FC Block explanation

I am having speed control issues with a machine. Its being driven by a danfoss inverter. This is getting its speed control via profibus. The high speed is triggered by a sensor which goes to #TEMP8 in FC91.
The inverter is not getting the signal to go to top speed but i dont know how to trace this.
Can someone explain what is going on in the Transfer Byte FC90 please.
Or has anyone a link to a help guide

Many thanks

What is the relevance of FC90 since the speed control you have shown is done using FC91?

Can you post FC91?

Post the code (create a library and then insert an S7 program into the library, copy the blocks to the library, archive and then attach to a post)