Analog signals in profibus
Here's a decent picture of a typical bus system: -



The system consists of a master unit and many slaves. The "wire" which connects everything up carries the digital data from all the slaves to the master. If the slave happens to be a monitor for some temperature sensor, it will digitize the temperature and at some point the master will interrogate that slave and collect that data. 

So, in short, only digital signals are sent over the wire but, some of those digital signals represent analogue values. 

I stole the picture from here. Below is another picture: -