Nowhere near as bright as those you can get today - at least an order of magnitude worse in terms of energy conversion efficiency. Why? The decade counters can drive the digits directly. I'm using shift-registers to move data from the counting circuit to the 7-segment decoding circuit. Then either a counter would have a 7-segment decoder and a parallel latch built in, or there'd be a separate latch/decoder. The decade counters would be, well, used for counting, so no multiplexing involved - they were just chained into a long chain of decade counters, as many as there were digits. The multiplexing for parallel latches was done on the bus side, naturally - each latch was an addressable write port. Most designs that didn't use CPUs wouldn't multiplex since the displays were driven by parallel outputs from counters typically, or from parallel latches. Why the 7400 chips? :) The MC14000 / CD4000 family was available back then.
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