ADLINK PCI-7442 - Manual (Page 39)

Operation theory 27
3 Operation theory
3.1 Isolated digital input
The PCI-7442/7443 card comes with 64/128 opto-isolated digital
input channels. The circuit diagram of the isolated input channel is
shown in Figure 3-1.
Figure 3-1: Photo Coupler
The digital input is routed first through a photo-coupler (PC3H4) so
that the connection are not polarly sensitive whether using positive
or negative voltage. The normal input voltage range for high state
is from 5 V to 28 V.