Advantech USB-4751/4751L - Manual (Page 21)

15 Chapter 3
3.3 Digital I/O Connections
3.3.1 Dry Contact Support for Digital Input
Each digital input channel accepts either dry contact or 0 ~ 5 VDC wet
contact inputs. Dry contact capability allows the channel to respond to
changes in external circuitry (e.g., the closing of a switch in the external
circuitry) when no voltage is present in the external circuit. Figure 3-3
shows external circuitry with both wet and dry contact components, con-
nected as an input source to one of the card's digital input channels.
Figure 3.3: Digital Input Connections