Can you tell me more about how can Z80 (or popular peripheral chips) "can output and input 3 states with a single pin"? Or maybe give some web links? I am prett

Can you tell me more about how can Z80 (or popular peripheral chips) "can output and input 3 states with a single pin"? Or maybe give some web links? I am pretty familiar with that chip, so hearing this surprises me. For the output side, I guess I you can tri-state the output, but this will by default keep the previous voltage, you will need some resistors to actually emit a voltage outer than "low" or "high". And Z80 cannot even control this on per-pin basis, since it has no integrated GPIOs. For the input side, I have no idea how would Z80 get more than two states. All the peripherals I know of (like 82C55 GPIO chips) also only provide two input levels as well. The only way to get 3-state input into a system I can think of are: - ADC input followed by software thresholding (this will be pretty slow, and works with hundreds of states, not just 3) - Use external voltage comparators to split a single input wire into multiple input pins. This no long is "single pin" though. - Some FPGA/CPLDs have multiple banks with changeable input thresholds. This lets you avoid external comparators. You'd still need multiple pins for each input wire.