PROFINET Forum

What is the advantage of sending IO data cyclically compared to sending IO data only when a data change occurs?

Webinar_Question wrote: ↑22 Mar 2024, 12:58 What is the advantage of sending IO data cyclically compared to sending IO data only when a data change occurs?

PROFINET was designed for cyclical data exchange. Ultimately, it is a question of philosophy.

In a machine, you normally have many sensors where the input information changes more or less quickly or actuators have to receive new values. Cyclical data exchange ensures several things at the same time:

• 1. The response time of the system is predictable.

• 2. In the event of a communication interruption, this can be recognized very quickly.

• 3. The network load of the system is predictable.

• 4. In the extremely unlikely (but theoretically possible) event that an Ethernet frame was corrupted during transmission and therefore incorrect data was transmitted, this error is automatically "forgotten" during the next update.
  • A: In the case of the controller, it is even very likely that the control program will not process the incorrect data at all, as the bus is always updated 5..10 times faster than the user program is called (and therefore the correct data is very likely to be available again).
  • B: In the case of the periphery, the output data will only be flat for one cycle, e.g. 1 ms, after which the correct data is immediately returned.

In the case of event-based updating, the network load can increase uncontrollably (rotary encoder) or many events that are read in simultaneously at the same peripheral station. In case 4, process would also continue with an incorrect status until the signal changes again, which means a new frame must be sent.

In general, all bus systems in automation work with bus cyclical updates.