There are two general classes of process control systems, local or machine control and large scale factory automation. Recently the barrier between these two classes of systems has been decreasing. Programmable Logic Controllers (PLC’s) are used for local or machine control and typically output or input voltages or currents for control and measurement of actuators, transducers, and sensors. Distributed Control Systems (DCS’s) are used for large scale factory automation and typically use 0-20mA or 4-20mA current loops for control of field instruments. Field instruments are sensors, actuators, and transducers that measure, record, and control physical variables, such as flow, temperature, and pressure. Field instruments usually include isolation and communication to a DCS. For communications, PLC’s predominately use isolated local communications, such as RS-485. Ethernet is used where communication is required to a central host. DCS’s predominately use HART superimposed on the 0-20mA or 4-20mA current loops for communication to the Field instruments and Ethernet for communications to the central host.