which is above the 64K limit of I/O).



> > > >
> > > 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
> >
> >
>
>

Peter Wallace
Mesa Electronics
Received on Sat Dec 15 2001 - 09:38:42 GMT