> What additional circuitry is on a PLC input that makes it "hardened" and 
> thereby more suitable for use in an industrial environment than a bare 
> microcontroller?

Circuits vary, but some of the things they have are: Low impedance to prevent false 
triggering, plus good filtering. Over voltage and reverse voltage protection. Often 
optocoupling for isolation and decoupling of electrical noise. Defined threshold 
voltages. Also not to be overlooked - good debounce software in the micro.

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Brent Brown, Electronic Design Solutions
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Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
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eMail:  brent.brown@clear.net.nz


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