PROFINET is not a star… but it can be when you want it to be – speaking of topologies, of course; in performance PROFINET is a star!
I read the article “Network Behemoth” by Dan Hebert at Control Design which featured this section:
Hard to Deal with Stars
Dealing with stars can be difficult, whether it’s the movie industry or industrial networks. “The biggest problem with Ethernet is the star topology. If I have ten devices, I need at least a 12-port switch, and that’s expensive,” explains ADCO’s Gilpin. “If each Ethernet-enabled device had a minimal switch built in, it would be simple to construct a ring topology network, and Ethernet adoption would progress at a more rapid rate.”
That “biggest problem” with Ethernet isn’t really a problem at all – PROFINET devices have had built-in Ethernet switches for years now. Some of the other Industrial Ethernet technologies used to pooh-pooh that idea, but are now following the lead of PROFINET. With PROFINET you can have any topology: star, line, tree, or ring… or wireless.
“But if I use a line topology and a device fails, the rest of the devices in the line are no longer available.” So observed a caller the other day as we discussed PROFINET topologies. True, I answered. And falling back on one of my favorite sayings, added, “There is no substitute for engineering.” If the rest of the devices in the line are impacted by the failure of the first device anyway, go ahead and use the line topology. If they are not impacted, perhaps a star, or more likely, a tree is appropriate. Do the engineering. (Attend one of our PROFINET one-day training classes for tips and techniques on topology.)
Even more appropriate might be ring topology, especially if communications is critical. PROFINET is the only Industrial Ethernet that provides several choices for ring redundancy, including one that is completely bumpless! Ethernet/IP has a “competitive” technology called Device Level Ring (DLR), but it takes milliseconds to respond. How many thousands of ball bearings are bouncing around the factory floor if the command to stop the line is three milliseconds late? With PROFINET you won’t have to worry about the answer. PROFINET simply sends the message around the ring in both directions. When a PROFINET device receives the second message, the one from the other direction, it does not use it or forward it, it discards it. Bumpless redundancy.
Speaking of stars, I visited the Rock and Roll Hall of Fame Museum in Cleveland in September. (This is related… really.) At the museum they had a video presentation highlighting all of the inductees and George Martin’s name flashed by. I wondered how many people knew who George Martin was. I knew, of course. If you don’t, check the Wikipedia article about him:
British record producer, arranger and composer. He is sometimes referred to as “the Fifth Beatle”—a title that he owes to his work as producer of all but one of The Beatles’ original records, as well as playing piano on some of The Beatles tracks—and is considered one of the greatest record producers of all time.
I was already listening to rock and roll when the Beatles rose to prominence. I was very young of course. My parents did not think much of rock and roll, preferring the big band sounds of their youth. If you put the two unlikely styles together (Beatles plus big band) you get a George Martin CD like “Off the Beatle Track.” Proving, as I listen to MP3s from that CD, that even rock and roll is good music.
Fieldbuses like PROFIBUS and PROFINET are like George Martin. They are not as visible as DCSs, PLCs, PACs, IO, drives, vision systems, valves, instruments, or automation in general. BUT without them producing, arranging and composing automation projects, the projects would be a little flat. You heard it here first: PROFINET, the fifth Beatle of automation and a star in its own right. Yes, that has a good ring to it.
–Carl Henning
More on topologies with this design guideline: