Looking at the knock system, I ran into an interesting detail. TPU register $14CE is used by the PCM to select which of the knock sensors in dual sensor applications is in use at the moment. There is only one bit active in the register: bit 14. This bit is probably used internally by the TPU but we can't see that because it is internal only. The TPU output is on pin 11 and connects to the TSide mux circuit through the ribbon cable on pin 25.

This is just a guess... It may be that it was intended that the TPU control the sensor select line based on the cylinder about to fire. If true, it was abandoned and looks to be all controlled by software through the register I noted above.

Here is a snip of code that alternates between sensors

Code:
* In dual knock sensor applications, alternate sensors each call
* PRU port a bit5 selects single knock sensor when set
* Sensor(s) are disabled when the "noise" detected by the
* knock module is below the noise floor set by calibration
loc_9435:
	bclr	<byte_004E $01			    ; KNOCK RETARD DISABLE FLAG

	brset	<byte_004E $02 loc_9447		    ; SELECTED KNOCK SENSOR L/R: TOGGLE	EACH CALL
	bset	<byte_004E $02			    ; SELECTED KNOCK SENSOR L/R


	ldd	#$0000				    ; SELECT KNOCK SENSOR 2 (LEFT SIDE)
	std	word_14CE			    ; TIO SHARED RAM: KNOCK SENSOR SELECT


	bra	locret_9450

loc_9447:
	clrb
	ldaa	#$40				    ; SELECT KNOCK SENSOR 1 (RIGHT SIDE)
	std	word_14CE			    ; TIO SHARED RAM: KNOCK SENSOR SELECT

	bclr	<byte_004E $02			    ; SELECTED KNOCK SENSOR L/R

locret_9450:
	rts
-Tom