Limit and home swithces with multiplexing with diodes and mux16 component?

Hi all,
I have a diy Plasma with Linuxcnc (it has some mechanical problems and never got it to reliably work... due to modification of the original project due to not having the planned space in my previous location... I will solve...), 

BUT I also had some wiring problems triggering limit switches...

AND, having all limit switches connected to one (or less than the total number) input in my 7i96,
I had to check every limit switch to find which wire was broken or had a loose connection (so triggering).

NOW, Hopefully in a few days, I'm going to be in another project (this NEEDS to work! ) adding Linuxcnc to a proper (not diy) milling machine by a Mesa 7i96S.

SO I WAS THINKING:

The Mesa 7i96S does not have enough inputs for 8 limits + 4 homing switches... SO

what if I make a diode schema to connect 8 limit switches (4 axis) + 4 homing switches to 4 7i96S inputs only and I multiplex these switches inside LinuxCNC?

I know there is a module called mux16 (or something similar, I have to check..), maybe I can connect it's output to the limit/home switches...

Has anyone tried that? Does it work?

This way if one limit or home switch get triggered when the machine is not there, I could already know the one to fix because the GUI would tell me which switch would be in error state...

Thanks for any reply,
Alberto.



 

Thanks Tommylight,
I know I can connect more limits/homing to the same input, but in case of a wiring problem/fault I will have a generic "Limit switch triggered error" and I would have to check all the switches wiring...

Instead if is it possible to multiplex limit and home switches inside LinuxCNC, by adding diodes connections between switches, it will reducing the all problem to 4 inputs only. This way, in the case of triggering I have the exact error like for example "Y Limit swtich triggered", so I can check only the Y wiring (in this example).

A disadvantage of multiplexing is that the switch detection will be slower (by a factor of the servo thread period times the multiplex depth)

You could also use the parallel expansion port to add 17 more inputs (though 5V only and probable should have 220 Ohm or so
series resistors for protection)

Thanks PCW,
that's interesting... so maybe I better think about an expansion card to connect more inputs... I Know 7i96 has RS-422/RS485 and there are cards expansion with many I/Os...
I will think about. My limit switches works at 24V but I can think for a solution if this card works at a lower voltage...

Thank you Tommylight,
looking at the 7i84 specs pdf, it seams the easiest solution.

Inputs and outputs works from 5V to 28V so it's ok (and yes you're right about the half/half voltage, as stated in the pdf).

There are two version of this board:

sourcing outputs: (7I84)
or sinking outputs: (7I84D)

I will use inputs only for now, but... I have to decide which one is better for any possible future expansion...

Any ideas? Better sourcing or a sinking outputs?

(Post Scriptum: probably Sourcing is better?)

Thanks

..AND, I see now, Mesa page says 7i84 is obsolite, there is the 7i84U now..


AND differences are: 
1. Field outputs can be set to sourcing, sinking, or push pull modes on a per output basis.
2. Field outputs do not require flyback diodes for inductive loads regardless of load current.
3. 500 mA output current

So, nice!

Thanks!