PWM H-Bridge signal

Hello ,

I have a question about the servo signals.
In the following example from etch servo :

net Xdir => parport.0.pin-06-out
net Xdir => parport.0.pin-07-out
setp parport.0.pin-07-out-invert TRUE
net Xpwm => parport.0.pin-05-out

As I understand :
the pin 7 (signal is always pin 6 inverted (signal A inverted), the pin 5 is the pwm (Enable signal).

In an H-Bridge there are the following condition :

When pwm is low : free running or motor stop (not takeing A or B in account).

When pwm is high : Signal A High , Signal B low is a forward condition.
Signal A Low , Signal B high is a reverse condition.
Signal A = Signal B (both high or both low) is a fast stop condition.

So because Signal B is always inverted signal A the third condition for a fast motor stop cannot be used.

This happens only in etch servo example ?
Is linuxcnc able to output separate A and B signals and it knows to make use of the fast stop function ?

Also with the current example configuration if a motor overshots , the software will command the motor to go back without a stop, so if the motor is high weight and still in inertia in the forward direction if we apply a reverse output we can damage the motor.

I am new to linuxcnc, searched a bit on this subject but didn't find out ,, so sorry if this have been discussed elseware .
Also excuse by bad English.

Have a nice day.

Having 2 direction lines is specific to the etch servo hardware (L298)
some PWM interfaces have just PWM and DIR or PWM-UP, PWM-DOWN or just PWM (50% = 0 drive)

Using the fast stop feature could easily be done with a L298 by adding "OR2"
and "NOT" components in the HAL file.

Note that fast stop can damage motors just like full reversal.
To prevent motor damage (if demagnetizing overcurrents are possible),
the H bridge must incorporate current limiting.

Hello again,

Thanks for your answers,

Mr. andypugh I am trying to make exactly that kind of "toy" but on 3 axis (the Z would be a stepper).
The stepper part is easy, the servo is more difficult.
By servo I mean simple DC brushed motor with quadrature encoder attached that would be controlled by L298N.

I have read the emergent.unpythonic.net/projects/01142347802 and he is saying about the function what Mr.PCW suggested :
" (In fact, I did write a tiny amount of code, but only because I was unware of a HAL component called 'or2') " but the emc2 configuration link on
that site is broken so I cant see how he implemented it in hal file.

Mr. PCW can you please give me an example implementing "OR2"
and "NOT" components ? (only if you have time.. ofc)

Mr. andypugh how exactly the frequency limit of lpt limits the operation? I mean, a 50% duty cycle on L298 imput with say 12V motor voltage that would be translated into 6v on motor pins, if I need higher duty cycle that cant be achived due to limits can't be passed by increasing voltage motor ? lets say 24V at 25% duty cycle it would mean the same 6V on motor pins with lower signal period?

I have read that the lpt frequency is a limit for the encoder reading if they are high resolution.

Thanks

Were you able to combine servo motors and stepper motors in one hal file? I mean, could you run servo and stepper motors on the same machine?