AJA FS1 Version 2.2.0.47 - Manual (Page 118)

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The following illustration shows typical external wiring to the GPI connector.
The GPI inputs require some kind of contact closure between the input pin and
the input ground pin to register the logic low that triggers the GPI input.
You can connect the outputs to TTL buffers that communicate the GPI output
logic levels to other devices. For example, you could use an opto-isolator
controlling a relay to activate other equipment as shown below.
Typical GPI Input and Output Connections
GPI Output 1
GPI
Input 1
GPI GND 1
+V
xmit+
xmit-
Optical Relay (SSR)
To Tally Lamp etc.