1. A connected sensor system comprising: a network unit for communicating with a host control apparatus via a field bus; and a plurality of sensor units connected to the network unit, the each 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 which includes a determination signal region storing determination signals obtained from the sensor units and a reception memory region including external input memory regions for giving external input signals to the sensor units, wherein the external input memory regions respectively correspond to the each sensor unit; a sensor setting information memory for storing setting information which is capable of being read and written by the host control apparatus through message communication; 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 executing the message communication with the host control apparatus to read and write the sensor setting information stored in the sensor setting information memory, and wherein the each sensor unit includes: a setting information memory for storing a setting of an external input function executed when the external input signal is received, wherein the external input function includes at least one of a zero-shift function for shifting the detection value to zero, a preset function for adjusting the detection value to a predetermined value, and an output fixing function for fixing the determination signal; 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.