Microelectronics | 16-bit ATX ISA BACKPLANE

Synopsis

I was thinking on restarting experiments with vintage computing hardware. Thus I decided to start working on an 8 slot, 16-bit ISA backplane that will fit either AT or ATX computer cases and will accept power from a modern ATX supply. Well I would've preferred to design it so that it could accept either AT or ATX power supplies. But I just couldn't find an online parts supplier for P8 and P9 male connectors to fit on the printed circuit board. Thus ATX it is.

I always wanted to experiment with BOOT ROMs. Back in the very early 1990s when I was routinely using an 80386SX / 25 MHz machine, 27c series EEPROMs were expensive and difficult to find. These days I have access to a lot of ROM integrated circuits at low prices from the local and online electronics stores. Needless to say that over the time I have recovered quite a few ROM ICs from vintage computer boards. I can erase and reprogram them for my experiments. Thus this backplane implements support for one 64K BOOT ROMs of type 28C64. Address decoding for each of these ROMs is implemented as well.

Adjacent circuitry is also carried on this backplane. Think reset and ATX power control circuits, and indicator LEDs.

The backplane should handle 8088, 80286, 80386, and 80486 CPU boards as well as any 8-bit or 16-bit ISA expansion card. So a complete computer can be easily built.

The design of such a backplane is no secret and despite all the many apparently complicated connection points, overall, the circuitry is fairly simple to understand and implement. Regarding the printed circuit board layout, I am manually routing all tracks in order to fulfill the best digital design constraints.

A historical fun fact: Back in the 1990s I was spending hours looking at computer printed circuit boards, observing the wire routing design, the choice of electronic parts, and the overall layout. I was seeing this as some kind of abstract art. In fact I was mesmerized by computer PCBs. I remember a few notable ones: an Adaptec AHA-1542CF SCSI ISA controller, ATI VGA WONDER-16 ISA video controller, an old 1989 CHIPS & TECHNOLOGIES 256K VGA controller, an unknown model NCR MFM ISA controller, a beautiful Western Digital WD-1004A-27X MFM ISA controller, a UNiSYS 80286-10 mainboard, and countless other that I cannot remember the model numbers. Looking at those PCBs I was dreaming about creating my own one day. So here I am, designing my own ISA backplane. I am planning other ISA designs in the upcoming couple of years.

At this moment, this project is still work progress.
I am still fine tunning this backplane design and now I just uploaded its VER. 1.4 REV. B variant. So there will still be activity and design files are subject to change up until I come up with the final design.

Schematics

Fig. 1: Electrical principial schematic.

Printed Circuit Boards

Fig. 2: Top silkscreen.

Fig. 3: Top layer printed circuit board.

Fig. 4: Bottom layer printed circuit board.

Fig. 5: Top silkscreen - simulation.

Fig. 6: Top layer printed circuit board - simulation.

Fig. 7: Bottom layer printed circuit board - simulation.