Found some info in haltech docs:

BAC Hi

BAC high is a form of BAC idle control that is used for 3-wire BAC valves with two coils. These valves are

sprung so that when no coils are energised, the BAC valve sits at 50% open. Two PWM signals are used to

move the coils. One signal moves the valve from 0% to 50% open, while the other signal moves the valve from

50% to 100% open. The BAC Hi output moves the valve from 50% to 100%. When the target position for the

valve is between 50% and 100%, this output will provide a signal from 0% to 100% duty.

All idle control tuning and setup is done with the BAC Hi output. Refer to section 3.13 for details on how to

tune your idle control.

BAC Low

The BAC low output is used to control the BAC valve when the target opening is 0% to 50% open. When the

valve target position is 0%, the output duty is 100%. When the target is 50% open, the output duty is zero to

allow the spring to return the valve to its default position.

BAC Inverted

For 3-wire BAC valves that do not have a spring to return the valve to the centre position, use BAC and BAC

Inverted to control the valve. Connect the BAC output to the coil that pulls the BAC valve open. Connect the

BAC Inverted output to the coil that pulls the BAC valve closed. The BAC Inverted signal is a complementary

signal to the BAC signal. If the BAC output is set to 70% duty, then the BAC Inverted signal is set to 30% duty.

All idle control tuning and setup is done with the BAC output.

pwm freq maybe in 200-1000hz range?

[original doc]