Pin 1 of the U1 connection (labeled GO) will be 24 volts in order to ground pin 7 of the TLP504A. What is the purpose of the diode?

 

Apparently to protect the internal LED from reverse voltage.
The diode limits the negative voltage across the LED to appx .7 volts.

 

How does it do that?

 

How does it do that?

Reread my post.

 

The means of protection against a negative voltage are provided by the diode BECAUSE as the input voltage would tend to increase in a negative direction, the diode will be biased into conduction and tend to direct the negative voltage into the "ground" instead of the A1 circuit connection. So it can also be called a "Negative Clamp" diode in this application.