Just wanted to say that I finally completed building the revision 4A board; most of the hardware was discussed in the previous log entry, but I've fitted the '688 comparator for address decoding as well as the TinyFPGA BX module. The result is shown in the pictures below.
I'm happy that, despite needing one bodge wire from U2 to +3.3V, it is otherwise fully capable of working from the POV of software development.
I've already made the changes to the design files to support revision 4B, which fixes all the known hardware bugs. Additionally, the nMigen files for the VDC-II has been configured to output 9-bit video, so all 11 output I/Os are known to work.
Besides getting revision 4B out the door, all that remains is a set of mezzanine boards: one for VGA, and one for PS/2 keyboard and mouse. That, combined with a reasonable GEOS-like GUI system, should be sufficient to make the RC2014 into a reasonably usable homebrew computer system.