[sdiy] just when you think DIY spirit is fading out

[Dave Brown]

I also grew up on a PDP8. I always wanted to build a system with the
Intersil 6100. I still remember the instructions. I did look at the PDP8
kits but what would I do with a larger system once running?

We have a PDP8 at the museum that was being restored just prior to Covid. We
need to get back in and finish it. There are enthusiasts that are designing
their own boards so you can use SD memory, etc. We want to bring up a good 2
person video game using a vector monitor eventually.

You can see it in the blue rack in this photo
https://vintagetek.org/tek-labs-pdp-1134/
It was used for hybrid laser trimming at Tektronix.

On a different note, I designed an 8080 processor card for Tektronix in
1976. I modified one to somewhat emulate a Sol-20 and to run those programs.
One program was music which did PWM modulation of the interrupt enable pin
on the 8080. You just connected it to an amplifier through a small RC
filter. That pin simply said if the interrupts are enabled or not, so the
code just toggles the interrupts.

I disassembled the code as I wanted to port it to a doorbell. The code
consisted of a complier to translate English symbols for the three voice
notes and created the "program". It had to execute in RAM as it was
self-modifying code. It was a work of art.  I built a doorbell consisting of
the three chip 8080 set, ram, and a 2708 eprom. I added Sarabande which was
one of the demo music scores as the default music. I built in what we called
an LDA downloader at Tek so I could compile new songs on my 8080 system and
upload them to the computer. That system obviously is long gone but the
doorbell has been running now for 44 years. Eventually the 8080 died and one
capacitor in the -5V supply generator also died. But the 2708 has retained
its program for all these years. It sits in a loop waiting for someone to
press the doorbell button. We live in the country so people rarely do. Once
some time ago I calculated the loop time and how many loops it has gone
through in all these years. I'm too lazy to recalculate but reading the
switch is probably 2 cycles, 1 cycle for the compare, and 3 cycles for the
jump, so 6 cycles at 2 MHz. Call it 200K loops per second. However, I had to
add switch debounce for noise on the doorbell line (ask me how I know). Say
it was 20K loops per second. That is something like 2x10^13 loops total. I
probably computed this wrong but it is still an impressive amount.

Dave

> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Jay
> Schwichtenberg via Synth-diy
> Sent: Tuesday, October 19, 2021 11:08 AM
> To: synth-diy at synth-diy.org
> Subject: Re: [sdiy] just when you think DIY spirit is fading out
> 
> What can you say, everyone needs a hobby.
> 
> There are people that do things the old ways in a number of fields just
> because they like too. Some people look at synth DIY and say why not buy
> a box or use a soft synth.
> 
> Then there is this. I'm going for a PiDP-8 sincethe first computer I
> programmed was a Classic PDP 8 and I worked for DEC for a bit.
> https://obsolescence.wixsite.com/obsolescence
> 
> Jay S.