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To synch or not to synch?

Feb. 21, 2023
Just as the phone call requests that one interrupt whatever they’re doing, synchronous communication on fieldbus requires a receiver/consumer cease what it’s “doing” and process the message.

That is the question, as I tried to persuade my very annoyed spouse. “What can’t people just answer their phones!” was her complaint. “They just texted me,” she exclaimed. Therefore, they should be available, one might conclude.

“I would just text them back” was my best suggestion, which was no comfort whatsoever. “I hate texting,” was her reply. She’s less enamored than me with features like “swiping” or text-to-speech. I’m confident that her incompatibility with texting isn’t going to be cured by me, but it did cause me to reflect.

Those of us old enough to be annoyed with texting probably recall a day when analog phones were the primary communication tool of business. On-wrist video chat was a prescient fiction dreamed up by Chester Gould for “Dick Tracy” (a comic printed in daily newspapers). Most folks in business had an “administrative assistant,” who would pick up missed phone calls and deliver a small pink slip with the name and number of a caller aiming to contact us. Another pink paper message would be generated if you called back, and your contact wasn’t at her desk.

If you were an individual whose duties required a timelier response, you might have carried a pager. I remember an office mate who felt his pastimes would render him unfit for driving into the plant anyhow; those calling his pager after hours could often hear it rumbling away in his desk drawer. But even in the current age of “5G” cellular service, some occupations still utilize pagers for the service’s more far-reaching and reliable network.

Voice messaging and the fax machine represented some progress, but the technology I found most liberating was email. Once all your goods and service providers had it, you could blast a single message out to all of them simultaneously. It saved me five phone calls when I changed the scope of a project for which I was soliciting proposals, or when I changed the due date for a bid. And everyone received precisely the same message.

Texting, I suggested to my wife, was liberating like email: it freed the sender from the trouble of connecting by voice. And now that all her contacts had a multi-featured computer in their bag (that included voice service) she need not interrupt them from what they might be doing – driving, for example. When I was frustrated by the lack of response from emails, even my less-than-uber-techie-boss suggested “try texting” – which worked.

Messages sent on industrial networks like fieldbus also can be synchronous (to a degree) or asynchronous. Just as the phone call requests that one interrupt whatever they’re doing, synchronous communication on fieldbus requires a receiver/consumer cease what it’s “doing” and process the message. The message might be an alarm, for example, that benefits from a precise timestamp and prompt display on the HMI. They resemble the network-connected pocket computers most of us possess, as our facilities have been filling up with digital devices for over three decades. Once digitally integrated, concerns about the timeliness or “determinism” of messages should be considered.

An instance of a PID controller receives a measurement. A PID controller, in addition to features like HMI and alarming, is essentially “just doing math” on the measurement it receives. Without customization, the math algorithm has no special accommodation for the timeliness of the message. It adjusts an output – potentially very swiftly these days – which then becomes a “message” to a final control element such as a valve. The loop works only to the degree that said move happens in a timely fashion and impacts the process, which impact is then measured by a sensor and transmitted back to the controller. If the messages become stale or have random latency, some closed-loop controllers may fail to function.

Random latencies were a worry back when present-day FOUNDATION Fieldbus was invented. That’s one reason why its creators valued control in field devices (PID solved in a device) and a “macrocycle” to ensure that the control cycle didn’t have random or unknown latency, at least owing to the network and function execution.

While the Zeitgeist favors asynchronous messages, the practice of “answering the phone when it rings” can be crucial for the performance of critical control loops. End users should ensure their choice of digital infrastructure accommodates their loops where deterministic execution is crucial.        

About the Author

John Rezabek | Contributing Editor

John Rezabek is a contributing editor to Control

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