Transitioning to an all-digital field

Oct. 28, 2020
Far from throwing a switch, industry’s adoption will unfold over time

As with any new technology intended to replace trusted, serviceable technologies and entrenched work practices, the adoption of Ethernet-APL—especially for brownfield facilities—will take time to gather momentum, even with the most compelling of value propositions.

Indeed, despite the commercial availability of fully digital, intrinsically safe field communication options for more than two decades now, the majority of process instruments currently installed still communicate their process variables via an analog, 4-20mA electronic signal officially standardized back in the 1970s. A just completed survey of Control readers indicates that in the typical process plant a full 70% of installed instruments rely on such analog loops, although many benefit from a bidirectional communication boost offered by HART, a 1980s-era digital protocol superimposed atop the analog, 4-20mA signal.

“We don’t believe that end users with FOUNDATION fieldbus or Profibus PA installations will start ripping out field instruments to install Ethernet-APL,” says Paul Sereiko, director of marketing at FieldComm Group, referring to users of the process industries’ first-generation digital field protocols. “We doubt a refinery unit will consider upgrading until their existing assets are at end of life. Early adopters are much more likely in more flexible process applications, such as life sciences, and food and beverage.” And for plants with sunk costs in analog instruments, I/O modules—and personnel comfortable with using them—wholesale conversion is a tough sell for them as well.

Brownfield inroads

Instead, the first incremental applications of Ethernet-APL within existing facilities are likely to be in the realm of monitoring and diagnostics, rather than for control or safety applications. Analytics-driven algorithms that rely on instrumentation data beyond the process variable are one arena where the higher speed of Ethernet-APL communications will prove particularly useful, according to Stefan Bollmeyer, R&D technology manager for fieldbus and I/O, ABB Measurement & Analytics. “Occasional diagnostics are fine, but if time-synchronous data beyond the process variable is needed, that’s where 4-20mA + HART sees its limits.”

For those plants seeking to perform primarily asset monitoring tasks with existing instrumentation, a best first step would be to take advantage of currently available technology—that is, adding HART multiplexers to the I/O, then using an FDI server to covert that data into OPC UA/PA-DIM for consistent communication with other enterprise systems. Then, for situations where the performance of Ethernet-APL devices is desired, simplify connectivity by adding your APL field switches to a pre-existing Ethernet I/O network. “You can mix Ethernet-based I/O with APL switches on the same network,” Bollmeyer explains. “Then, you can talk directly to your APL devices, and save the extra engineering in between.”

Once confidence has been gained in Ethernet-APL’s use in monitoring applications, one may choose to start wiring new devices to this infrastructure instead of expanding traditional I/O, ultimately swapping out existing I/O in favor of Ethernet-APL. On the positive side, you should be able to tap into your host system using whatever Ethernet-based protocol it already supports. “The availability of existing protocols over APL on the host side will help speed adoption,” Bollmeyer notes. “There are no prerequisites.”

“Within a traditional fieldbus plant, Ethernet-APL will be a good choice for a new plant area, or possibly a major unit revamp where barriers and protectors can be swapped against APL switches,” Bollmeyer adds. “In the second case, at least you shouldn’t have to run new cables, since existing fieldbus cabling should support Ethernet-APL.”

Guidelines for cable re-use

Speaking of cable re-use, Ethernet-APL requirements for IEC 61158 Type A shielded, twisted-pair cables are in line with established fieldbus practices, notes Andy Kravitz, flow transmitter marketing manager and APL working group representative for Emerson. “The APL working group is preparing a set of engineering guidelines to help users select the correct cabling for a given APL application,” Kravitz says.

“As fieldbus cable has been designed for 31.25 kBit/second, but not all existing cable can be used for APL at the full cable length,” adds Michael Kessler, executive vice president, components and technology, Pepperl+Fuchs. “Therefore, the APL port profile specification defines for different categories supporting spur/trunk cable length of 50m/250m, 100m/500m, 150m/750m and 200m/1,000m. Cable manufacturers have to specify their cable according to this classification.”

Toward a unified architecture

Ethernet-APL vs. current fieldbus protocols doesn’t have to be an all or nothing proposition, adds Michael Kessler of Pepperl+Fuchs.

“Ethernet-APL switches have been demonstrated to provide Ethernet-APL spur interfaces with dual functionality, e.g., Profinet-APL and Profibus PA,” Kessler says. “Theoretically, this is also feasible with a FOUNDATION fieldbus instrument where its data can be mapped on any Ethernet-based, real-time protocol. This dual-functionality is important to migrate existing plants to Ethernet-APL-based infrastructure. During the first years of market introduction of Ethernet-APL, existing FOUNDATION fieldbus or Profibus PA instruments may fill the gap of missing instrument functions with an Ethernet-APL interface.”

Ultimately, Ethernet-APL is the key enabling technology to deploying OPC UA and PA-DIM in the field devices themselves, Kessler adds.

“This will finally allow real plug-and-play since the device will come with an embedded information model—that means no need for any kind of device description. FDI, meanwhile, will allow the use of PA-DIM for legacy instruments. This will help with plants migrating from HART or fieldbus to Ethernet-APL.” 

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