The GPO pins are open-collector current sink outputs. The GPOs can handle a maximum of 15mA and 40V with respect to ground. To use them with TTL inputs, the GPO output lines must be pulled high with a 1K ohm resistor.
Example 1: TTL Output
The figure below shows the GPO outputs from the camera being used to output a TTL signal.
Example 2: Synchronizing multiple cameras
Here's another example whereby the GPO from one camera being used to trigger a second or more cameras. See Tech Note 001 on camera synchronization using this connection method.