Have you wondering...is Arduino rough enough for an industrial environment?...I have seen some blogs and forums with endless mixed opinions....some who think it can be...some definitely not. I incline to not take the risk and avoid put the Arduino (or based Arduino boards) on industrial environments but....after make some reverse engineering...let's rethink this topic. On the hardware side I use to believe that the PLC's have some very specialized MCU or a very complex design with some filter design on their inputs, chokes or super electrical protection (ok...maybe the high end PLC family, the very big boys on industry it really has this specialized architecture with redundant processors and ultra high noise filtering ) but for the most common applications... the medium or base family of PLC's, this is not that high end tech. For example...this is what you can find on a base Zelio Logic PLC (who has a good performance on the field applications)... ok...more close...more close... first you can note a known friend...yes an ATMEGA128 MCU...so not a very specialized IC. Second thing I want to point...some time ago when I was trying to design an input board for the Arduino; I made some reverse engineering from an OMRON input module and I take the optocoupled input design for 24Vdc inputs...but in this case you can see that in the Zelio Logic it's enough with a resistor arrangement and a simple RC filtering circuit...and that's all...this goes directly to the MCU pin...so you can say what you want...this works and works fine... on the output side not optocoupled again...some simple classic transistor-relay driver and that's all... so in the hardware side you can see that a very simple design can do almost all of the simple (and not so simple) work...just use a good reliable power supply and that's all...and again you can choose the optocoupler way to keep in peace your mind...but again...of course the Arduino can do de job for good sake!....there are almost not difference in the components and the design is not out of this world.... But....the really risk relies on the software...some bug on the program and maybe you can see the magic smoke!!!....the PLC brands have some stable software platform to do the job just right...so is always a theme of test and debug your software before installing....but to be realistic a lot of applications are so simple that you can do a very good, simple and reliable program and for those high end applications were other equipment could be on risk just use a PLC from a big boy players on the market....see you soon!!!