Tachometer Driver
ECU Output: Tachometer Drive
The tachometer drive is used by the ECU to tell the tach on the dash what it should be displaying. It should be obvious to even a casual observer that the dashboard tach is computer controlled given the little redline 'dance' the tach performs when the ignition is first switched on. The tach is controlled by the following output:
As an aside, the tach needle is driven by a tiny stepper motor inside the dashboard unit. A stepper motor is a special electric motor that is designed to be computer-controlled. While most motors just spin when power is applied, a stepper will rotate its output shaft in discrete 'steps' (hence the name), starting and stopping rotation at any of the discrete step locations located throughout its full rotational circle. With proper software, the Aprilia tachometer stepper moves the tachometer needle to a position corresponding to the speed of the engine.
Another aside: as mentioned earlier, typical electric motors are designed to make their output shafts spin when you apply power to them. To get a standard electric motor to stop with its output shaft in some precise location takes a bunch of electronics and software. In contrast, a stepper works exactly in the opposite fashion. If you simply apply power to a stepper motor, instead of spinning, it will hold its output shaft from turning at all. To make a stepper motor actually spin requires a bunch of electronics and software! Fortunately, the dashboard contains all the required hardware and software.
The bottom line is that the dashboard needs to know what RPM to display via the RPM pointer attached to the stepper motor. It gets the info from the ECU's TAC signal. The ECU generates TAC by toggling port J0 three times per revolution as the crankshaft spins. This generates an output frequency on TAC output that the dashboard uses to determine how fast the engine is spinning. The dashboard interprets the frequency of this signal to decide where to have the stepper motor set the RPM pointer to.