Spindle speed?

Just to explain what was going on previously.

The config is set up with closed-loop control of spindle speed, using a PID component. You can look up PID elsewhere, but most references will tend to overcomplicate it. They have existed since long before digital computers. You run a PID controller in your head when you drive a car. Feedback is looking at the speedo, the command is how hard you press the pedal, and the output is how much power the engine makes.

To stretch the analogy, in your linuxcnc config you have a speedo that goes negative, but you are stupid (as you are a computer) and when told to do 70mph in reverse you see -70mph on the speedo, think "that's 140mph less than 70mph" and press the pedal to the floor.

You have to ensure that the domains of the signals are the same. If you are using single-sided speed command (spindle.0.speed-out-abs for example) then the PID neds to see a single-sided speed feedback, typically achieved by passing the measured encoder speed thorugh an "abs" component in HAL.

You might find it instructive to browse through the available HAL components, they are fundamental to the flexibility of lInuxCNC as you can do all sorts of manimpuations on the input and output signals.

You can see the list here:
linuxcnc.org/docs/stable/html/
if you scroll-down to "Realtime components and kernel modules" and (to a slightly lesser extent in this context) "Commands and userspace components"