I think going directly from the Touchpanel PC to the converter would make more sense in most cases where the system does not need be such that it need to be able to run only PLC (without the PC powered on).

So if your DMX-512 lights need to react to the PLC controlling (for example react to PLC inputs or control program in it) and you want the system to be able to work without the PC, then trying to make things with PLC makes sense.

If your system can rely on PC being on all the time you want to control the DMX-512 lights, then I think interfacing the DMX-512 converter directly to the PC makes much more sense.