New approaches in Profibus DP and PA components (Snapshots from the near future: Part 2)

An undergoing migration

Wandering around at the Hannover Messe, I visited the Phoenix Contact booth. The vast product portfolio of this company is somewhat overwhelming and covers a lot more fields of application than humanly possible. Being my focus on Process Automation, I checked out their new offerings on PROFIBUS networking.

The current trend in the PROFIBUS world is a migration from PROFIBUS DP to PROFINET. As in any other segment of the automation market, PI is positioning PROFINET as the solution for Real-Time Ethernet Industrial Network applications for the Industry 4.0 era.

But this migration will take some time to be complete, if it ever is, and there are quite a few PROFIBUS PA applications that are still connected to PROFIBUS DP networks. The plants where these networks are installed will continue to operate in the foreseeable future, requiring supervision, maintenance, support for expansions and spares.

So, there is still a market for components for PROFIBUS infrastructure. And this market is becoming more price conscious since a whole lot of alternatives to the traditional solutions have appeared in the last 3 years.

PROFIBUS DP modular hubs

One of the components that have become commonplace in PROFIBUS DP installations are the PROFIBUS hubs. Like their Ethernet counterparts, they allow the use of star topologies in RS-485 networks, providing flexibility and allowing the segmentation of networks. A PROFIBUS DP Hub works like a modular repeater. There are two kinds of hubs: the monobloc hubs with a fixed number of DP derivations and the modular ones, which typically use a modular backplane that allows a variety of configurations, like mixing RS-485 repeaters to fiber optic links (FOL) and even offering the option of advanced physical layer diagnostics.

Since fiber optics links are transparent for PROFIBUS DP networks, they are the best solution to solve potential equalization problems in large installations. Using FOLs to create single or redundant ring topologies and switching to copper RS-485 cabling locally, it becomes possible to avoid traditional current loop issues that are common in area extensive installations, where potential equalization is complex or too expensive to achieve. PROFIBUS Hubs usually offer galvanic isolation between the connected segments, therefore offering a simpler solution than to deal with expensive potential equalization cable runs.

The Phoenix Contact modular hubs can be customized by mixing various kinds of repeater modules, using the standard DIN rail backplane common to other products from this company.

You can mix copper RS485 repeaters, FOLs and telephone cable repeaters in any combination possible, up to ten modules in the same backplane.

The PSI-REP-PROFIBUS repeaters have dual RS-485 ports, so with one module you can have two electrically isolated segments. The integrated bus diagnostics detect corrupt PRFIBUS DP telegrams and filters them out.

The PSI-MOS FO modules offer fiber optic breakage detection, which is a useful resource to localize failure points in fiber optics PROFIBUS DP installations. They are available in versions compatible with polymer-based or HSC fiber optics, glass or HSC multimode fiber optics and fiberglass optics in mono or multimode varieties

The Profibus Extender module PSI-MODEM-SHDSL/PB allows the extension of a PROFIBUS DP segment for distances up to 20 Km. This is accomplished by the use of copper telephone wires and the SHDSL (Single-pair High-speed Digital Subscriber Line) protocol. This is an evolution of the DSL protocol employed by ISPs around the globe but optimized for industrial applications.

All the modular PROFINET Modular Hub have Zone 2 / Cass I Div2 approvals. The FOLs have the rather new II (2) G [Ex op is Gb] IIC approval, making it possible to do fiber optic runs into Zone 2 / Cass I Div2 classified areas.

The third path for PROFIBUS DP/PA integration

For PROFIBUS PA applications Phoenix Contact and Procentec offers the same modular high-speed transparent segment coupler solution called FB-HSB-DP/PA or Commbricks (according to each supplier). This solution was the first alternative available in the market to the traditional Siemens and Pepperl+Fuchs approaches (DP/PA Link and SK3 Segment Coupler, respectively).

This base solution consists of a head station module, a PROFIBUS DP Repeater Module and a DP/PA Link/Coupler, all mounted and interconnected via a DIN rail mounted backplane. The backplane can be extended to accommodate up to 9 DP/PA Link/Coupler Modules. If more than 4 link couplers are required, a 6 A power supply module – new at the show – should be used.

The PA Link/Coupler module can be configured to work either as a semi-transparent link or as a fully transparent coupler via a switch. This puts an end to the decade-long discussion of which option is the best: a transparent coupler or a non-transparent one. The Commbricks answer to that question is both, in a single module. Use either option according to your application requirements.

This solution can use external DC power supplies to provide up to 500 mA per PA segment at a variable Voltage range (11 V to 28 V), working in this case as a PROFIBUS PA power conditioner. In coupler mode, an external PA power supply can be employed, leaving the module’s DC input unpowered.

System integration can be done via GSD files or via a Comm DTM and an FDT compliant Framework. In coupler mode, the device is fully transparent up to 1,5 Mbps DP transfer rates. Especial timing settings must be used for trouble free behavior. Additionally, Physical Layer diagnostics are optionally available for the head station module. This feature can be accessed through the device’s embedded web server.

The only missing feature in this solution (from my point of view) is the lack of redundant PA link/coupler functionality. This is a defining factor for critical PA installations, but it usually is not necessary for smaller or non-critical applications.

Isolated device couplers, with a twist

For PROFIBUS PA field infrastructure, Phoenix Contact presented an interesting twist on the field barrier concept.

Field barriers, also called isolated device couplers, are PROFIBUS PA distribution blocks that feature short circuit protection and intrinsically safe galvanic isolation for the spurs. When mounted in an adequate housing, they can be mounted up to Zone 1, with the spurs being able to reach up to Zone 0. This kind of devices are the best way to install intrinsically safe PROFIBUS PA field devices in hazardous areas. Since the trunk is not IS, no trunk length limitations exists. Proper mounting requires Ex e installation for the trunk and Ex e housings for the couplers.

Their FBxSP-ISO isolated device couplers, available in 8 and 12 spurs versions have some interesting features:

They have separate isolation zones, this means galvanically isolated spurs in groups of 4. This is an advantage over other isolated device couplers that usually feature a single galvanic isolator for all the spurs. This characteristic allows the device to perform a sequential start up in 4 spur stages, reducing the startup load strain on the power supplies.

Since up to 16 field devices can be connected to the same segment, and currently, some devices can use more than 25 mA at startup, this means a total load of up to 500 mA. A sudden load change from 0 to 500 mA can cause severe strain in the fieldbus power supply. This strain can reduce the lifespan of the power supply in the long term. The sequential startup technique used in the FBxSP-ISO device couplers prevents that kind of risk.

In the event of a spur failure, the device coupler attempts to restart the field device connected to the spur every 10 seconds, until the fault condition disappears.

In the event of a spur short circuit, the faulty spur is shut down in order to save power and minimize power supply strain.

A way to integrate HART field devices into PROFINET

Another interesting device presented by Phoenix Contact is the Universal Ethernet Hart multiplexer. This device works as a traditional HART multiplexer but, instead of using an RS485 serial connection to the Asset Management Host, it has an Ethernet output which can be configured to provide communication via HART IP, Modbus TCP or (and this is nice) PROFINET.

In PROFINET mode the device works as a PROFINET IO modular device. The GSDML device descriptor is generated by a software tool (each HART device is mapped to a device slot). The configuration can be done via an embedded web server or using a Comm DTM and an FDT Framework.

The Universal Ethernet Hart multiplexer consists of a gateway module with an Ethernet connection. This module can be expanded via either 4 or 8 channel Input or output add-on modules.

A maximum of 5 expansion modules can be added to a gateway, making possible to connect up to 40 Hart field devices to a Host, which sees them as a single modular PROFINET I/O device. Like many other devices, the Universal Ethernet Hart multiplexer is certified for Zone 2 / Class I Div 2 applications having an Ex nA IIC T4 Gc approval.

This allows the integration of legacy HART devices into a PROFINET network, working like a HART to PROFINET proxy.

There were a large number of innovations at the Phoenix Contact booth, but the ones previously described were those who caught my attention.

A mature market, still thriving

We are going to work with PROFIBUS DP networks for a long time. It is nice to see that their users can optimize their current installations with considerable more options and fewer constraints that we had to face just a few years ago.

Issues like the lack of equipotential grounding in large installations can be overcome by fiber optics, availability issues are covered by redundant FO ring topologies with copper drops as close as possible to the application. Flexible topologies are now commonplace thanks to the wide range of hubs and repeaters.

The PROFIBUS market has become a mature one. Perhaps not as exciting as when it started, but it has reached the point where it just works, if you just follow the good practices recommended by PI.

I just wish I had access to all these new options a few years ago…

I’d like to thank the help of Mr. Arnold Offner from Phoenix Contact for the valuable time he spent with me at this company’s booth.

Mirko Torrez Contreras is a Process Automation consultant and trainer. He and the AUTEX SA team are looking forward to receive you at the upcoming PROFINET trainings at the PITC/PICC (Profibus International Training Center / Profibus International Competence Center.

Please check the training schedules at www.profibus.com.ar

You can find the first article of the series here, and the second one here.

This series will continue next week.