[En-Nut-Discussion] protecting inputs

[William Baker]

I am trying to protect the inputs on my Ethernut.  Under normal 
circumstances, the inputs should be wired to a nice protected TTL 
friendly 5V circuit.  However, this is an industial environment and 
there is a lot of 24V circuitry on the device.  I am sure that circuits 
will be misconnected/shorted during assembly/maintenance/testing of the 
equipment.

I'm happy to add a friendly daughter-board with screw connectors, but am 
unsure about the circuitry.  I need the device to survive a temporary 
24V misconnection to any of the digital inputs.  Zener's and resistors?  
Unity gain op-amp?  I'm a software guy with only passing knowledge of 
hardware.

Any recommendations?

I realize this an electronic question and not 100% on topic.  Please excuse.

bbaker