Homebrewing a classic

From: Chuck McManis <cmcmanis_at_freegate.com>
Date: Mon Oct 5 18:58:06 1998

Some thoughts on this homebrew stuff...

Front Panels:
-------------
I've been mulling over the idea of building a "generic" front panel.
Clearly there is a lot of resonance with the blinking lights of the
PDP-series, the Altairs and IMSAIs etc. However if I did this it would have
to be both durable and cheap. Having rebuilt a couple of PDP-8 KC8's I
think the rocker over the slide switch can be cheap but it isn't durable.
I've been sketching ideas based on a rocker that is threaded on a rod. The
back of the rocker contains a 'paddle' which when the rocker is down,
breaks an IR/detector gap. This has a couple of benefits. One the rockers
are on a rod so they don't have small pins to break off, and two the switch
mechanism is optical and thus not subject to oxidation decay that
mechanical systems suffer. Of course I would like the front panel to be
usable for a variety of hardware designs and that leaves open the question
of 'generic' switches. Clearly there are the following switch requirements:
        Run/Stop
        Single Step - momentary
        Deposit - momentary
        Examine - momentary
        Load Address - momentary
        switch register - what size? Could have 8 to 32 bits if we wanted
        Key switch - Off/On/Panel Lock

Some panels adopted a dual action switch for 'examine'/'examine next' and
'deposit'/'deposit next'. Clearly that's harder to do with my simple
optical switch.

Compound Computers
------------------

I've about half decided to build something PDP-8 like out of PIC chips.
They are relatively cheap, easy to program, and quite fast. I hadn't
thought about using them as base units (microcode sequencer, ALU control,
etc) until this discussion began, but it also raised the point of
'emulators' and I began to wonder what if you build a Z80 system that
emulated a PDP-8 with one of these custom front panels. That turns out to
be more of a software effort than a hardware effort.

Transistor based CPUs
----------------------

You can get PC boards made fairly cheaply, if you wanted to do a transistor
based CPU I would definitely consider having a few hundred 'flip chip'
equivalents made. It wouldn't have to be 1:1 DEC replacements but they
showed it could be done. Wiring up 12 flip flops by hand on a piece of perf
board (4 transistors each!) would be extremely painful.

TTL Logic based CPUs
--------------------

First, find a 50Amp 5 volt supply. :-) The thing about TTL logic that
always amazes me is how much power it draws. I built a digital PLL out of
TTL chips once (about 40 chips) it was easily drawing 3-4 amps at speed.
74HC logic might be a good compromise here.

The use of CRTs
---------------

There is a really clever homebrew display that was influenced by early
computers called the 'beer budget graphics display' and was featured in
BYTE magazine in the late 70's. Basically this was two R2R D/A ladders
driving the X/Y inputs to an oscilloscope. If you make a 12 bit machine you
could build a 4K x 4K resolution display fairly easily. (You will be core
limited in terms of points to plot but what the heck right?) This type of
output was featured in several different early computers.

Anyway, just some thoughts...
--Chuck
Received on Mon Oct 05 1998 - 18:58:06 BST