Technical Overview of ProfibusProfibus as a family of protocolsProfibus is a family of protocols originally designed by Siemens to provide communications from real world sensors and actuators to area controllers. These protocols were donated to the Profibus Trade Organization to maintain as an open standard.

Profibus was initially designed as a remote I/O subsystem to provide a communications hierarchy for a PLC system, primarily in discrete manufacturing and building automation. Profibus has evolved to be used in process industries for peripheral processes such as packaging lines and control of prime movers such as motor and pumps in a motor control center.

Extensions to the original Profibus architecture were implemented to extend the market of Profibus to process automation and other areas. Profibus now consists of four separate protocols grouped under the common umbrella name of Profibus. These protocols are: Profibus FMS Profibus DP Profibus PA Profibus PAE.

In addition, there are a number of totally proprietary protocols used by Siemens that are called Profibus.

This analysis will focus on the comparisons of Profibus PA versus FOUNDATION Fieldbus.

1

ProfiBus PA: Summary Comparison to FOUNDATION fieldbus Below is a summary of reasons why Profibus PA is not considered an optimum protocol for process automation when compared with FOUNDATION fieldbus.

ProfiBus FOUNDATION FieldbusDesign IntentProfibus was originally designed as a remote I/O bus for factory automation (discrete on/off signals). Profibus PA is an additional layered protocol

intended to address process automation applications.

FOUNDATION fieldbus was designed from the ground up for process automation. It focuses on leveraging the vast increase of data embedded within intelligent devices, such as transmitters and final control elements. Ff has ease of use and functionality features that make it much better suited for process automation applications.

Number of ProtocolsProfibus is not a single protocol. It is a family of protocols. Many claims are made about the large installed base of Profibus, but this includes all types of Profibus.

The largest percentage of applications is DP. A very small percentage of the total is PA.

FOUNDATION fieldbus is a single protocol. All articles that have been published regarding

FOUNDATION fieldbus test sites and commercial installations refer to a single, comprehensive, standard protocol.

On-line FunctionalityIn the past, new devices could not be added to a Profibus DP or PA segment without shutting down the process and reconfiguring the segment. Siemens has invested much effort to add this functionality to the existing protocol.

The results of this effort are delivered in PCS7 V6.0. Some on-line additions are now possible but only if: Place-holders are pre-configured into the

control strategy The system uses their newest PCS7

hardware/firmware (on-line adds are not possible on the pre-V6 equipment)

An objective comparison reveals their layered solution falls well short of control systems that were designed from the beginning to accommodate on-line add functionality.

New devices can be added to a running Foundation fieldbus segment.

Some process control systems are designed to automatically sense the addition of new devices, making the procedure seamless and easy.

Device AddressingProfibus addressing requires manual hardware and configuration settings. Errors can stop the segment from working.

Some process control systems are designed to allow Foundation fieldbus to assign addresses automatically: Eliminating human error and saving

configuration time.

2

ProfiBus FOUNDATION FieldbusGatewaysProfibus PA requires a gateway to Profibus DP before it can be brought into a control system. This increases cost, reduces performance and adds failure points to the solution. It also increases engineering time, drawing complexity, and configuration effort.

Profibus functionality changes depending on the type of gateway used between the Profibus PA bus and the Profibus DP bus. You can have higher speed at higher cost and lower functionality, or lower cost and higher functionality but at low speed. If the wrong gateway is selected up front, the cost to change includes not only new gateways, but major rewiring costs.

No gateways are required. FOUNDATION fieldbus information is delivered directly to the process control system via the H1 or HSE modules.

With FOUNDATION fieldbus, performance is consistent and predictable and there are no gateways required.

Message StructureProfibus requires many more messages to control a loop than Foundation fieldbus. This is because Profibus uses a master/slave communication technology. Master/slave technology is appropriate for

high-speed discrete buses that carry a small amount of information per device.

Profibus-PA uses this technology because it was designed as an add-on to Profibus-DP.

Master/slave increases the communication load and therefore limits the number of devices it is practical to have on a Profibus PA segment to 8 or less. Users have reported that at 10 devices per segment, the segment overloaded. Users have had to rewire their plants to get around the communications bandwidth problem.

FOUNDATION fieldbus uses publisher/subscriber communication technology.

FOUNDATION fieldbus allows a maximum of 32 devices per segment, although 16 devices and 4 complete control loops are the limits currently in common use to ensure sufficient speed when fast control loops are required.

Communication TimingProfibus runs without timed execution because it was designed initially for discrete control, which does not typically require timed execution.

FOUNDATION fieldbus incorporates an accurately timed execution schedule that eliminates the jitter found in the Profibus communication architecture.

Because of this, Foundation fieldbus will give higher performance and better quality of control than Profibus PA. This reduces variability and improves plant

profits.

3

ProfiBus FOUNDATION FieldbusControl in the FieldProfibus PA does not support control in field devices. It only supports control in the master (host).

Because intelligent field devices have microprocessors with high levels of computing power, it is cost effective to run control loops in the field devices, rather than in the host. This allows control loops to run independently in the event that communication to the host is lost.

Installation SavingsProfibus claims greater savings than Foundation fieldbus based on the combined savings of Profibus PA for continuous and Profibus DP for discrete functions.

Discrete busses such as AS-i bus, DeviceNet, or Profibus DP can easily be integrated with Foundation fieldbus. Foundation fieldbus clearly represents the better solution since: Ff has numerous advantages over PA DP can be used in either solution AS-I bus and DeviceNet are offered on most Ff

solutions whereas Siemens is reluctant to use non-Siemens protocols.

Configuration ToolsThe Profibus Trade Organization claims that a single configuration tool can configure any Profibus PA device. This is not the case. A single configuration tool can configure any PROFILE. Only a configuration device provided by the device vendor can configure the device manufacturer-specific parameters. This means the user will probably require one configuration tool for EVERY device vendor, and maybe more, in the plant. This is similar to the HART protocol, where Universal and Common parameters are generic to all HART devices, but Device-Specific commands are determined by each individual manufacturer and require a “device description” file in the host for access.

If you have multiple Profibus PA segments joined by link boxes, you must connect your configuration tool directly to each segment to configure devices. Therefore, configuration cannot be done directly from the host system.

FOUNDATION Fieldbus has been fully integrated into some process control systems, such as Emerson’s DeltaV: Providing a single engineering interface AND

configuration tools.

Other process control systems, such as Honeywell’s Experion and Yokogawa’s Centum systems, layer FOUNDATION Fieldbus over previously existing engineering platforms and tool sets. This provides a single engineering interface, but: Multiple engineering tools are required to

configure, Unique function blocks, and Map the Ff data in the system’s existing point

structures

Function BlocksThe Profibus Trade Organization claims Profibus supports a wide variety of function blocks, including those for control. Profibus PA currently has ONLY input and

output blocks. Any other block claims are for planned or

potential future deliverables.

FOUNDATION fieldbus supports a wide variety of function blocks, delivering to the user a high level of flexibility in designing process automation schemes.

4

Independent Consultant Predictions:

ProfiBus PA versus FOUNDATION Fieldbus

The prediction by ARC is that process fieldbusses have reached the mainstream and are viable solutions for large and critical applications.

“Process fieldbus… have all moved into the mainstream of process automation and are being installed in large plants for critical applications.”

“Process fieldbus… have all moved into the mainstream of process automation and are being installed in large plants for critical applications.”

“Process fieldbus… have all moved into the mainstream of process automation and are being installed in large plants for critical applications.”

“Process fieldbus… have all moved into the mainstream of process automation and are being installed in large plants for critical applications.”

Furthermore, ARC predicts Foundation will become the dominant bus, overtaking the lead that Profibus DP and PA established early in the race for market acceptance.

FOUNDATION Fieldbus will likely outstripProfibus over the next 3 years

FOUNDATION Fieldbus will likely outstripProfibus over the next 3 years

FOUNDATION Fieldbus will likely outstripProfibus over the next 3 years

FOUNDATION Fieldbus will likely outstripProfibus over the next 3 years

5

6