Emulators of Classic Computers

From: Holger Veit <holger.veit_at_ais.fhg.de>
Date: Wed Jan 21 02:52:09 2004

On Tue, Jan 20, 2004 at 01:44:52PM -0700, ben franchuk wrote:
> Holger Veit wrote:
> ^^^^^^^^^^^^^^^^^^^^^
> > Tssss. Not to discourage you, but a TTL computer with bazillions of ICs
> > is not really suited for newbies, although there is plenty to learn -
> > namely on those nasty aspects like circuit timing, delays, glitches,
> > noise on power and signal lines. TTL is to a large degree analog, not digital,
> > circuit development ;-)
> So are TUBE computers better?

The effects occur if you run the system at "high" speed - the definition of
"high" depends on the quality of construction. Wire-Wrap and breadbaords are
susceptible to cross-talk - wire lengths are a factor unless you run
almost static (the mentioned breadboard with a 2901 controlled by switches,
debounced with RS-Nand-FFs, is such a case.

Tube computers are larger and thus effectively run at by far lower speeds,
but they are likewise analog technology as discrete flip-flops and
RTL logic.

> > You might not directly want to start with a monster like Computer 74 or
> > EGO, but perhaps with a simple ALU/register device: get a 2901 circuit
> > from BG-Micro and play a bit with it (a breadboard with lots of switches
> > and LEDs is sufficient). And then extend it with a 2909/-10/-11 from the same
> > source and build the micro control for that toy. This is already the
> > basic step towards an own homegrown system.
> Using LS TTL ( also from BG-MICRO ) I expect I'll need about 32 chips
> for a 12 bit ALU and 32 chips for basic control. I suspect about the

And this can be had with just three 2901. Surely you could go the hard way
and assemble the ALU out of three 74181, some 74175s for the accu register,
and some glue logic as I did as well 20 years or more ago. Besides more
power consumption, more points of failure and larger boards you don't
gain very much.

> same for front panel logic. Since I don't have a PROM or PAL burner
> a bit slice design out of the question for me.

You don't need any PROM or PAL burner. You might put your microcode into
a fast cache RAM (from old PCs) and have some small serial shift register
logic to load it from a PC parallel port (a similar circuit BTW also allows
to program EEPROMs and flash ROMs from the parallel port). You would not
use PALs, but CPLDs which are bit serial programmable through a JTAG port -
provided you accept the use of modern circuits to replace TTL graveyards.
The first microcode I built for such a homegrown CPU I did with a diode matrix
after finding out that the 74S188 PROMs that were available those days were
too expensive for bug fixing (they could BTW also be programmed manually with
a simple socket and a number of switches).

Received on Wed Jan 21 2004 - 02:52:09 GMT

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