A code skeleton written for a modbus PLC and an atmega128 is in place. So the ideas are there but the new PCBs are on their way and the components so a PLC is going to be built and mounted in an enclosure before the 128 code is converted in sections into that for the 328. I will use a modified 9 pin D female connector on the enclosure which will not accept an rs232 cable.
My programmable load uses modbus via a 9 pin D connector, rs232 but TTL level. They warn against connecting a real rs232 cable as it would damage the interface. My experience and knowledge is rudimentary but I really think that is crap. Make connectors physically unique as if there is a way to blow something up people will find it and it will not be there fault.