Hi svoicum.
You can download to the OP77B via MPI or Profibus.
You probably have to setup the port on the OP77B to the correct Profibus settings. Upon powerup, I believe that you can choose to go to a special menu for changing settings.
If your programming device is a serial or USB PC adapter, then it needs 24V from the port on the PLC.
So you need a Profibus connector with a PG socket on the port on the PLC. You connect a normal (purple) profibus cable from the connector on the PLC to the OP77B. And you insert the PC adapter on the PG socket on the connector on the PLC.
You can then program both PLC and OP77B.
Edit:
I realise that you must have an IM151-7 CPU + a Profibus Master interface module attached.
Is there any particular reason to that the OP77B is attached via Profibus ? You could have connected it via MPI. Do you use the Profibus interface for other things, maybe i/o ?
Anyway, you can freely choose to connect via MPI or Profibus.
You can connect both PC adapter and OP77B via MPI, or connect both PC adapter and OP77B via Profibus.