Basic Robotics

Hello, I have a conveyor system and a 6 axis robot . I want to know how will the robot know if there is a box waiting on the conveyor ( ofcourse there is a sensor).
How will it communicate with the PLC which controls the conveyor. Is there a interface (devicenet?) between plc and robot ( if the sensor goes high the plc send the data to devicenet and then device net informs the robot to start the program for pick and place ? i am not sure) i am asking this question because it was asked to me in an interview for robotic engineer . i would also like to know how the communication works. Are there any standard command that the devicenet sends to robot ( eg. 1 - start run, 2- stop run). It would e helpful if anyone can answer any of the questions.
THanks PRatik
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