1. A connected sensor system comprising: a network unit for communicating with a host control apparatus via a field bus; and at least one sensor unit connected to the network unit, the sensor unit obtaining a detection value according to a physical amount detected from an object to be detected, the sensor unit outputting a determination signal upon comparing the detection value with a previously-defined threshold value, the connected sensor system being constituted by coupling the sensor units with each other, wherein the network unit includes: a host communication interface for connecting to the field bus; a cyclic communication memory for storing data for performing cyclic communication for sharing data with the host control apparatus via the host communication interface, the cyclic communication memory including a transmission memory region and a reception memory region including a external input memory region for giving an external input signal to the sensor unit; a sensor communication interface for communicating with each sensor unit coupled with the network unit; and a control unit for executing the cyclic communication with the host control apparatus, as a result of the cyclic communication, the control unit transmitting the external input signal stored in the external input memory region to each sensor unit via the sensor communication interface, and wherein the sensor unit includes: a setting information memory for storing a setting of an external input function executed when the external input signal is received; and a control unit for receiving the external input signal transmitted from the network unit and executing the external input function stored in the setting information memory.