USB should be able to handle real time but think it is complex which might be a 
reason to avoid it and there are no support in Linuxcnc. There are however a 
lot of cheap development boards available which may make it a very cheap and 
good choice. There are standard protocol for keyboard and mouse but do not know 
about this kind of input you want.

Switching noise might be a problem if connected to power electronics but not to 
hand held device.

There is Modbus available in Linuxcnc. It might be a little bit slow nowadays 
but expect it more than good enough for buttons. Think it is also simple and no 
problem to build a device using one of these development boards available but 
otherwise a Modbus Remote IO Module might be a good choice.


Emergency stop, any plan for this?


> I have actually been wondering about the same/similar thing, on how to 
> feed information into hall.
> 
> On 6/4/20 2:33 PM, Chris Albertson wrote:
> > I'd like to build a handheld pendant controller.   I have some ideas about
> > how it should work.  One thing I want is an LCD screen.   This could evolve
> > into a "smart pendant" that can do things like move along an arc, rather
> > than just one axis at a time.
> >
> > I think the best physical interface from a handheld device to the computer
> > is USB.    Is there a way to get data from USB to HAL?   I could use a USB
> > virtual serial port but then the question is getting serial port data to
> > HAL.
> >
> > Maybe the best interface is wireless, so how to get data from a UNIX socket
> > to HAL?
> >
> > Note that we don't need "hard" real-time to the computer.  Latency of a few
> > tens of milliseconds is acceptable for a human interface so this could be
> > done in userspace.  If so, then maybe all I need is a way to get data from
> > an arbitrary Linux application to HAL.
> >
> > It just occurred to me that this "arbitrary Linux application" could be the
> > Apache webserver.  Then you run a browser on the tablet.
> >
> > Summary:   How to move data from USB or serial or socket to HAL?


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