Solution:

Please refer to the PCI-1750's manual pg.13 figure 3.1. When use wet contact method, it is recommended to couple a resistance with the input voltage. And the external input voltage specification are as follow: High => 5V ~ 48V DC Low => 0V ~ 2V DC As you can see in the Figure 3.1, when the external voltage in the ID0 is higher than 5V, the current will not flow from 'internal ISO 5V' to GND. Then the photo transistor will not enable. On the other hand, if the external voltage is lower than 2V the current will flow thought the diode, and the transistor will be enabled. But at this time, it needs a path for the current to flow into GND, if the internal impedance is too high then we need a extra path to bypass the current, that is why we suggest the user to couple the 5Kohm resistor when using wet contact method.