SBC6120-RBC: Re-issue of Improved Classic SBC6120-2
Many PDP-8 aficionados were interested in obtaining SBC6120 boards, but none were available for some time. Andrew Bingham and the Retro Computer Group took this up and reengineered the original design to eliminate some unobtainable memory chips and make the PCB two layers only. The kits came out in 2016 after Andrew was able to find a considerable number of working HD6120 chips.
My SBC6120-RBC
As I still had three HD6120s left I got three boards and the necessary programmed GALs (to make life easier) from Andrew and proceeded to assemble them. Very well made, the first two worked fine - after correcting a minor error (mine) on one. The third had to wait as I found one HD6120 to be defective and did not have more spares.
Looking on Ebay, I found a Chinese vendor selling pulls and I test-ordered one - it worked. So I proceeded to get some more, now there are some spares if anything happens... The third unit could now be built as well and again no surprises. Thanks Andrew & group for the excellent work!
One of my SBC6120-RBC boards.
What next?
The current project with these new boards is to implement an old dream: drive an OMNIBUS backplane with a reduced set of bus signals, similar to what I did many years ago for my PDP-8/I (see the document here). The difficult part is again coming up with the correct timing signals TP3 and TP4 which are needed for the peripheral cards but not available as such from the HD6120. The mostly wire-wrapped PDP-8/E card is almost complete and waits for initial tests with the piggy-back SBC6120-RBC in place. Hopefully I will be able to describe another successful project one of these days...
