Abstract: A communication apparatus inserted in parallel in a two-wire communication line of a two-wire measuring instrument which consists of a transmitter and a receiver connected to each other through the two-wire communication line and transmits a process value detected by the transmitter to the receiver as an analog signal, the communication apparatus being adapted to communicate with the transmitter, comprising a switching device for separating a closed loop constituted by the transmitter and the receiver into a first loop including the transmitter and a second loop including the receiver, a power source for supplying power to the transmitter in the first loop, and dummy output means for outputtting a dummy signal to the receiver in the second loop.

Abstract: A communication system is provided in which a plurality of communication devices communicate with one another over a plurality of transmission lines. One of the communication devices operates as a master station, while the remaining devices are slave stations. The master station periodically transmits transmission line specifying signals which specify or identify which of the transmission lines is to be used by the devices for communicating with one another. The master station, when operating normally, will periodically change the transmission line in use so that each of the plurality of transmission lines is used in turn and for substantially similar periods of time. At such time as any one of the slave stations detects a problem in a transmission line in use, that slave station will transmit an abnormality detected signal.