Originally Posted by
BobM2
Where does STS come from?
My quote was definitely faulty. Speed trim comes from the flight control computer. See:
B-737 Speed Trim System
Edit: Reading that detailed reference suggests two points. 1. STS and MCAS run on a single FCC at a time, which alternates between flights. 2.
AFAIK the autopilot runs on both FCCs, but either is crew selectable while inflight.
The autopilot can be disabled, but STS and MCAS are used in manual flight, and cannot be disabled. How this all fits together raises many more questions (completely outside my scope).