May 2, 2023

Entrex - Interface Board -- sending to the host

The ROM code writes to this port 13 times. It drew my interest as a central part of the interface board interface (how about that phrase?) before I was able to study the schematic. It would be fair to call it a "master control register" for the interface board.

One point for those who may be new to all of this. There is also an input port with the same address (0x78) -- it is entirely unrelated. Enough said.

Experience with Modern microprocessor designs would lead a person to expect some chip like the 8 bit register 74LS374 at the receiving end of this port. No such thing is done on the Entrex interface board. I am not even sure if 8 bit interface chips like the LS374, LS373, LS244 and such had been invented at the time the Entrex was being designed. What we see instead is a variety of 74LS74 flip flops and we aren't even clocking data into their D inputs. What we see on the interface board is gates and inverters that are setting or resetting flip flops that control the circuit. Something I have not seen before. Clearly this was designed at a time when engineers were comfortable with TTL logic, but just getting used to working with microprocessors.

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