A communication system uses controllers for bi-directional protocol conversion. There is a plurality of remote signal inputs comprised of supervisory control and data acquisition (SCADA) signals, and/or remote terminal unit (RTU) signals, and/or instrumentation monitoring signals, and/or instrumentation control signals having different protocols. The signals are either in synchronous and asynchronous data formats and include voice inputs. Multiplexing such signal inputs into a composite signal is effected for transmission over electronic and physical communication media using X.25 or higher protocol, at an aggregate transmission data rate of 9.6 Kbps or faster. One or more controller can be configured as the master of remote units. Such units can perform remote signal input emulation; master/remote apparatus emulation, permits user-defined signal port control access; and provides operational diagnostics of the units and their associated communication link.