The STEbus writes output port bytes to an edge-triggered latch, and reads input port through an 8-bit buffer.

The STEbus writes command bytes to an edge-triggered latch.

The microcontroller writes response bytes an edge-triggered latch.

The actions set flags to say a latch is full, and clear them when a latch is read.

This communication through one-byte back-to-back latches is extremely simple and can easily be used for communicating with any other slave processor. There are six in the BBC Micro's Tube interface. To keep things simple, I would look at J.G. Harston's tube-through-a-single-channel software.