Spindle speed control, how does yours work?

I'm working on controlling spindle speed control, input / output
expansion, and I2C for TurboCNC.

Looking for input on how your spindle speed is controlled.

Is it serial? (baud rate?)
0-10v DC? (is it linear? rpms/v ratio?)
Contact closures?
Variable resistance?

Thanks in advance for your input.

Regards,
JJ

ps. I was going to call it Spindle Universal Control Kit / System, but
then I realized, that sucks.

pps. Excuse the post to two groups.