Do the Engineering

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I realized I had a pet phrase to go with my pet peeves: “There’s never a substitute for doing the engineering.”  I realized it was a pet phrase when Gary Mintchell noticed I said it in a comment to his post, More on Industrial Protocols and Standards.

hammer and screwA corollary might be “choose the right tool for the right job; the right network for the right application.”  Much as I like PROFINET, it is not always the right network. Hence my earlier post, Complementing PROFINET.

In doing the engineering, I think you should choose PROFINET for the network backbone, but there may be other networks needed.  In the process industry, PROFIBUS PA, Foundation Fieldbus, and WirelessHART make sense.  For a string of on/off devices, AS‑interface makes sense. For intelligent sensors and actuators, IO-Link makes sense (even though it’s not a network, but digital communications over point-to-point wiring). Fortunately integrating these into PROFINET is easy using proxies.

Doing the engineering includes choosing the right topology.  With PROFINET that will be line, star, or a combination.  Star requires more wire, but you don’t have to worry about one device going down and disconnecting other parts of the network.  In a line topology the line continues through a device but continue means through an internal Ethernet switch… which needs power.  Do the engineering such that devices in a line might as well all go down of one does.  Otherwise, use a star.

Doing the engineering means deciding to use redundancy if the process should not stop or stopping is expensive.  The average cost of downtime is probably $10,000 per hour; your plant might be more or less.  With PROFINET you can use ring topology so a single network break will not shut down the process.  You can also use dual ring topology.  And redundant controllers.  And redundant devices.

Doing the engineering means assessing the safety requirements.  If they cannot be met with guards and signage, you may need PROFINET with PROFIsafe to ensure functional safety messages are properly handled.

Doing the engineering means doing the engineering for your specific requirements.

When doing the engineering, remember that cost is a legitimate design parameter (another pet phrase).  Oh, “cost is a legitimate design parameter.”  Sounds like the title of a future blog post.

–Carl Henning

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