Here is a snippett of my schematic drawing showing how the input citcuits would work. This just shows 2 inputs wherein the actual board has something like 32 inputs.



Note that the SP721 is an ESD clamp device, the SMD050-2 is a surface mount polyfuse and the CCLM3500 is a 3.5 mA constant current diode. Input circuits like this work nicely with a wide range of supply voltage bias from about 4 volts to as high as 35 volts. If you look at the typical industrial I/O board that has ruggedized inputs with opto copuling they use a big resistor in series with the opto coupler and this limits the input voltage range since the opto diode current varies with input voltage level.

I have used similar circuits on indistrial equipment for years and years. In the past however I had typically just put the high and low sides of the opto coupler curcuit to the I/O connector and let the user wire up the inputs to voltages and drivers as approproiate. Here I am trying to simplify the external wiring so that there is just one wire per I/O line.

Michael Karas