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Buses 301

FOUNDATION fieldbus H1 or Profibus PA?


Different origins, different attributes

Physical layer

Master-slave or peer-to-peer?

Control in the field


Information integration

Linking devices or couplers

The FOUNDATION fieldbus approach

Address assignment

Device tags and tag search

Real-time clock


Does it really matter whether I use Profibus PA or FOUNDATION fieldbus?
Fieldbuses are the most appropriate type of field networks for control and diagnostics in process
operations. That's because fieldbuses provide highly reliable two-way communications between
"smart" devices and systems in time-critical applications.

you have to choose the right one. was designed specifically to address the needs of process automation — including providing safe and effective real time process control on the bus. packaging lines. watch for answers to these questions: • How does physical layer of FOUNDATION fieldbus compare to that of Profibus PA? • Where is control executed in FOUNDATION fieldbus? in Profibus PA? • How does each achieve interoperability? Different origins. The Profibus family of protocols was designed to provide a communications hierarchy for a PLC system. To maximize fieldbus benefits. and material handling systems. Some of the advantages are determined by the physical characteristics of the bus. Built on top of these two earlier protocols. The greatest differences between Profibus PA and DP are in the physical layer. The two most widely used fieldbuses for process automation are FOUNDATION fieldbus and Profibus PA. Let's compare the ways Profibus PA and FOUNDATION fieldbus deal with these potential advantages — including differences that can affect your choice of bus. FOUNDATION fieldbus. Profibus PA is intended to extend the use of Profibus to the process industry. PLCs. Hint: As you go through the topics in this course. building automation. Because both Profibus PA and FOUNDATION fieldbus are digital buses. connecting user interfaces. and so on. . This course can help you decide which is right for you. they have the potential to provide certain advantages over traditional point-to-point wiring. on the other hand. Profibus DP is used for primarily discrete processes such as assembly lines. different attributes Some of the differences between Profibus PA and FOUNDATION fieldbus reflect their different origins.But different fieldbus technologies have different attributes. and some by the software implementation of the bus. primarily in discrete manufacturing and building automation. motor control. It includes three different protocols that have different capabilities and applications: • • • Profibus FMS provides a backbone for PLC systems. CNC machines.

Devices can communicate with each other without a host. as well as discrete and sequential control.Physical layer Profibus PA and FOUNDATION fieldbus H1 use the same physical layer as defined in ANSI/ISA-50. This means the host must be present for control to function. The host must also forward the results to other devices. Part 2-1992. its execution is synchronized with that of the host. A field device is a slave that can only respond to a command from a master.02. a host will still be required to tell the device to process the function block and communicate the results. independent of a host system. it can send an alarm. Both FOUNDATION fieldbus and Profibus PA support the requirements of intrinsic safety. electrical signaling. or partially in both. For example. it can't report the problem unless the host specifically asks. Advantages brought by the physical characteristics of the bus include reduced wiring due to multi-drop. . control can currently reside only in the host. That includes both basic and advanced regulatory control. With FOUNDATION fieldbus. The Profibus Users Organization has work under way to distribute control to field devices. if a FOUNDATION fieldbus device experiences a problem. since peer-to-peer communications is not supported. Master slave or peer-to-peer Profibus PA is a master-slave protocol. However. This means the media (twisted pair). control can be in the field device. since Profibus PA is a master-slave protocol. or the host. Control in the field With Profibus PA. and reduced control room space due to fewer I/O cards. This peer-to-peer communication also enables FOUNDATION fieldbus devices with the appropriate function blocks to execute control in the field. and they can initiate communications without a specific host command. which is the same as IEC 1158-2. FOUNDATION fieldbus is a peer-to-peer protocol. While control in the field devices runs independently of the host. including the new FISCO model. That means if a Profibus PA device experiences a problem. and other physical layer issues are the same.

The PlantWeb advantage Because all Emerson Process Management fieldbus devices and the DeltaV and Ovation automation systems use a common set of function blocks. That's because the devices can maintain real time deterministic control over the bus. all of the functionality of all the devices is available. By design. Or you can combine control function blocks in the host and in the field devices. You can also move control simply by reassigning it to a different location — no reconfiguration. Careful planning can result in graceful degradation of control strategies when the host is not operative. Control function blocks can also run in the host with only I/O blocks in the fieldbus devices. As a result. FOUNDATION fieldbus addresses interoperability by a combination of device descriptions and function blocks. you can disconnect the host without halting the loop — or your plant. If you want to use a field device that has additional vendorspecific functionality. no re-mapping. And you may need several different tools to cover every device or every vendor in the plant. without loss of functionality. For example. one of the PIDs in a cascade loop can run in the field device and the other in the host. Work is under way to address these issues. If a host can understand a FOUNDATION fieldbus device description. . And a single host application can configure and access all device information and functionality. you should carefully evaluate the functionality available using your specific host system.When you run control in field devices using FOUNDATION fieldbus. no changing the engineering environment. controllers can't use vendor-specific advanced functionality in the field device without special software. your control strategy operates the same way — with the same dynamic response —. and require vendor-specific engineering tools. But until a universal solution is defined and implemented. Interoperability Interoperability enables you to interconnect and operate devices from different manufacturers on the same fieldbus. as well as the cost and complexity of using multiple tools. Profibus PA allows a host system to use a standard set of basic functionality for a device (called a standard profile). this functionality may be available to the control host only through a vendor-specific extension of the matter where you choose to locate these control functions.

Information integration: Linking devices or couplers Most Profibus control hosts don't accept Profibus PA inputs directly. The information available through these two techniques is very different.25 Kbits/sec. so the host doesn't have access to vendor-specific device information. such as diagnostics or quality information. The linking device has the advantage of allowing the PA segment to run at 31. to run at a . In such situations. This means a centralized. The linking device.Information integration Getting the full benefits of a digital bus requires that information be available wherever it's needed. To access non-profile information such as device configuration information contained in the device description. The control host uses profiles to access device information in the control environment. which attaches the PA segment to the host. Most linking devices can only pass information contained in the standard Profibus PA device profiles. a linking device or coupler converts the Profibus PA protocol into Profibus DP protocol. called the V1 enhancements. the only way to extract and use all device information may be to use multiple hosts. Profibus PA uses two different technologies to make device information available: • • The device configuration and management host uses device descriptions to configure and interact with the device. isn't available to a control host that only uses the standard profile. Often this non-standard device information is available only in control host systems from the same vendor as the device. the configuration tool must be attached at the PA segment. With Profibus PA devices it's important to know the capabilities of the devices that are available using device descriptions. Instead. and the host accepts Profibus DP. to co-exist with the control host on the same DP segment. while allowing the Profibus DP bus. and the capabilities of the device and control host combination using profiles. A configuration and management host must implement a set of revisions to the Profibus protocol. Device-related information not in the standard profile. device-description-based device configuration and management tool can only be used with a linking device if the tool implements the V1 enhancements to the Profibus DP protocol.

or lower communications throughput and higher bus cost by using the coupler and more DP buses to connect PA to the host. The coupler. and drag-and-drop commissioning. and applications. A coupler converts the physical layer between Profibus PA and Profibus DP. Information integration: FOUNDATION fieldbus approach FOUNDATION fieldbus uses device description technology to make all the information available to other devices. System management tools provide auto-recognition. These differences leave the Profibus user with an architectural choice when considering Profibus PA: • • higher data throughput by using a linking device and fewer DP interfaces. This is usually a register mapping exercise and may require significant design. engineering. Address assignment In Profibus PA. and testing. The device addresses must be entered into the host for the host to locate and recognize the attached devices. device addresses are set using several different methods. However. including setting DIP switches or user-entered software addresses. The fastest is about 92 Kbits/sec. The linking device may require configuration to map the device profile parameters from the PA segment to the DP bus.much higher speed. the entire Profibus DP network is limited to the converted speed. host systems. The centralized device management host still must implement the V1 enhancements to the Profibus DP protocol. This allows a centralized device management host to configure PA devices through a coupler using device descriptions. The link vs. The PlantWeb advantage PlantWeb with FOUNDATION fieldbus offers a single configuration tool for device and control configuration. It has no knowledge of the message content. A typical installation technique is to connect several PA segments to one DP segment and have that DP segment bring the information into the host at high speed. This means the host may require many Profibus DP networks to connect the PA segments to the host. auto-addressing. coupler tradeoff. .

The tag database is manually entered into the host. The connection between the host tag and the device address must be entered manually. and control execution. It also means that configurations for FOUNDATION fieldbus devices can be compared with the host database based on tag. The device address can be manually or automatically assigned. All devices on a segment have the same real time. time stamping. commissioned. all reserved and used addresses should be protected against use or reuse. This is one reason why all bus communications and future control execution are directed by the host. Real-time clock FOUNDATION fieldbus provides a distributed real-time clock on the bus. This means that any FOUNDATION fieldbus device can be located by asking for it by its tag. Profibus PA does not provide a real-time distributed clock. . enabling highly deterministic communications. Profibus PA supports tags in the host. The net result is that FOUNDATION fieldbus is appropriate for real-time control on the bus with or without a host.) FOUNDATON fieldbus devices can be added. Profibus devices are accessed by bus address. Configuration errors may not be detected until segment startup. Profibus PA is appropriate for host-based control only. and the segment must be taken offline to correct errors. FOUNDATION fieldbus uses a special message to detect and identify new devices added to a running bus. Then the segment is restarted.To add a new device to a Profibus PA segment. Device tags and tag search FOUNDATION fieldbus supports both device and function block tags in the field devices. and used while the rest of the segment remains in operation. (If the host automatically assigns the address. the segment is shut down and the device address and other configuration parameters are configured in the host.