A comprehensive control system and method of great flexibility. Submodules include the specialized circuitry needed to interface to particular types of field inputs and outputs, but present standardized interfaces to modules or other devices on which they are mounted, reducing overall system cost. Modules can be mounted on backplanes, which suppress outputs to module connectors when modules are absent. An extension to the IEEE 1284 Enhanced Parallel Port standard allows units to be connected in a flexible manner. Submodules include a current-limiting universal digital input submodule able to sense AC signals or DC signals regardless of polarity, and an analog input module offering great flexibility in choice of input signals.