As for the application - what data needs to be transported and how often influences the choice of protocol. Sometimes you need to be a little creative in thinking of a solution. One app I had was a sensor that outputted distance data on a regular basis and I needed to do processing that relied on the relative timing of that data. My solution was to use a queue and map that onto a set of modbus registers. The PC would read that block of modbus registers that would extract the data from the queue - thus I wasn't relying on the timing accuracy of the PC - the queue would soak up the difference.