Homebrew SC/MP system from 1980

Post by **Phil_G** » 27 Sep 2020, 21:21

I recently came across a bunch of friendly retro-computer nerds on the net and this has rekindled my interest in old 1970-1980 homebrew micro systems. Back in 1975-80 commercial kit was expensive and for most of us homebrewing was the only affordable way. I built this from a project designed by Andrew Aitken and published in Practical Electronics April 1980. It was a very limited system and programming was a very tedious operation using just binary keyswitches and LEDs. So I modified it quite a bit, adding extra memory, more I/O and a simple OS called 'Kitbug' in ROM with an RS232 interface to a terminal. I'd used a 4118 static RAM so it was easy to 'piggy-back' a 2716 EPROM since the pinout is identical.
Kitbug itself is very crude, having only three commands - 'T' to type out the memory contents, 'M' to modify memory, and 'G' to go to a program. I did some simple extensions to Kitbug, giving hex memory dump, tape load & store, and inline ascii string handling. Ultimately I intended to add NIBL which is a Tiny-Basic interpreter by National Semi, but at the time I couldnt find a ROM dump for NIBL so it never progressed beyond Kitbug. Very soon after this I built a Nascom so the SC/MP saw little use really but it still works here in 2020 and I occasionally fire it up just for old times sake. Its weird how op-codes committed to memory 40 years ago are somehow easy to recall from the old grey matter, yet I cant remember which day to put the bins out.

The "My Scamp loves me when I speak NIBL" scrawl on my 40-year-old notebook is a skit on the David Ahl BASIC games book which declared "my computer loves me when I speak BASIC"

The Tandy 100 is a recent addition, I've only had that for 30-odd years

Originally the SC/MP ran on an 'Elekterminal' which I built from an article in Elektor using the Thompson-CSF 96364 video chip. The Elekterminal is still up in my loft somewhere.

Incidentally, anyone who fancies building a really simple 1980-style BASIC machine should check out the "Jeff Graham Maximite" https://geoffg.net/OriginalColourMaximite.html.

More to follow, next up for 'show & tell' are the homebrew 6802 & Z80 systems...

Cheers
Phil

homebrew_scmp.pdf: (4.31 MiB) Downloaded 287 times

Update:
Well, the 'bit 7 set on echo' problem with the 'kitbugged' SC/MP is fixed, tonight the problem got to me and it had to go! I was only joshing when I said I might strip bit 7 with a PIC, but - never one to go looking for nutcrackers when theres a sledgehammer right there...
Terminal echo is perfect now, no funny characters makes it much more pleasant to use!

Video:

Cheers
Phil