Friday 30 April 2021

Further work on a Microbee 1248-6 mainboard

This is the definition of a long-term project. I have a bare Microbee 1248-6 mainboard. It's one of the later revisions of this board. Being blank, it's a good candidate for replication.

Some years ago I created a schematic for it in Protel, and made a PCB. The PCB wasn't particularly accurate, as I mainly just threw the autorouter at it. So (probably) functional, but not particularly exciting.

Success at getting KiCad to do beautiful curved traces reignited the spark of that particular project, so I pulled out the files and had another look.

I imaged the board at 600dpi using a scanner. Then cropped a section of that and fed it into the gimp. I played around with levels to increase the contrast between tracks and boards.

Then I put the result of that into the KiCad bmp to component converter. This tool is designed for making fancy logos, and converts a bitmap image into a bunch of vectors that can be used for gerber plotting. The file goes on the silkscreens. I have seen a board made by one of the guys at the microbee software preervation project that used this technique to create gerbers directly, but there were issues with getting board houses to accept this.

So now the next step. Trace over the image in KiCad using proper components, tracks (15 mil) and vias. It takes a while, but it's enjoyable bead therapy work. There's no need to accurately follow the original. Indeed around corners it's better to leave the round the corners plug in to do it neatly.

For the bottom layer, just process the scan the same way, then convert it to a silkscreen component, and flip it when you place it.

This makes a real board, that can be DRC checked against a netlist (when I get around to putting the schematic into KiCad), to ensure that it's correct. Finally, run the board through the round the tracks plugin to smooth things out and create absolutely stunning artwork, even better than that used to make these computers in the first place.

Edit: After a few days work, it's looking pretty cool:

And with the soldermask obscuring all the pretty curvy traces:

Finally, I did the schematic entry in KiCAD. This meant that the PCB now had net information, which then allowed me to do a meaningful DRC. There were _a lot_ of problems. Firstly, the original board has some unused gates, with floating (=oscillating if CMOS logic is used) inputs. There was also duplicated designators on the board (C33), some bits just not connected per the schematic - notably some of the clamp diodes around the serial port don't actually clamp, plus the emitter of TR4 is not connected to anything on the PCB.

Anyway, I fixed the obvious things, and mainly made the schematic agree with the PCB where it didn't matter (swapped designators, swapped pins).

So here's the final glorious thing, in a form that I can get one made. 15 thou tracks and 9 thou spaces (I know, very assymetrical). Totally ready to build.

Of course now it's digitised, it's trivial to change. Like for example replacing all the keyswitches with easy to buy Cherry ones, plus replacing the unobtanium IC16 82S123 PROM with an easily available GAL16V8...

1 comment:

Someone said...

Excellent stuff Suzy!
I remember when David Hall of microarts laid the original boards. The original artwork comprised of Bishop Graphics "PUPPETS", "PREKUT" & tape on plastic film at 4X scale (i.e. 2xX and 2xY dimensions). From this full scale negatives used for PCB exposure.

In Australia, RCS Radio, Bexley was the distributor of Bishop Graphics.
Here is a link to a photo of the Bishop Puppets.

Around 1984 at Microbee Systems Limited, an IBM PC compatible installed with P-CAD was purchased and used for PCBs by Paul Wilmshurst. Paul spent most of his time on the Delta & StarNet products.