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Actually I found a mistake in my grub command line.  Now my histogram is better, in that there are only a few excursions to debug now. 
 

It looks like only interrupt that seems to fire at roughly the same frequency is IRQ 20 which is described as

IO-APIC 20-fasteoi pata_pdc2027x[0000:03.05.0]

It is interesting that it fires on CPU1.  I thought I had a command in my grub setup to disable hyperthreading.
 

Fair enough. I thought I had gone through all the steps but I guess I missed it.

I just got a touch probe and want to set it up alongside my tool height setter.

Turns out the tool height setter I bought years ago is NC.
Without even thinking about checking I bought a NO probe which if I am thinking about it correctly if I wanted to wire them into the same pin it would have to be NO.

From what I can tell in reading though it seems like I should be able to wire them into separate pins on my 7i76e, distinguish one as NO, the other as NC, and get it to function properly correct?

And from reading it does sound like LinuxCNC still only has one probe function right? So in the configs I tie both sensors/pins to the single probe function correct?

That is going to be it. I'll make the change and check in the morning.

do you need a bob to use it?

What bob do you recommend me?

There will be some info that mentions modifying config.txt, as this is what the raspi-config utility does.
The HDMI cable that comes with the RPi s doesn't "feel" like the best of quality, so Tommy may have a good point.
The RPi docs have a good explanation of the config.txt file, but just remember that the location of that file on the images is /boot/broadcom/config.txt

Okay, I finally have the rtai version running, but I seem to only be able to run the latency-histogram as sudo.  Is that the way you're supposed to use the rtai version?

Some notes on the install, as the directions leave out specifics of a lot of steps.  (Half the words in 7.2.4 are separate steps without spelled out commands.)  Hopefully this helps some others.  I followed a lot of forum.linuxcnc.org/9-installing-linuxcnc...tall-rtai-got-errors

Also, I'm not sure if these were done in the correct order.

After installing from the live cd and booting into the new os, I ran

sudo apt-get update
sudo apt-get dist-upgrade

I'm not sure if I did this next, or it was somewhere else on the way.

sudo apt-get purge linuxcnc-uspace
sudo apt-get purge linuxcnc-doc*

Next I updated /etc/apt/sources.list.d/linuxcnc-uspace.list adding 2.9_rt to

deb [arch=amd64,arm64 signed-by=/etc/apt/trusted.gpg.d/linuxcnc.gpg.key.binary.gpg] www.linuxcnc.org/ bookworm base 2.9_uspace 2.9-rt
deb-src [arch=amd64,arm64 signed-by=/etc/apt/trusted.gpg.d/linuxcnc.gpg.key.binary.gpg] www.linuxcnc.org/ bookworm base 2.9_uspace 2.9-rt

(Note I think this is what the linuxcnc-install.sh script called out in the directions is supposed to do, but instead it adds a linuxcnc.list to the sources.list.d, which causes a conflict with what is in linuxcnc-uspace.list and means that apt and the Synaptic Manager stop working.)

Next I did another:

sudo apt-get update

Next I installed the real time kernel:

sudo apt-get linux-image-5.4.258-rtai-amd64

Note: You have to put in the version number. You can tab to find the right one to finish the line. It should have rtai.

REBOOT!!!! Here you have to reboot into the rtai kernel (I think). When the blue screen comes up press down to the second option to choose the rtai kernel.

Next I installed the modules and linuxcnc:

sudo apt-get install rtai-modules
sudo apt-get install linuxcnc

Here I don't think I needed the version number, but if you get an error you can try that.

I also installed headers at some point. I don't know if that was necessary:

sudo apt-get install linux-headers-5.4.258-rtai-amd64

I think I ran the linuxcnc install again after, but I don't know if it was necessary.

sudo apt-get install linuxcnc

The reward for all this... It looks like my min/max base thread latency is about the same as the preempt_rt version.
 
Any other ideas what to try?

Like I said it's the thread period that is limiting your speed.

This is so specific, I'm guessing I've got a bone-headed mistake.
I've attached my .ini and a couple of screenshots.
My machine lines are showing going to the back of the Y=0 line, not in front.
All my homing and jogging and machine movements are correct.
I do have GEOMETRY = X-YZBC, but the red machine lines are out the back of the machine if I turn that to XYZBC, and when I set it without the -Y, movement in the display is mirrored in Y
My Y home sensor is in the back, X to the left, and Z on top, so the machine is oriented:
Z: Top is zero, Down is Negative; X: Far Left is zero, right is positive; Y: Far Back is zero, forward is positive.

Can I keep those orientations and get the red lines to show around the actual working space?
The inconvenience here includes that the measurement lines are inside the machine space, but waaay back from where it's visualizing the program (the white spiral lines).
The white spiral lines are in the right place, but the measurement lines and the axis lines and the red machine outline are all backwards with respect to Y.

(I'm on linuxcnc-uspace 1:2.9.4, and QtDragon)