Every once in a while I get someone that just can’t understand why one EtherNet/IP node can’t talk to another EtherNet/IP node. It’s a common objection to CIP (Common Industrial Protocol) in general and EtherNet/IP in particular.
Why do they want to do that? Well, for lots of reasons. If one node detects a quality problem, other nodes may want to act on that. If a downstream impurity is high, there is no sense in injecting more expensive raw materials into the process. The best thing to do is to shut that injection process down.
CIP, EtherNet/IP, DeviceNet and the other protocols based on CIP are PLC-centric. That means that they are nearly always PLC based. Those systems have a PLC getting inputs from the network, processing logic and sending new outputs on the network.
Essentially, what these people are saying is that they want to eliminate the PLC. They don’t want to eliminate the logic – they still need the logic. They just want to move it farther down the automation architecture.
This is possible, even desirable in some applications. In fact, that’s what the CoDeSys Open Control Tool is designed to do. CoDeSys is an open PLC language that can be easily (within reason) ported to an embedded platform. You can then write logic for drives, valve controllers, IO blocks and anything else having the CoDeSys control engine.
OK, you do this and now you’ve eliminated the PLC. Great! Hooray – let’s all cheer. But now you’ve introduced a whole host of new problems. You need to keep track of all this low level code that’s distributed around the network, you need to have access to the I/O of the other nodes on the network – remember; you’ll need to make sure that outputs are only driven from one source. And how are you going to synchronize code that is running in 30 different processors.
Now, there is technology that addresses a lot of these problems. And there are people in universities and research labs that are trying out these solutions. Lucky for them, they have lots and lots of time and usually money to buy all sorts of stuff. But the short answer for the rest of us is to just use a PLC. Until there is a real easy way to address the backup, synchronization and management of distributed control we’ll just keep doing what we know works and has the least risk.
Appreciate your PLC more now? If so, just drop out to the production floor and give it a little hug…After all Valentines Day is nearly here.