If I’m incorrect in the question, please correct me. From what I’ve read and gathered the stab trim system essentially did the same before hand? It relies on AoA? And if so, did the computer for that take information from one AoA at a time like MCAS did? I’m pretty sure I’m wrong anyway, this is super confusing for me, but that’s my understanding as of now, so basically Boeing needed the trim system to make a further command than the previous system, correct?
There are several ways through which the stabilizer can be commanded:
Manually via the trim switches.
Manually via the trim wheels.
Automatically via the Speed Trim System (STS). A helpful explanation of the STS can be found in this answer. STS feeds back from stabilizer position, thrust lever position, airspeed and vertical speed.
Automatically via the Autopilot trim offset. This is only applicable with the relevant autopilot mode engaged.
The Mach Trim function also modifies the trim characteristics. However, it does not actuate the stabilizer and repositions the elevator centering and feel unit instead, which backdrives the column.
MCAS' requirement is completely different from those of the STS or the Mach Trim.