Topic: Any reason for using both 3rd and 4th bits of #fe port?
As utz and I figured out, behaviour for MIC and EAR ports on the ZX Spectrum 128k +3 with Amstrad 40077 Gate Array produces completely different output in comparison to a classic Speccy.
For the Speccy, values: 00, 01, 10, 11 of bits 4 and 3 produces 0.39, 073, 3.66, and 3.79 volts respectively.
For the +3 however, values: 00, 01, 10, and 11 would produce ~0, ~5, ~0, and ~5 volts, so the result level is driven by (bit4 & bit3).
So the question is: what is the reason for using all 4 levels (both MIC and EAR bits)?