Does PROFIsafe use up bandwidth?

This is a question we received at a recent PROFINET training class.

Does PROFIsafe use up bandwidth?

The answer is: “NO”.

For a full description of PROFIsafe, read the PROFIsafe System Description. For a short refresher, watch this video:

PROFIsafe is used to exchange data between a safety device (F-Device; e.g. emergency-stop button) and a safety logic controller (F-Host; e.g. PLC). The device and controller exchange I/O data just like any normal controller and I/O device. There are no extra messages for this. This is why PROFIsafe doesn’t use up any extra bandwidth. All PROFIsafe is doing is ensuring messages sent from one end of the network to another arrive at their destination. PROFIsafe is a protocol-independent Application Profile. The communication, transport, and physical layers are transparent to the PROFIsafe message.

A good analogy is writing a letter, sealing it up in an envelop, and sending it to your friend. When your friend gets the letter, he/she breaks the seal on the envelope and reads your message. How the letter gets there is unimportant to you and your friend. Perhaps it got there via truck, or airplane, or maybe even a mule/donkey train. All that matters is the letter got there. If the envelope seal had been compromised, your friend would know right away.

The end result is a few extra bits in a PROFIsafe telegram (as compared to a regular I/O telegram), and not extra dedicated telegrams.