The ComNet FDC80 Series contact closure transmitter and receiver units provide transmission of up to eight independent supervised or unsupervised dry switch or relay contact closures over a single optical fiber, or any two-wire RS-485 copper circuit. Microprocessor-based logic transmits the contact closure information in packets that are ordered and encoded, ensuring extremely robust and reliable transmission. Packets that are garbled, or packets out of sequence with transmission bit errors will not cause random changes of state in the FDC80R receiver unit relays. The supervising function in the FDC80T transmitter allows for the detection of short and open circuit conditions, and normally open or normally closed contact inputs are supported. System faults are displayed via LED status indicators, and user-programmable summary fault alarm contacts are triggered in the FDC80R receiver unit. The FDC80R receiver is offered in both latching and non-latching relay configurations. External series and shunt resistors are included for each of the 8 contact input channels, for contact supervision.