The ilustration below focus of how you daisy chain Profinet (or Ethercat) using a dual ethernet device and optionally create a fully redundant circle.
This design has some popularity in classic PLC designs, but it comes with some pros and cons:
Pros
- It is easy to connect device to device at 100Mbit and even 1Gbit speed.
- Low collision rate even with a high number off devices.
- Fully redundant as it will still function if a cable or device break.
- No extra SW except config on the device.
- Supported by “plug & play” components.
- No need for a Ethernet Switch.
Cons
- More expensive Ethernet interface per node.
- Latency increase with number of nodes in the chain.
As each node act as a switch it typically take 25us to forward a message, so with 10 nodes chained you get 250u latency end to end. Profinet RT uses layer 2 MAC protocol to be faster than TCP/UDP, but it is only in the local net and still suffer this latency issue.
If you have any experience communicating with aPLC you will know that it typically iterate on 1ms and slows down the loop as more code is added, so any signals with less than 1ms latency is basically a 1 ms latency. And 1ms is actually very fast in a good design, so this extra low latency numbers are most often of no value. Higher bandwidth compared to CAN is of value assuming you need it + it is easier to (less work) to code a Ethernet system.
Dual Ethernet cost ca 5.- USD extra and save you the Switch, meaning it save some space. More important is actually the option to interract in a Profinet network if you want to communicate with a Siemens PLC or similar + the nodes can offer full redundancy through various schemes.
Code for a minimal Profinet RT modul is less than 100 lines of C code.