An environmentally controlled transportable container monitoring system including a data acquisition unit mounted to each container monitorable by the system, and a central microcomputer system located at a container storage site, e.g. a container ship or land based terminal, which communicates over existing on-site power wiring with the data acquisition unit of all containers plugged into the on-site power. The container data acquistion unit includes electronics forming a microcomputer adapted to store current container parameters, prepare a history of the container's past condition, and alert a system operator or other persons of an "out of limit" condition, a possible container malfunction, and to diagnose the source of the malfunction. A signal transfer circuit is provided in the data acquisition unit and a separate signal transfer unit is provided in proximity to the central monitoring microcomputer system wherein serial data outputs from the system's microcomputers can be conditioned for transmission over the existing on-site wiring to permit bidirectional communication over such wiring. The system provides for minimizing noise induced operating problems and for detecting those problems when they occur; it also preferably generates information for identifying the precise location of each container within a particular storage area by means of a location transmitter provided at each defined on-site container storage location.