The DG NOVA is an interesting story in and of itself, designed by some DEC expats as a straightforward extension of PDP-8 style technologies into a 16-bit minicomputer, but a couple of days ago I learned something quite interesting about its LSI microprocessor implementation, the microNOVA. For more information on the NOVA, there are previous threads on this forum here and here.
I was reading the book Minicomputer and Microprocessor Interfacing, by John Cluley (1982), and it mentioned the DG microNOVA. I was aware that it existed, and that it was an implementation of the DG NOVA much like the LSI-11 was an implementation of the DEC PDP-11, but it had an intriguing description of its architecture. It said that the microNOVA, in order to fit a standard 40-pin package, had a 16 MHz serial I/O bus!
Some digging turned up more information on this. (There’s a good page on the microNOVA on CPUSHACK, for example.) It turns out that the external I/O interface was actually two pins sending 8 serial bits each, in parallel!
The book does mention that the complexity of interfacing this bus was high enough that it was used only by the CPU and special transceiver circuits (I believe the mN603 I/O controller, from this publication from Data General) on the motherboard or backplane, which converted the serial bus to a parallel bus for consumption by external I/O devices — possibly using the same format as the larger NOVA minicomputers.
What an interesting design choice, and particularly for 1976. Even many keyboards were not yet serial in 1976! (Of course, terminal units and teleprinters were.)