Why does Basler GPIO need to have an external powersource + resistor to its GPIO output
Without external power supply you only have 3.3V and 0V logic levels. Which is fine if you're talking to an Arduino or Raspberry pi, but any other industrial controller won't see a logic high at 3.3V, they need at least 10V according to IEC 61131-2.
Hence you connect an external supply in the range you need. And the circuit will operate the same, except the logic high level now is the external supply range.
There will not be any damage due to the diodes inside.
Be aware of the resistor, the output shorts pin 3 and 6 together, this means the resistor is the only element limiting the current.
The industry name for this is &quot;open collector&quot;.