"The elephant in the room" – with this metaphorical idiom, James Powell, Senior Product Specialist Industrial Communications at Siemens, describes in his blog post 'Is Profibus or 4-20mA More Accurate?' in detail the issues with 4 mA ... 20 mA signal quality that everyone is aware of, but nobody wants to talk about.
Interview with Long-Standing Member of the PNO Advisory Board, Juergen George about the Past, Present, and Future of Fieldbus Technology
PROFIBUS & PROFINET International (PI) celebrates its 25th birthday in 2015—an organization in which Juergen George from Pepperl+Fuchs was involved from the outset. Over the decades, he has influenced the definition of standards in fieldbus technology through various bodies, committees, and working groups. In celebration of PI's anniversary and George's retirement, we invited him to an interview: What does the future hold for PI? What will he remember most from his eventful career in service of "Mission Fieldbus"?
...and, the goose!
As I was reading John Rezabek's recent article "Can Improved Physical Layer Diagnostics Save Christmas Dinner?", a few thoughts came to mind.
John pointed out that keeping the installation — junction boxes, instrument housings — sealed from moisture ingress is one of the big challenges for both fieldbus and 4 mA … 20 mA installations.
Your fieldbus technology questions answered...
What’s important to you and your operation? Is it a reduction in costs, easy maintenance, or simply just working within the confines of your aging infrastructure? If you answered yes to any of these questions, fieldbus technology might be something you should consider.
Recently, ControlGlobal.com featured an article by John Rezabek: Why Industrial Couplers Aren't Commodities?
When designing FOUNDATION™ fieldbus or PROFIBUS PA networks, it is extremely important to make sure that the devices on those networks have enough power to start up properly and operate normally.
The fieldbus physical layer specification IEC 61158-1 defines the min/max voltage levels for FOUNDATION fieldbus and PROFIBUS PA. The minimum voltage required for any fieldbus device is 9 V and the maximum voltage allowed on a fieldbus network is 32 V.
To calculate the voltage drop along a fieldbus cable, you must know the number of connected instruments and their current consumption, as well as the resistance of the cable, cable length, and power supply voltage. Typical fieldbus devices consume 15 mA to 20 mA. The exact number can be found on instrument data sheets. A standard “type A” fieldbus cable has a typical resistance of 44 Ohm/Km and a typical fieldbus power supply supplies 30 V.
Knowing the total current draw, supply voltage, cable resistance, and cable distance of a segment, and applying Ohm’s law, allows you to deliver maximum distance and/or voltage drop at a certain distance.
To simplify these calculations, companies like Pepperl+Fuchs have created free-of-charge segment design tools like Segment Checker.
First let’s define exactly what a fieldbus terminator is. A fieldbus terminator is a component of a fieldbus system that is the equivalent of a 1 µF capacitor and a 100 Ω resistor in series with each other. The main functions of fieldbus terminators are to shunt the fieldbus current and to protect the fieldbus signal against electrical reflections.
The short answer is: although both “buses” are very similar, they are not the same!
Here is what’s different: PROFIBUS is the overall name for a process fieldbus solution that includes the application and industry-specific PROFIBUS DP, PROFIBUS PA, PROFINET, and PROFIsafe. In the process industry, we see mostly PROFIBUS DP and PROFIBUS PA.
(For additional information visit: www.profibus.com)
PROFIBUS DP (decentralized peripheral) is the RS485-based high-speed (up to 12 Mbit/s) solution for connecting remote I/O systems, MCCs (motor control centers), or similar externally powered devices to a PLC or DCS control system.
PROFIBUS PA (process automation) is defined in the IEC 61158-2 standard and used predominantly in process plants. It is suitable for general purpose and hazardous locations and delivers power (up to 32 V) and communications (31.25 kBit/s) on the same cable (bus powered) to PROFIBUS PA instruments such as flow meters and temperature or pressure transmitters.
Typically, PROFIBUS PA applications are connected to a PLC/DCS control system via a PROFIBUS DP network, which requires a DP/PA Segment Coupler. These coupler devices are necessary to connect the two different physical layers of the high-speed, non-bus-powered DP to the lower-speed, bus-powered PA. The Segment Coupler includes the power supply modules needed to condition 24 VDC bulk power for use on PROFIBUS PA. PROFIBUS PA does not support CIF (control in the field), where instruments can talk to each other even if the connection to the host is lost.
Just like PROFIBUS PA, FOUNDATION fieldbus is based on the IEC 61158-2 standard. This means that both networks are based on the same physical layer and used in similar industries and for the same types of application. The physical layer defines the cable type (Type A fieldbus cable with specific R, L, and C values, twisted shielded pair) as well as the min/max power levels (9 V … 32 V) and the transmission rate (31.25 kBit/s). Since PROFIBUS PA and FOUNDATION fieldbus are based on the same physical layer, the same type of field distribution devices like Segment Protectors and FieldBarriers can be used for both networks.
We assign the term fieldbus junction box to an enclosure mounted in a remote location, near the process, containing some type of electronic distribution module for use in a digital ‘fieldbus’ application.
The module mounted within the enclosure typically provides short-circuit protection, current limitation, isolation, and other electronic monitoring to the connected field instruments/sensors and to the control system.
In a digital fieldbus network like PROFIBUS PA or FOUNDATION fieldbus, it is very important to provide protection against inadvertent shorts or other faults. FF and PA fieldbus networks share a single shield twisted pair for power and communication, so a fault of this type could potentially affect the entire network if the individual instruments are not isolated. This is the main reason it is NOT recommended to daisy chain PA and FF devices.