Steve Blackmore wrote: > Just tried something with a very surprising result. > > PC is dual boot with Windows XP, ran Mach3 at 35kHz Kernel speed and > it's capable of seeing the index pulse and giving an accurate rpm > reading to 1000 rpm! I find it odd that Mach can use it but EMC can't > above 200 rpm? Any ideas? > > Mach's dispatch rate can go a lot higher than EMC's, on the same hardware, due to the way Art Fenerty built his ring zero driver, it runs MUCH closer to the core of the CPU than EMC's rtai kernel modules. He saves very little of the system status to enter his interrupt service routine, we have to save a LOT of status for a formal context switch. We can do a lot more things, and do them in a lot more flexible manner, but with all that flexibility comes a reduction of performance (raw interrupt service rate). Really, if you are coming close to the limit, you ought to be using some kind of hardware assist for this. At the absolute bottom, you could put in a one-shot chip to stretch the index pulse to be at least twice your sample rate, than you should never miss a pulse. This could be a 74HC123 and one resistor, one capacitor.
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