I don't know any specific Function Blocs for DMX-512 generation for any PLCs. If the PLC has RS-485 output that is capable of 250 kbit/s data rate then it might technically possible to wrie such for some PLC... (someone who knows well programming that specific PLC should do that)

One idea that might be technically more sensible is this: Take a RS-232 to DMX-512 converter (commercial or DIY model). Those typically take simple commands (few characters) through RS-232 port to control the DMX-512 channel data inside the converter. Then your PLC sends out those specific command characters through it's serial port when it need to change the DMX-512 data that it sent out... The converter handles all the DMX-512 specific things (RS-485, repeated sending and needed data, accurate data signal timings etc..)