On 10/14/2016 09:50 PM, Chris Albertson wrote:
> No, it would not get you to ns levels.
>    I've
>> wanted to set this up on X86 through the parallel port for
>> some time (since about 2002, in fact) but it would take some
>> serious hacking on the PPMC driver.  It looks like it might
>> be easier with the uspace / rt-preempt kernel than with
>> rtai.  It would reduce latency/jitter to ns levels.
Yes, actually, it WOULD.  All the Pico Systems motion 
control boards can latch the position when the internal 
timer ticks.  So, the position latching would be accurate 
down to the jitter of the crystal oscillator.  (If multiple 
boards were used, then the latching would be accurate to the 
difference between the clocks on the different boards, so 
jitter would increase to 1 us, maximum, or 100ns if the 
encoders were updating at 10 MHz clock.)
The master board sends the latch command on the parallel 
port bus to other boards on the bus, as well as the 
computer.  The computer would have the luxury of responding 
before the next timer tick, nominally 1 ms.
As long as the computer never took more than the timer 
period to complete its work, it would never cause ANY 
latency at all!  The computer's updated velocity info would 
not become effective until the next timer tick.


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