If you have ever worked with Profibus PA networks, you surely will know about the so called Profibus DP/PA couplers. If not, please allow me to give you a brief resume:
The IEC61158-2 standard
The IEC 61158-2 standard, finally approved in 1999, allowed the coexistence of two different technologies for process fieldbus communication employing a shielded and twisted two wires cable using the Manchester Bus Powered encryption (MBP). The two fieldbuses were Foundation Fieldbus and Profibus PA.
This standard was a compromise solution that tried to appease the industry suppliers, who had created their own buses, in order to take advantage of the fieldbus concept.
Both protocols employ the same physical layer, feature a transmission rate of 31,25 kbps, use the same cable specs and offer the possibility of implementing intrinsic safety if it’s necessary. Due to the MBP based protocols that they use, the same cable can provide both power and bidirectional communications.
They were usually known as H1 fieldbuses by the IEC’s Fieldbus Committee in order to differentiate them from the H2 fieldbuses, which were higher speed protocols that employed different physical layers such as RS485 or Ethernet.
The function of H1 fieldbuses was to connect field devices with the control system, while H2 fieldbuses purpose was to allow the connection of more complex devices like VFD’s, RIO nodes, sensor buses, HMI´s, etc.
The promise to ditch the decades old 4-20 Ma standard seemed within reach.
But the digital grail proved to be as elusive as the Holy Grail was for King Arthur ’s Knights of the Round Table, or as John Razebeck’s states:
“Our normal vendors, whose pursuit of market dominance has given us irksome dualities like Foundation Fieldbus, Profibus, WirelessHART, ISA 100.11, etc. in lieu of common platforms and open solutions”
Profibus PA
Profibus PA (Process Automation) entered into the market a bit earlier than Foundation Fieldbus (1995 vs 1996), and from its conception was designed as the way for Profibus DP (Descentralized Periphery, the H2 level of Profibus) to reach field instrumentation.
This year of difference and the lack of backwards compatibility requirements with older protocols enabled Foundation Fieldbus to offer a few interesting advantages over Profibus PA, being the most important automatic network addressing and the availability of the so called user layer.
This latest feature enables Foundation Fieldbus to work in a consumer-subscriber mode, which makes possible the implementation of the Control in the Field concept (CIF). CIF allows the processing of control loops directly in and between the field devices, without the controller’s intervention. Profibus PA is strictly a client-server protocol, control loops are solved in the controller and while addressing can be done remotely it’s not by any means automatic.
As with almost all things in life, everything comes at a price:
The Control in the Field concept required to deal with macrocycle issues plus the automatic addressing and the Virtual Communication Resources necessary for the peer to peer data exchange meant a higher use of the controller´s processing power, so real life Foundation Fieldbus segments were usually limited to no more than 16 field devices, being the common practice not to pass the 12 devices limit.
Profibus PA on the other hand can comfortably support up to 24 field devices per segment.
According to the standards, Profibus PA and Foundation Fieldbus do support 32 field devices per segment, but if we consider the commonly accepted table of number of devices vs. spur length, we find out that segments with more than 24 devices can have spurs of only 1 meter.
Given that the typical topologies use device couplers to connect the field devices to the trunk, spurs longer than 1 meter are the norm. So, in real life IEC61158-2 segments, the practical maximum number of devices per segment is 24 (23 PA field devices plus a PA master in Profibus PA; 24 field devices in FF, from which you can have 1 or 2 devices that act as LAS and Backup LAS).
Most important, Foundation Fieldbus requires the function blocks architecture typical of DCSs, while Profibus PA could be easily implemented in any PLC.
Some PLCs do support Foundation Fieldbus, but they need to perform protocol translation, usually via Modbus TCP.
Profibus DP requires higher data transmission rates, so it became faster as time went by. Eventually Profibus DP reached transfer rates of up to 12 Mbps, although in most applications the usual limit is 1,5 Mbps. Cable length is the variable to take into account for maximum transfer rate limitations
The coupler
The obvious question that appears is how to connect the Profibus PA side of the network to the DP side.
The initial solution (developed by Siemens) was named coupler (a.k.a. segment coupler) and received the model code IM-157. This is a device that acts as a network transition gateway: it provides power (400 mA@32V) to the PA side and routes communication between the DP and PA sides.
But since Profibus PA is limited to 31,25 kbps, the DP side has to be limited also in data transfer speed, otherwise the PA data would not get in time to the DP master and hence to the controller. So the first generation PA couplers forced DP to work at 45,45 kbps.
A second supplier (Pepperl + Fuchs) offered another coupler (model code SK1, 400 mA @30 V) that enabled DP to run up to 93,75 kbps, but anyway this still was a constrain.
Transparent couplers
One of the benefits of keeping Profibus DP’s transfer rate slow was that the DP master could exchange data with the PA slaves transparently, this means that both DP and PA devices shared the same addressing space and allows the exchange of datagrams with a size of up to 244 bytes per node, thus enabling the use of advanced diagnostics.
But the market quickly demanded Profibus DP networks running as fast as possible and the need of a device to connect, or link, PA segments to fast DP networks became a necessity.
High speed couplers
So the DP/PA link devices were developed by Siemens. These devices employed the following arrangement to work: the already available low speed couplers (IM-157) were connected via a backplane to a device that worked like a gateway (IM-153), packing all the data generated by the PA devices connected to the couplers in a single DP datagram.
This modular device was called a Link; it requires a Profibus DP address and was the Profibus version of the IEC 61158 definition of a linking device: something that could connect H1 segments to H2 networks.
Now the interaction of PA networks with fast DP networks was possible, but there were some trade offs.
The high speed coupler’s dilemma
Since the DP/PA Link works as a DP node, it requires a DP address and consequently is limited to exchange up to 244 bytes per cycle with the DP Master. Although you can physically connect 32 PA devices to a PA segment (31 slaves and 1 PA Master) and (since it´s possible to connect up to 5 couplers to a link), in theory this implies that you can have 31×5= 155 devices per DP/PA link. In real life, the PA segments have to comply with the 124 device address limit. And the 244 bytes datagram size limit is usually reached with less than 40/45 devices.
This is caused with the minimum data volume that Profibus DP can handle, which is 1 byte. Any change of state in a Profibus device requires the use of 1 byte.
A typical PA device needs to have at least one transducer block active to be able to enter into the synchronous communication mode with the DP Master, in order to transmit the PV in floating point format, which implies at least 4 bytes and an additional byte for PV status.
This means that the absolute minimum data generated by a PA device is 5 bytes. If you add basic diagnostics, multiple variable data and advanced diagnostics, the data volume increases sharply. So you get the idea: you can run out of space in the DP datagram very quickly, especially with modern multivariable devices with advanced diagnostics.
But in the early 90’s Profibus was basically a single supplier’s solution, so if you wanted fast DP networks combined with PA segments you had to use the DP/PA link whether you liked it or not. Additionally, the first generation of Profibus PA devices did not do much more than its analog counterparts.
Then, in 1995, the same guys that had created the 93,75 kbps SK1 transparent coupler (Pepperl + Fuchs) shocked the market presenting a fully compliant, high speed, multi brand compatible, transparent coupler. Its modular design offered the option to connect 5 PA power supplies (400 mA @30V) to a head station.
The SK2 (as it was called) was a killer product and a game changer, because it opened the doors to Profibus PA for any control system supplier that had a DP master available. The original DP/PA Link could be integrated with other control systems but it was an awful task that nobody wanted to do nor maintain.
This device sold like hotcakes, it was even sold in a brand labeled version by ABB.
Although more expensive than the DP/PA Link, it was so user friendly that it became an industry standard.
To be fair, each solution performed better in different scenarios: The Siemens solution was better suited for applications that featured large numbers of simpler devices and the Pepperl + Fuchs approach was better suited for large numbers of multivariable devices. Also the Siemens solution allowed theoretically more PA nodes under a single DP master. The Pepperl + Fuchs solution was limited to 124 PA devices per DP master. Cycle timing was also faster in the Pepperl + Fuchs version.
Two standards that were influenced by this new device were the development of the FDT/DTM technology and the ample acceptance of the Profibus DP-V1 protocol.
DP/PA Links require to be configured by the integration of PA gsd files into the gsd of the Link. If you want to perform Asset Management or remote parametrization on the PA devices you have to use Siemens’s PDM software, which is based in EDDL technology. PDM is so deeply integrated with other Siemens software packages that its use as a standalone solution makes little sense.
EDDL has advantages over FDT, like the possibility of automatic firmware upgrades in the field devices, but EDDL support outside of the DCS world is scarce.
Transparent couplers allowed the use of DTM’s for device configuration using a FDT framework, greatly enhancing the user’s experience and allowing easy access to newer PA device’s enhanced diagnostics.
The advent of the DP-V1 standard, with advances such as Class 2 Masters, asynchronous communication and device profiles made Asset Management accessible for PLC users, having been for a long time patrimony of the DCS world. Every system’s integrator could now use remote parametrization and Asset Management.
FDT has become so popular that you can acquire third party DTM’s designed to enable the integration of a DP/PA Link in a FDT/DTM architecture.
What is really curious is that for a long time no other supplier of Process Automation equipment offered a third option. So the PA game featured two players for almost 20 years.
Just to be fair, there has been a third alternative since 2011: a gateway that connects Profibus PA to either Control Net or Ethernet IP networks, made by Hiprom (later acquired by Rockwell) but since this article´s subject is specifically about Profibus connectivity I´ll leave that device for another history.
Equally curious is that the Siemens didn’t try to work out the of the DP/PA Link´s limitations, they simple lowered the price so they could remain competitive.
Third generation couplers: advanced physical layer diagnostics
The guys behind the high speed transparent coupler (Pepperl + Fuchs) developed a successor of its killer device creatively called SK3, which remains being the market standard.
The SK3 is more modular, features the option of redundant power supplies by load sharing (500 mA @30V), offers optional head station flying redundancy and optional integrated advanced diagnostics for the physical layer. This additional diagnostics data can be transmitted through Profibus DP, using the asynchronous data transmission method available in Profibus DP-V1, or via Ethernet.
This diagnostics information is made available for end users via FDT/DTM technology in a Master Class 2. The DTM employed for this is one of the most advanced available examples of this standard.
IMHO, the real value of advanced physical layer diagnostics lies in its use while doing the commissioning of the segments, afterwards it’s mostly overkill, although a portable advanced diagnostics module is a must have for Profibus PA maintenance.
But the tech behind the SK3 is amazing: it works transparently, like the SK2, because it was basically a Profibus PA proxy server.
It emulates a DP slave that sequentially transfers the data from the PA slaves through a PA Master. In this way, PA devices are viewed by the DP Master as DP Slaves, so the 244 bytes limit applies to each PA device.
That means no restrictions in the amount of data that can be send by any PA device. But it also limits the number of PA devices per DP master to a maximum of 124.
Although theoretically one could connect up to 124 DP/PA Links to a DP master and, as a consequence, thousands of PA devices to a single DP Master, in real life this was no possible because of timing issues.
Due to additional physical layer constrains (spur length), the practical maximum number of PA devices that a SK3 can handle is about 90. A DP/PA Link can handle about 45. Every application has different requirements, of course, so your mileage may vary.
The Siemens approach
Siemens finally improved the DP/PA Link in the middle 2010´s, offering more features like allowing the option of power supply redundancy by the hot stand by method, (power was increased to 1A @32V). It also offered head station system redundancy, but only when used with Siemens hardware.
Finally, the company offered the option of a ring topology for PA segments. This was only possible by employing Siemens own proprietary device couplers (AFD for safe and Zone 2 applications and AFDIS for Zone 1 applications, with IS spurs reaching Zone 0).
They kept the FISCO version of their coupler (110 mA@12 V), although the market has definitely adopted the High Power Trunk approach for IS applications.
Pepperl + Fuchs, being the original developer of the High Power Trunk concept, does not feature the option of FISCO power supplies. They also do not consider justified the added costs and complexity of ring PA topologies.
A third player appears, finally
And finally in 2014 a small company called Procentec launched a long awaited third option for linking DP and PA networks. Based on the modular design of Profibus DP gateways, repeaters and advanced diagnostics modules of the Combricks family of products.
This new coupler features the ability to change its working mode from transparent low speed coupler to a non-transparent high speed coupler to a fully transparent high speed coupler by flipping a switch.
Up to 8 PA couplers can be connected to a single DP head station (it provides 400 mA @30V). The DP connection can be done via RS-485 or fiber optics, and can provide advanced physical layer diagnostics through a DTM or via an embedded web server.
It lacks redundancy, which is offered by the other two contenders. You can also get it also in a brand labeled version from Phoenix Contact.
Meanwhile in the Foundation Fieldbus world
In the Foundation Fieldbus’ world the game was totally different. Although the Fieldbus Foundation had initially planned to use a H2 network level, the quick advance of Ethernet technology made this H2 level irrelevant. So the Foundation Fieldbus H1 segments were supposedly going to be connected via the HSE protocol (High Speed Ethernet) to the plant’s network, making H2 network levels (which usually relied on RS485 based protocols) obsolete.
But HSE never won enough acceptance in the DCS market and the usual approach from DCS suppliers was and continues to be to connect Foundation Fieldbus H1 segments to their own proprietary Ethernet implementations.
As a consequence, Foundation Fieldbus never had to deal with datagram size issues.
The availability of controllers with higher performance CPUs also reduced the limitations on the allowed number of devices per segment, although this factor has not been reflected in the Fieldbus Foundation documentation.
The coming of Profinet
Back to the Profibus world, the advent of Ethernet gave birth to Profinet technology. Profinet is Profibus over Ethernet (it does not use the TCP/IP stack but is based on the UDP protocol)
Profinet enjoys a growing acceptance in the Factory Automation world and it’s slowly gaining traction in the Process Automation world. But the obvious device needed to finally enter this world, i.e. a Profinet/PA coupler, does not exist in the portfolio of its main supporter.
There have been talks about a Profinet to Profibus PA proxy since 2009 and the official Profibus International development path still shows it as part of its future. A recent Profibus International whitepaper called “Profinet for PA” talks about the development of different proxies to allow the interaction off Foundation Fielbus, Hart and Profibus PA devices directly with Profinet.
The current available solution is to use a Profinet to Profibus DP coupler (IE/PB Link) and a DP/PA Link in cascade, a solution that while viable, depends on the old RS485 based DP H2 layer.
A German company called Softing announced a Profinet to Profibus PA gateway last year at the SPS IPC fair, the device can connect up to two PA segments directly to a Profinet network.
It works by combining up to 32 PA gsd files (16 per segment) into a gsdml file. It does not provide power to the segments, but it is mechanically compatible with Stahl’s Fieldbus power supplies. Of course you can also use other power supplies compliant with IEC 61158-2. Additionally, it is compatible with the FDT/DTM standard
Another way to connect Profibus PA segments to Profinet networks is the one proposed by Ifak Systems with its isNet product line. This is a modular system that features several gateways, one of them enables PA segments communication to Profinet.
As the Softing solution, this device requires the integration of the PA devices’ gsd files into one gsdml file. This file is used for the hardware configuration. The equipment can provide power to the PA segments (300 mA@22-24 V per channel for the one channel module, 200 mA@22-24 V per channel for the 2 channel model and 100 mA@22-24 V per channel for the for channel module). It can also work as a passive gateway and use external power supplies compliant with IEC61158-2.
The device is compatible with FDT/DTM and includes advanced diagnostics.
The future
As was shown in last year’s Hannover-Messe, there is a new player in town: Ethernet for Process Automation, also known as Advanced Physical Layer. This technology would allow a special version of Ethernet to reach the field, carrying power and bidirectional communication.
In this concept, Hart, Foundation Fieldbus and Profibus PA protocols are left as means to connect devices to a new kind of device coupler.
Whatever happens to be the result of these developments, for sure the future is going to be interesting and plenty of options so I’ll be expecting the next chapter of this story with great curiosity.
Final thoughts
It has been really interesting to compare the two methods that have been used until 2014 to provide connectivity between Profibus PA and Profibus DP networks.
The first one, employed by Siemens, is based in a vertical proposal totally integrated (no pun intended) both in hardware and software. If you accept that kind of integration, you get a robust and trusty solution, but not exactly flexible.
The second one, employed by Pepperl + Fuchs, is based on a proposal that looks for greater compatibility, interoperability and employs open standards such as FDT/DTM.
A large amount of the growth reached by Profibus PA can be explained by this open approach.
My personal preference goes for the second way. Most of the technological advances of the last decades owe more to an open approach than to proprietary ones.
Only time will tell which method succeeds in the new scenario that the IioT paradigm has created.
Scoop!!!!!!!
While reviewing this article, I found out the latest Process Automation newsletter from Pepperl + Fuchs.
In this newsletter appears a new Profinet to Profibus PA coupler, which claims to be backwards compatible with the SK3 series and for the first time supports systems redundancy. That would mean the existence of an easy upgrade path for the integration of Profibus PA into Profinet networks. I suppose that, following Pepperl + Fuchs’ tradition, this device will be shown at the 2016 Hannover-Messe or later this year.
I´d like to thank Andrew Morse and Dennis Brandáo (PITC Brazil) for their collaboration in the writing of this article.
Mirko Torrez Contreras is a freelance Process Automation consultant who has a keen interest on the evolution of Process Automation technology. He has been playing a “catch me if you can” sort of game with the industry product news for the last week in order to post this history and avoiding it to be instantly obsolete, at least for another week.
