Well, it's not a secret. SimDash even displays the frequency of the signal, in pulse per minute.
Anyway, the rev burner signal is at 12V, it's a square wave and the frequency varies from 0 to around 580 Hz.
Same think when the rev burner is using the firmware for the speedometer.
As I said with next version of SimDash, the firmware for the speedometer will create a signal with a max frequency just above 1Khz.
I hope this helps.
Cristiano