There was an old TV commercial for oil filters that warned you to change the oil filter or risk damaging your engine which would cost you more later. The tag line: “Pay me now or pay me later.”
This relates directly to choosing an Industrial Ethernet network. Well, maybe not real directly; in fact, it’s a bit of a circuitous path. Let me lead you through my reasoning…
This was triggered by an article in the Industrial Ethernet Book, “Technical Article: Modbus, Real Time, Determinism and Reality in Plant Floor Control” about Modbus. It starts with some examples of “real-time.” The examples run from 250 microseconds for motion control through about 10 milliseconds (or less) for discrete automation and about 100 milliseconds for process instruments. No real quibbles here except maybe a little one – there are discrete automation processes that need closer to one millisecond speeds.
The article goes on to provide a “Modbus perspective,” concluding: “Modbus protocol, leveraging the Ethernet TCP/IP network solution, meets 90% of control applications…”
Another little quibble, this time with the percentage. Modbus does not do motion control. I’ve heard it said “Use Modbus with motion control, not for motion control.” ARC did a survey of the discrete market a couple years ago and found that 38% of the “discrete market” was some form of motion control, from CNCs to general purpose drives. Discrete applications that need closer to one millisecond response times are also not good candidates for Modbus.
The last sentence in the article is: “Why pay for what you don’t need?” The implication is that Modbus is a cheaper solution that’s good for most applications. Here’s the “pay me later” part: If you install any network that does not do 100% of what’s needed, you’ll wind up adding a second network to do that part, be it motion control, wireless, safety, or whatever. This forces your engineering staff and your maintenance staff to learn two systems (after you buy a second set of configuration and diagnostic tools, interfaces, etc.). PROFINET is the 100% solution covering discrete and process, motion control, safety, wired and wireless, integration of legacy networks, peer-to-peer integration, and vertical integration. The integration of legacy networks feature is important, especially if you already installed a 90% (or less) solution. You can connect it to PROFINET using a proxy. Pay once.