At home, I play with arduinos sometimes; at work I play with PLCs. I often think about bridging the gap, but that strikes my intuition as an unwritten (or at least unknown to me where it is written) taboo. But is it? I have seen other users here mention arduino here, but just in passing. I have seen one-off proprietary circuit boards installed in machines, which I highly doubt were put through the years of testing that PLCs tout. So is all that testing and certification really a requirement for a circuit/device to be embedded in machine logic? Or is it up to the discretion of the machine builder? Could a machine legally be controlled by an arduino? Would it be unwise? What are the insurance ramifications? On the technical level, I don't see any reason why not; I have put arduino in harsh conditions and had one running a process since 2010 without failure. But still, if I were a machine builder, I would be reluctant, for reasons that escape me.