On other computers the 4 directions plus fire are input, which are connected to gnd when active.
On Enterprise these are output, and the common line the input.
The program select on the B5h port which direction needed to read, and the state of selected direction can be read on one bit of the B6h port.
In a simple wired joystick adapter the common line connected to joy GND. It is work for simple mechanical switchs. But when autofire or or other circuits use as power GND then not working.
In Boxsoft interface, the 9 pin joy connector use standard pinout, the GND are GND.
The 74LS32 are OR gates, make OR operation with the EP direction outputs and Joy outputs. For example the EP want to read Left the the Left output are 0. If the Joy Left also 0 then the result are 0. If any of them then the result are 1.
The diodes make a AND operation with the all directions, if any of them are 0 then send 0 to EP common input.
The extra thing on EP: not only one common line, it is have a 3! (Keyboard J,K,L)
Then totaly 5*3=15 directions/buttons can be readed in one Control socket, totaly 30 on the two socket. For example 6 standard (4 dir + fire) can be used...
The Boxsoft interface use the standard 5 bits, and another one for the second button.