The connection from one MCU to the other is a single #signal #wire, plus shared #power and #ground. It's a point-to-point link; there's no addressing involved. It's simplex, so no collision detection or anything complicated.

In fact, it's dirt-simple, and extremely low-bandwidth. The secondary MCU transmits a single #byte of data to the primary MCU approximately every 120ms. So ~70 bits per second, max.

The signal line has a #pullup #resistor at the primary end.

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