Hi,
If You ask about RS485 I assume that You want to communicate via Modbus RTU. In this case, with raspberry pi I prefer solution:
Use usb/rs485 converter (like this
https://www.amazon.com/JBtek-Converter-Adapter-ch340T-Supported/dp/B00NKAJGZM - it has drivers included into the kernel) or better - some industrial usb isolated usb/rs485 converter)
To communicate with the PLC you have to know it's Modbus ID and registers addresses.
What brand of PLC You want to communicate with?
I use RPIs to communicate via rs485 with Power Meters and it works without problems.
Best regards,