Incentives to migrate fieldbus to Ethernet-Advanced Physical Layer (APL)
What was the original motivation for installing fieldbus, perhaps more specifically Foundation Fieldbus? Three decades ago, there was a vision among members of the ISA SP50 committee that a single, unified standard for a two-wire bus could be specified to create a broad standard for integrating “smart” field devices. It was akin to 4-20 mA, only digital.
The committee wanted it to be hazardous-area-capable and able to go long distances—maybe a mile or more. They wanted several devices to communicate on one twisted-pair copper wire, and use the wire already installed in their process plants. Meanwhile, end users were dreaming of a migration path, where field devices could be migrated to fieldbus incrementally.
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But why bother to replace 4-20 mA with pure digital?
By 1992, Honeywell’s TDC-3000 distributed control system (DCS) had been supporting digital integration of field devices with its proprietary Digital Enhanced (DE) network communications protocol. From Honeywell’s perspective, it facilitated “bundling” its highly adequate transmitters with systems sales at a time when many process plants were migrating to DCS-based controls. The move instilled terror in its competitors, whose more popular transmitters couldn’t “speak” the proprietary DE protocol. However, some end users enjoyed key features of digital integration.
Transmitter configurations were stored centrally in the DCS, and could be entered before the devices were wired up. Transmitter installations/replacements were made simpler, as stored configurations could easily be downloaded to new field devices. Secondary variables like transmitter temperature were easily accessed, avoiding freeze-ups and overheating. Also, there was no loss of accuracy due to analog-to-digital conversions. Unfortunately, microprocessor-based valve positioners had some of the most interesting data, but no one made a positioner that was “DE” capable.
Read more about Ethernet-APL: What can Ethernet-APL do for you?
Honeywell intensified the appetite for digital integration, and the ability to acquire additional information from field device diagnostics. Unlike its proprietary, closed offering, Honeywell also created the appetite for a single, unified, vendor-agnostic communication protocol.
By now you might be thinking, yes, at last we have a field communication standard (in Ethernet-APL) that fulfills the vision of fieldbus pioneers and early adopters. For some protocols, Profinet, for example, APL can and has begun facilitating a migration for standards that already support an Ethernet backhaul. Another feature of APL is that it’s not only vendor-agnostic, but also protocol-agnostic. Unfortunately, few industrial control systems support an Ethernet backhaul for Foundation Fieldbus.
There was an abundance of visionaries in the early days—maybe too many—and they conceived of “Hunk 2” (H2), a high-speed backhaul for fieldbus H1, which was later modified to high-speed Ethernet (HSE). The SP50 committee determined that a fieldbus should move many functions of the host system to field devices.
In the 1990s, computing power, memory and speed were catching up to the needs of real-time computing. Updates from microprocessor-based transmitters didn’t necessarily sync with controller cycles, which didn’t sync with final element (valve) positioning. Uncertainty about latency led to functions beyond square root extraction, scaling and linearization being instantiated in fieldbus standards, including PID. Unfortunately, the large installed base of recently updated DCSs weren’t easily adapted to this new, foreign, function-block standard. Foundation Fieldbus had to wait on a generation of controls systems to refresh in a way that would allow integration of all the Foundation Fieldbus features.
Microprocessor and communications technologies evolve quickly. In the 25 years since Foundation Fieldbus entered the early-adopter phase, most process-industry-focused control systems now offer fieldbus capability to varying degrees. The deliverables of ISA SP50—digital integration of field devices, multiple devices on a twisted-pair, hazardous-area backhaul, and access to device diagnostics —all those boxes appear to have been satisfied by Ethernet-APL.
Earnest, early adopters deployed a substantial installed base of Foundation Fieldbus devices, so now we have a template for a plausible path to migrate them to APL—FieldComm Group’s FCG TR10365 Ethernet APL migration concept. If hardware and software offerings emerge to address the infrastructure needed, Foundation Fieldbus users must sell the investment to their management.
