D-Link DCS-2102 - Manual (Page 57)

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DI/DO Input specifications
DO Digital Output:
Normal: open circuit
Trigger: short circuit
After powering on or rebooting the DCS-2121, the camera will be in Normal mode by default.
DI Digital Input:
Normal: no current
Trigger: 12V current
As the Digital Input signal is controlled by an external device, we recommend that you set your device to feed no current normally, and feed a 12V current only to
trigger a DI event on the DCS-2121.