[Mark1960]

Interesting method of operation but I guess that is how the Elektor Hexio worked so you are forced to do something similar for compatibility. The use of three priority encoders for the keyboard input is odd, but I guess reduces software complexity at the cost of more hardware.

I quite like the display driver using memory scanned by hardware as that would stop the display from blanking or showing a single bright digit like the MK14 when a user program is executed.

You might consider putting the right hand decimal point displays upside down. That would give a left hand decimal point at the top instead of bottom of the display, which might look better if software is implementing a left hand decimal point. You just need to reassign the segment positions to show the correct characters when the display is inverted.

Its nice that you created a document to describe the circuit.

Note the 74172 was an 8x2 register file with three ports. The 74870 and 74871 are both dual 16x4 latches and also the dual port addressing is a bit strange.