Hi Russell,

Seems the consensus is that the circuit on the E_x path but with a
Schottkey diode is the best way to go other than perhaps also adding a
second Schottkey for negative polarity signals.

The L_x path will quite rightly toast the diode if the power supply is
low impedance.

I've gone to a 74LS245 type driver simply because the pins allow the
signals to go straight through the package as compared to the 74LS240
driver at the extra cost of only a few pennies.

The circuit is from the ELS schematic block called Filters.sch and the
two lines of interest are the ESTOP input and LIMIT switch Input.  Full
schematics, layout, BOMs and Gerbers will be published on the
E-Leadscrew group in the near future.  Processor is a PIC18F4680.
Software, also to be published under GNU license is compiled with MCC18.

John Dammeyer


> 
> 
> >> Let's see.  12V one side of the diode, VCC the other.  Either VCC 
> >> will
> >> get pulled up towards 11.4V or VCC holds its own, the diode goes 
> >> boom
> >> and goodbye protection.
> 
> http://www.pacificsun.ca/~john/ELS/photos/IO_Circuit.jpg
> 
> Assuming that _I means input and _O means output, as is clearly the 
> intention then:


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