The Entrex comes in two varieties. The once we are dealing with here we have nicknamed the "Trapezoid" because of the external shape of the terminal. Inside, it is distinguished by the main logic board (which we call the "processor board) having an 8080 CPU (actually an AM9080 in the boards I have seen, but this is pin compatible with an Intel 8080).
There is nothing at all unusual about a terminal from the 1970's with an 8080 cpu in the middle of it (or a 6502 or Z80 for that matter). What does seem unusual about the Entrex is the serial interface. It is not RS232, and does not look like any of the many synchronous serial protocols that came along shortly thereafter. It looks like its own beast and that is part of the puzzle.
The other part of the puzzle is the circuit. AJ went to great lengths to photograph and then import the actual circuit boards into KiCAD, from there he worked up a netlist, and from there a schematic. I have left all that hard work to him and my job now is to make sense of the schematic, be alert for any errors and omissions, as well as to analyze the code in the pair of 2708 ROMs. Note that a 2708 ROM is 1K byte, so a pair of them is only 2K bytes. Not much code, but that just makes our job easier.
The Entrex Trapezoid has several earlier relatives, in particular the Entrex "Datascope".
Tom's Computer Info / tom@mmto.org