Is this Base Thread Jitter ok and which GPU would you suggest?

Hi there!

I am currently designing and building my first homemade CNC machine. After some discussions with experienced enthusiasts I decided to go with a gantry design.

Since the CAM software I was using for the test setup does not natively support gantry squaring, I ended up stealing an old mainboard and testing out LinuxCNC which I installed from the downloadable image.
My employer was kind enough to gift me an old GA-B85M-HD3 (rev. 1.x) mainboard with some old Intel(r) Pentium(R) CPU G3250 @ 3.20GHz. Some of its manufacturer specifications:

Onboard GraficsIntegrated Graphics Processor - Intel® HD Graphics support:

1. 1 x D-Sub port, supporting a maximum resolution of 1920x1200
2. 1 x DVI-D port, supporting a maximum resolution of 1920x1200
   * The DVI-D port does not support D-Sub connection by adapter.
3. 1 x HDMI port, supporting a maximum resolution of 4096 x 2160 @ 24 Hz / 2560 x 1600 @ 60 Hz
   * Support for HDMI 1.4a version.
4. Maximum shared memory of 1 GB

 Expansion slots

1. 1 x PCI Express x16 slot, running at x16
   (The PCI Express x16 slot conforms to PCI Express 3.0 standard.)
2. 2 x PCI Express x1 slots
   (The PCI Express x1 slots conform to PCI Express 2.0 standard.)
3. 1 x PCI slot

I have watched my fair share on tutorials about LinuxCNC by now and found out that using a dedicated gpu could reduce the jitter by a lot, and using a shared-memory onboard gpu may render the machine useless for LinuxCNC.
Furthermore I read that using NVIDIA/RADEON/ATI and other major companies with their proprietary drivers might be a bad thing, therefore I decided to buy a Matrox M9125 PCIe gpu.

The problem with the Matrox M9125 is, that its most recent drivers can not be installed with the latest xorg/xserver version that is shipped with the LinuxCNC image, and I do not feel like downgrading to an older version.

Using the onboard graphics and the HDMI slot @ 4k resultion I "stress tested" the machine while running the HAL Latency Test. I believe the high resoltion has an effect on the jitter, but it might only have been by chance (I later tried 1920x1080 res and the jitter appeared to be way lower).

"Stress testing" was basically just playing ~20 youtube videos and ~5 glxgears windows. I doubt that this counts as a proper stress test, but when this box is controlling the cnc machine I do not plan on having anything else running but LinuxCNC.

Using 4k resoltuion, the result was 31292 max base thread jitter. In idle, the max base thread jitter was usually around 18000-20000.
Using FHD resolution, the result was 21000 max base thread jitter, in idle, the max base thread jitter was usually around 10000-12000.

1. I am not sure if these values are any good. Is this a usable machine, or should I aim for better latency values?
2. What dedicated (PCIe x16) GPU would you suggest, that also works (has drivers available) for the latest LinuxCNC image?

I assume with these values I could run LinuxCNC on the onboard graphics at a 1920x1080 resolution or is using the onboard gpu generally a very bad idea?

Regards




 

There is nothing wrong with using the onboard GPU. 20,000-30,000, is generally fast enough for software stepping with lower resolutions (1000-10000 steps/inch or 100-500 steps/mm) and moderate feed speeds. (More than adequate for most DIY home router builds.)

As far as a driver for your Matrox card, I wouldn't bother trying to install it's proprietary drivers, just use what ever driver the Linux installation detects and thinks it should use. (Unlike Windows, you don't usually need to install a driver from the manufacturer in Linux.) I would only go there if you can't get it to work. Trying to install 3rd party drivers in Linux can be a can of worms (very squirmy ones.)

Allright, thank you! Happy to hear I don't have to find another setup.

For this first build I don't mind the machine going slow, my goal for now is to get it running, hopefully with somewhat useful results.

The Matrox M1925 was not automagically detected by the OS, when I used it in the PCIe slot the screen wouldn't receive any signal.

@tommylight

Yes and no: there's a LPT port on the board, but for now I am using a parport card which I put in one of the mainboard's slots. I made a random config with the Stepconf Wizard and used a multimeter to find the voltage between the 5V source and the signal pin of the breakout board and it appears to be working.

I use the 9100F+AMD RX580.The RX580 does not require any additional drivers, and the jitter is excellent. It's less than 10us.