Hi Chuck -
One of the reasons for basing it on the RCA CDP18S651 is so I can use UT71 and RCA MircoDOS intended for the RCA MicroDisk Development System MS2000 - see RCA MPM-241. I managed to find an original Disk for this and the associated BASIC on eBay.
I'll need to make an additional memory card to go with this as well since that system used a 2k ROM at 8000 with RAM filling the rest of the addressable space. My ELF-ish currently only has 16K RAM and a 4k ROM at 8000. This will require disabling the memory-mapped I/O devices I have in upper memory, but I included a jumper in the system to do allow this.