Can I/What can I do…?

Would it be possible to configure a Revolution Pi device to continuously run a PHP program, and is there documentation regarding the installation of PHP in RevPi?

Yes, take a look here:

(661) PHP in RevPi - The Industrial Raspberry Pi Forum (revolutionpi.com)

I would like to access the built-in RS485 interface of the RevPi Connect - which interface is the right one?

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