Robot ABB IRB120 and PLC siemens via Profibus

Hello everyone.
 I am a student doing self-study for ABB IRB 120 robot that communicates with PLC siemens via profibus. But the robot has not received the command from the PLC. Please share with me how to program the robot so that it receives commands from the PLC. With the simple problem that the PLC controls the conveyor and the sensor reaches the end of the conveyor, the sensor detects it, then the PLC sends a command to the robot to pick up the product and put it in the box.
Thanks very much