MITS 2SIO serial chip?

From: Peter C. Wallace <pcw_at_mesanet.com>
Date: Sat Dec 15 11:33:07 2001

On Sat, 15 Dec 2001, Richard Erlacher wrote:

> Well, that doesn't change the fact that if you AND /IOR with /IOW you get a
> useable /IORQ. Whether you feed the /IOW forward doesn't matter.

Except that you need to have a valid read signal before IORQ...

>
> I stand corrected on my presumption that the M1 and IORQ were used solely for
> the int handshake. I never liked the Z80 peripherals and never even once used
> one in my own work, though they were everywher in commercial hardware. Even my
> HP plotter has one it it, though it doesn't have a Z80.

        Actually I thought the Z80 peripherals were quite nicely
integrated with the Z80, with the status affects interrupt stuff, daisy
chained interrupts and other nice ideas. (many of which had been done
before in minicomputers) They also had consistant reset polarity's and
basically look like they gave a lot of thought to System Design, not just
chip design. When we moved from designing Z80 stuff to embedded PC stuff
it was like going back to the stone ages. The Intel stuff being a rag-tag
pile of ****

        The Z8000 stuff was even nicer, with Multiplexed data/address bus
so the all peripherals could have 256 directly addressed registers without
losing valuable pins on the 40 pin chips...



>
> I believe the reason they were so widespread in their application was that the
> Zilog and Mostek application notes seemed, unlike those from Intel and MOT, to
> work without any additional effort, so anybody could copy one and use it, to
> wit, the Ferguson Big Board, and Big Board II, among others just like it.
>
> The reason I didn't like 'em, BTW was because the peripherals limited the rate
> at which you could run your system without major effort, unless you didn't mind
> giving up the features for which you'd bought them.
>
> Dick
>
> ----- Original Message -----
> From: "Peter C. Wallace" <pcw_at_mesanet.com>
> To: <classiccmp_at_classiccmp.org>
> Sent: Saturday, December 15, 2001 8:38 AM
> Subject: Re: MITS 2SIO serial chip?
>
>
> > 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
> >
> >
>
>

Peter Wallace
Mesa Electronics
Received on Sat Dec 15 2001 - 11:33:07 GMT

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