Hi,
You can generate a "square wave" with the FP-DO-410, but this is not normally the way this module is used. This module contains "static digital output", which means you can only write true or false, in other words, you cannot exactly control the frequency of the output signal, if you want to have control of the frequency you normally use a counter module (FP-CTR-50X) or even better a FP-PG-522... these modules have their frequency hardware controlled...
You can create a square wave with the FP-DO-410 but in that case you will have to create the square wave via software and the frequency will be software dependent, connection dependent and so on... the final result will be very poor (no practical use)...
So, if you want to create square wave I
would advise the FP-PG-522.
I hope it helps
Best Regards
Andre Oliveira