Panasonic S-LINK transmits 128 points on two signal lines, and "T"-branch multi-drop system enabling flexible cable layout. Large voltage amplitude (24 V) and wide pulse width (35 μs) signal transmissions make for units less prone to impulse noise effects with no code errors. This high level of noise proofing enables them to be used even in worksites with conventional, high-priced optical communication remote I/O units. In the event that verification cannot be obtained from an S-LINK I/O unit, such as if the main cable is cutoff, the address of the particular unverifiable S-LINK I/O unit is specified and displayed allowing equipment recovery time to be greatly reduced. Labor-saving hook-up connectors are used enabling multiple "T"-branch hookups. It goes without saying that cascade wiring (bus wiring) as well as multiple branch wiring (star wiring) is also possible.