servo thread limit for mesa card?

Hi,

I'm using combo 5i25+7i74+7i84+7i83.

Today when i tried decrease servo thread from 1ms to 125 us, Some warnings occured, they are " Unexpected realtime delay on task 0 ..." and " Smart serial port 0 : Doit not cleared from previous servo thread ".They are still appear when i disconect remote serial daughter cards They are just not appear when i set servo thread at 250us.

Is this limit for 5i25 card or problem come from my PC? i did a latency check and saw PC is still ok

Will be better if i use PCIe card?

Usually 4 -6 KHz is about as fast as you can run smart serial devices/PCI cards on normal PCs
Basically the latency and access time cause overruns.

Note that the latency test only checks dispatch latency, not hardware = memory or I/O latency which can be much higher
than the dispatch latency.

To get an idea of actual latencies take a look at the *.tmax parameters:

halcmd show parameter *.tmax

(these numbers are in CPU clocks so need to be divided by the CPU clock frequency to get seconds)

thang wrote:

Hi,

I'm using combo 5i25+7i74+7i84+7i83.

Today when i tried decrease servo thread from 1ms to 125 us, Some warnings occured

Why do you WANT to run your servo thread at 8 KHz? Do you actually need 3-4 KHz servo response bandwidth?
Usually, servo bandwidth of a few hundred Hz is adequate for machine tools and such. Having vastly excessive servo bandwidth can just lead to a lot of trouble in tuning.

Of course, if you are steering laser beams with mirrors or something like that, then you may be exactly right to turn up the servo rate. But, know that there are limits to how fast a typical desktop computer can go. For running a servo thread in this range 8 KHz+, it may be necessary to use purpose-built real time controllers, rather than ordinary PCs.

Jon

i'm controlling AC servo in analog speed model, so i want to increase servo-thread to increase PID's performance

There is little to be gained by running velocity mode servos at faster than about 1 KHz servo thread rate
unless you have very high acceleration requirements or require very high precision at high speeds

The drive will close its local velocity loop at a much higher rate (perhaps 4 to 20 KHz), and the high bandwidth parts of system performance depend more on the drive tuning than LinuxCNCs position mode tuning

Hello,
Hope that this isn't hijacking this thread too much. I have a Intel D945 motherboard with onboard graphics and 5i20 & 7i33 setup.
There is no base thread, only servo thread at 1 ms. I haven't done a latency test but the question is what is acceptable latency and jitter for a servo only system with this hardware?

TIA,
Dave.

100 usec jitter is normally not an issue with such a setup and more (up to say 300 usec) can be tolerated using firmware with a DPLL module to decouple the encoder sampling time from LinuxCNC's read time.