Re: [Emc-users] How fast can EMC read an input pin

[Tom E. <to...@bg...>]

Thanks Jeff and Stephen.  We were thinking of using it for home.  So, if the index pulse is going to be different each time, then how do physical limit switches work any better?  It is a digital input, no?  

Also, Jeff mentioned moving at a snails pace...we will be moving slowly during homing.  We would get near home, find index pulse while moving slowly...

Tom


On Nov 30, 2010, at 12:17 PM, Stephen Wille Padnos wrote:

> Tom Easterday wrote:
>> We are building a plasma/router table and are using Gecko 320X with servo motors.  The Gecko's ignore the index pulse of the encoders and so we want to send the index pulse to EMC.    We are using a D510 with parallel port and simple parallel break out board.  How fast can EMC poll an input connected there? We can build a circuit to latch the index pulse but it would be nice if we can just run it straight to one of the inputs....
>> 
> EMC reads the inputs as often as the thread you put the parport.read 
> function runs.  So if your BASE_PERIOD is 25000 ns, you can read 40000 
> times per second.  Note that jitter and latency will affect the minimum 
> pulse width you can reliably detect.  Given that you're probably 
> generating step pulses with the parallel port as well (based on the 
> hardware you describe), you should be more or less OK.
> 
> Now, what you can do with that index input, I don't know :)  The 
> software stepgen doesn't have any way of using the index input (to latch 
> the current step position, for example).  If you're trying to home to 
> index, it won't work right.  The stepgen rawcounts parameter isn't 
> usable in HAL (since it's a parameter rather than a pin), and the counts 
> and position outputs are generally updated in the servo thread since 
> they use floating point, so although you will see the index in the servo 
> thread (you can pulse-stretch in HAL, as long as the base thread is fast 
> enough), the position would be off by some small amount.
> 
> - Steve
> 
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