A smart field-mounted control unit, for controlling a process, receives signals and sends a command output over a two-wire circuit which powers the control unit. An input section receives the signals, which can be instructions representative of commands or instruction sets, process variables sensed by external control units or setpoints representative of a desired process state. The instructions are representative of a control requirement of the process and adjust a controlling section in the control unit to generate the command output in conformance with the control requirement. The command output can be a function of the difference between the process setpoint and a process variable, or a function of a linear combination of a process variable and its calculated time integral and time derivative functions. A sensing section in the control unit can sense and scale a process variable for generating the command output as well. The control unit can include a regulator section, controlled by the command output, which regulates application of a mechanical, hydraulic, pneumatic or electromagnetic force applied to the process.