Re: I/O latency for systems not using a RTOS



On 20/01/2012 03:42, langwadt@xxxxxxx wrote:
On 19 Jan., 17:08, pip...@xxxxxxxxxx (Jack) wrote:
David Brown<da...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
choices here. Linux is never "hard" real time, but it can certainly be
pretty "solid" real time and good enough for uses like this. In

well, actually you can use some patch to make linux hard real time.
For example Linux-RTAI<https://www.rtai.org/>


There are a few systems that let you use Linux in a hard real time system. But they are, in a sense, a cheat - they work by using an independent hard real time OS on the bottom, and running Linux as a low-priority task on the RTOS.

The normal Linux RT options (such as kernel preemption) are good enough for most uses - if you need something with tigher control you are usually best with a separate microcontroller for the critical stuff.


usb will always have a latency of atleast 1ms, thats the frame rate.

must be possible doing hard realtime in windows too, cnc programs like
mach3
does 10s of kHz on the parallelport


No, that is not hard realtime. You can do sort-of real time on Windows, and there are third-party extensions that make it a bit better (though never "hard"). Remember, there is a big difference between "running fast" and "realtime". Yes, you can run the parallel port fast on Windows (or Linux) - but you can't guarantee that it will /always/ run fast. Sometimes the program will be delayed unexpectedly - hence it is not real time.


-Lasse

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