Author Topic: PMDX-424 and constant Motion Underrun Error  (Read 127 times)


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PMDX-424 and constant Motion Underrun Error
« on: July 04, 2019, 12:06:26 PM »
I've recently upgraded my home made CNC to the PMDX-424, Mach4, and new drivers and power supply, so essentially a full electronics rebuild. I've been slowly working through constant issues with the upgrade, and got the machine functional, but now I'm running into an extremely persistent 'DEVICE ERROR 'SYSTEM ERROR(7): Motion Underrun'.

I've already attempted to increase my buffer from the 150 default to 450, resulting in no change in the error frequency. And the error certainly is frequent, there are times when it will pop up every tenth of a second, and I just hit enable, cycle start again, the machine will twitch to the next line of code, then the error will pop up again. However, there are times when it will run perfectly fine for hours. For instance, I ran this two hour job last night, and had the error maybe 5 times. I'm halfway through the exact same job this morning and I've probably had it 200 times, and now I'm at the point where I won't even be able to finish it because it is so constant. The error doesn't typically happen when jogging, though it has occurred before.

My initial thought was the PC I'm using to run my whole CNC is just too slow and underpowered, although it worked great for months before upgrading to the PMDX-424 and Mach4. ( It is a small fanless PC I thought would be good for a dusty environment, but with only 1.92 GHz and 4gb of RAM. I thought adjusting for the buffer would help with this, but it doesn't have seemed too. I'll upgrade if I need to, but I want to make sure this isn't something I could adjust in the settings or script or whatnot.

I understand typically this could occur when there are many small rapid movements, but I'm seeing it when it is cutting a straight path along one axis only as well.

I'm at a loss as to what could be causing this inconsistent, but crippling, error. I've seen postings about maybe optimizing my Mach4 for my particular lead screws and/or steppers, but I'd like to get at least some sort of direction from someone better informed than I before I leap down yet another rabbit hole to get my rig running like I hoped it would after spending $400 on the PMDX/Mach4 upgrade.

I'm running Mach4 Hobby, version, build 3804, screen set wx4.set.
PMDX Plug In Version 0.53.263
PMDX Bootloader Version 1.13.73
PMDX Firmware Version 0.60.208




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Re: PMDX-424 and constant Motion Underrun Error
« Reply #1 on: July 06, 2019, 10:33:45 PM »
Yeah, having a fanless PC does sound like an advantage.  If you can upload your profile I can try it out on my PC and see how it runs.  Or if you have another PC you can try it on, see if it runs any better there (you can run without a license for a couple of minutes - I've forgotten the exact timeout).

I don't know about running Mach4 and/or plug-ins on an ATOM-powered PC - I've never tried it.  According to the Amazon page you linked to, your PC has a base freq of 1.44 GHz and burst freq of 1.92 GHz.  Newfangled says the minimum requirements are 2 GHz ( Your burst speed is "close enough" probably, but it depends how long it can run at the "burst" rate)  I do know that PCs with graphics on the motherboard have had issues running Mach4 due to memory bus usage between the program an the graphics.  I'm not sure but I think that issue manifested itself in screen update issues.  Newfangled  recommends a separate graphics card for that reason.

One thing you could try is turn off the WiFi and unplug the Ethernet cable (if attached).  And make sure Windows power settings are set for "desktop, always on" mode - go to control panel, "Power & Sleep" then "advanced settings", change from "balanced" to "high performance".  See if that makes any difference.


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Re: PMDX-424 and constant Motion Underrun Error
« Reply #2 on: July 24, 2019, 05:50:41 PM »
I just ran across this post. I am in the process of building a new cnc with the PMDX424 board and Mach4 software. My thought was the same as yours to run with a fanless pc. I had installed a ITX motherboard with a quad core integrated atom cpu. I have been battling this setup for weeks and getting underruns in Mach4. I was thinking this was a issue with the OS as I was running a Win10 OS and have installed the OS several times. I finally swapped out the pc to another pc that was a ITX motherboard with a AMD 6 core cpu and i am not seeing the underrun issues anymore. Hope this helps.