Tach to Pcm

[davepl]

So a pull-up resistor is just the positive side of a pulsed ground system for when it doesn't have ground potential? And the ECM value for Tach Output is 12/resolution value?

Seems shorter when I say it ;-)

Thanks though, your explanation did clarify things! Still wondering what'll happen if I hook a logic probe up to a floating circuit (seriously!).

[dayj1]

Originally Posted by davepl

So a pull-up resistor is just the positive side of a pulsed ground system for when it doesn't have ground potential? And the ECM value for Tach Output is 12/resolution value?

Seems shorter when I say it ;-)

I'd think that's a fair description in this example. The pull-up resistor is used to make the "default" value of the signal high.

Also, there is such a thing as a "pull-down" resistor that does just the opposite (defaults the signal to low).

Originally Posted by davepl

Thanks though, your explanation did clarify things! Still wondering what'll happen if I hook a logic probe up to a floating circuit (seriously!).

If you have a fancy logic probe that graphs an output it wouldn't graph a floating signal at all as it's undefined (i.e. neither high nor low).

I use an older logic probe with just red (high) and and green (low) LEDs. The raw tach signal without a pull-up would show as a flashing red light. In other words periods where neither LED is lit.) Adding the pull-up resistor would cause the signal to be represented as alternating red and green lights with one or the other LED being lit at all times.