G76 problem

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20 Dec 2022 13:45 #259953 by Ismacr63
Replied by Ismacr63 on topic G76 problem

Before, I used mach3 with more speed than in linuxcnc and I didn't have problems of losing steps.


It is possible that Mach3 wasn't managing the requested step times (ie, it was lengthening them). What step time and step space are you using?


here I have what came by default and based on period maximum jitter I have the maximum which is 50000, I don't know if it is correct. I leave some images.
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20 Dec 2022 14:04 #259954 by andypugh
Replied by andypugh on topic G76 problem
The second image shows some very bad jitter numbers.

What does "uname -a" say?
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20 Dec 2022 17:34 #259970 by Ismacr63
Replied by Ismacr63 on topic G76 problem

The second image shows some very bad jitter numbers.

What does "uname -a" say?


Do you mean what I show you in the image?

Forgive my ignorance but I'm new to this.
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20 Dec 2022 18:19 #259973 by andypugh
Replied by andypugh on topic G76 problem
Yes, that. (though you should be able to copy and paste)

The latency / jitter in the previous screen shot was so bad that I wondered if the machine had booted into a non-realtime kernel.
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20 Dec 2022 18:29 #259974 by Ismacr63
Replied by Ismacr63 on topic G76 problem

Yes, that. (though you should be able to copy and paste)

The latency / jitter in the previous screen shot was so bad that I wondered if the machine had booted into a non-realtime kernel.


These are the PC specs.

I put the image because I write to you from my mobile phone, because the network cable of this PC has broken and I have to put one.

what could be the latency problem?

do you think i would need a higher pc?
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20 Dec 2022 19:11 - 20 Dec 2022 19:11 #259975 by Todd Zuercher
Replied by Todd Zuercher on topic G76 problem
The PC should be more than fast enough. Linuxcnc doesn't require any particularly high performance CPU (even a Raspberry pi can be adequate.)

The problem is more likely due to BIOS or mother board configuration settings that are poorly configured for real time performance. Make sure things like Hyper-threading are turned off and power saving features are disabled (C-states set to their lowest levels.)
Last edit: 20 Dec 2022 19:11 by Todd Zuercher.

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20 Dec 2022 19:34 #259976 by Ismacr63
Replied by Ismacr63 on topic G76 problem
I have disabled some bios options that I think are energy saving, (nothing has happened with latency).

I have also changed the processor for another one that I have and the latency has gone down, but it is very strange.
before with the other processor linuxcnc didn't warn me of anything, and now that it has much less latency it does warn me.
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21 Dec 2022 01:19 #259990 by andypugh
Replied by andypugh on topic G76 problem
The servo thread latency there is still terrible.
1.1ms latency on a 1ms thread can't work.
It's strange that the base thread is OK.

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22 Dec 2022 18:29 #260088 by Ismacr63
Replied by Ismacr63 on topic G76 problem
I have been doing all kinds of tests with the pc at the hardware and bios level.
Latency has improved a bit but not enough. servo and base are still maintained between 100,000 and 200,000

Is there a pc to buy that you recommend?

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22 Dec 2022 19:29 #260095 by Todd Zuercher
Replied by Todd Zuercher on topic G76 problem
Are the PCs you've been trying Laptops by any chance?

Laptop PCs are almost never good candidates for running real time applications. They usually have too many built in hardware intrups for power saving features that can't be overridden or turned off.

To be honest I haven't set up a "new" Linuxcnc PC for nearly a decade, and I can't make an experienced guess at a "good" current PC or motherboard. The last Linuxcnc machine I set up I used a used HP PC with an i5 CPU I claimed from our IT depts scrap pile. It gave OK, but not great latency. It was good enough for moderate software stepping with the Preempt-RT kernel.
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