Reading a tachometer signal

I want to read spindle rpm into LinuxCNC. Based on this thread and the LinuxCNC Encode documentation which says:.
I am planning to tap into the spindle pickup that feeds a display on the mill and connect to the 7C81 card with the default 5ABOB program as shown.
 

From the documentation it sounds like the Quad-IDX input is a time base signal for counting. Must this be connected, or is it not needed in mode 1?

Setting mode 1 means put this line "encoder.0.counter-mode = 1" in the HAL file?

Thanks,

Gene

The index input is not used unless you are doing spindle synchronized motion

The software encoder documentation you quote is not correct for the
hardware encoder in the 7C81. The hardware encoder does use the B input
in mode 1 (for count direction) . On the 7C81 if B is left unconnected, it will be high
= count up.

If you started with a pncconf created hal file you probably have the

setp hm2_7c81.0.encoder.00.counter_mode 0

or

setp [HMOT](CARD0).encoder.00.counter_mode 0




statement already in your hal file, so you just need to change the 0 to 1