Originally Posted by
GarageYears 
If you're referring to the Speed Trim System (STS) it has it's own speed sensing system based on pitots located at the rear of the aircraft.
Hmm.........no.We really need engineering input to try and get somewhere here but........ FCCs command the STS, AP STAB TRIM functions.
The air data inertial reference unit (ADIRU) sends these signals
to the FCC for speed trim calculations:
* Computed airspeed (CAS)
* Mach
* Inertial vertical speed
* Roll angle
* Angle of attack.
AMM 22
Each ADIRU sends these air data reference (ADR) signals to its
onside FCC:
* Uncorrected altitude
* Baro corrected altitude number 1
* Baro corrected altitude number 2
* Mach
* Computed airspeed
* Maximum allowable airspeed (VMO/MMO)
* True airspeed
* Static pressure
* Total pressure
* Indicated angle of attack.
AMM 22
From above
L ADIRU sends CAS to FCC A. FCC A commands the STS signal via AP stab trim motor,just like when it commands AP stab trim
Inputs to ADIRU?
The pitot static system is comprised of three separate pitot tubes and six flush static ports.Two pitot probes and four static ports interface with the air data modules.The remaining auxiliary pitot probe and alternate static ports provide pitot and static pressure to the standby instruments.The air data modules convert pneumatic pressure to electrical signals and send these data to the ADIRUs.Each pitot air data module is connected to its on-side pitot probe;there is no cross-connection.The air data module connected to the Captain's pitot probe sends information to the left ADIRU,while the air data module connected to the First Officer's pitot probe sends information to the right ADIRU.
FCOM 10.20.13
Elevator pitots have no interface with ADIRUs.They feed the elevator feel computer.
FCC A and FCC B
The FCC can supply these commands:
* A/P trim nose up
* A/P trim nose down
* Speed trim nose up
* Speed trim nose down.
If the autopilot is engaged, the FCC can supply an A/P trim
command. If the autopilot is not engaged, the FCC can supply a
speed trim command.
AMM 22
Column Switching Module
When the main electric trim is in operation, switches in the
column switching module open. This does not let the FCC clutch
signal go to the trim actuator. The FCC and the main electric
trim cannot control the trim actuator at the same time.
If the pilot moves the control column forward or aft, signals go to
the two FCCs. These signals do not allow the FCC to supply trim
commands that do not agree with the pilot.
AMM 22
Speed Trim Stability
The speed trim controls the stabilizer to oppose any change of
airspeed. An increase in CAS causes a nose up trim command
to the stabilizer. A decrease in CAS causes a nose down trim
command to the stabilizer.
AMM 22
STS is not there for fun.Its needed for certification.By retrimming it gives you more elevator authority,particularly important in high AoA low CAS
conditions.Nothing absurd about it.