From the testing that I have done (not Miata specific) the Injector pulse is triggered by grounding the injector signal. I have a really neat picture of and o-scope screen showing injector versus spark timing. I will see if I can post it a bit later tonight.
I wrote a short paper trying to explain in simple English the relationship between spark and injector pulses for a collage course if you would like to take a look at it.
INJECTOR PULSE WIDTH AND TIMING
Injector pulse width is the signal that is sent to open the fuel injector. The injector is triggered by dropping the injector signal to 0v; otherwise it gets a 12v ? 15v signal from the ECU. This is illustrated by the oscilloscope screen posted below.
By the use of the oscilloscope I measured the width of injector pulse at idle. In order to determine injector duty cycle you must use the following equations.
If we take Idle to be approximately 2400 RPM?s and the injector pulse width to be approximately 4ms we find the injector duty cycle to be 1.6%. This tells us that the injectors are being used very little at idle and that is to be expected. The time between injector pulses? per RPM is shown below.
The best fit curve was found by using a power function in the form f(x)=x^y. The exact equation that I came up with is f(x)=103708x^-0.9804. This is only a preliminary equation and with further testing it may clean up to something more reasonable such as the equation f(x)=103700x^-1. However it is more realistic that with more precise testing and more data points the equation will become a more exact number and the exponent could be taken out to more decimal places.