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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.

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

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

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

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

much higher speed. auto-addressing. The PlantWeb advantage PlantWeb with FOUNDATION fieldbus offers a single configuration tool for device and control configuration. System management tools provide auto-recognition. This means the host may require many Profibus DP networks to connect the PA segments to the host. The device addresses must be entered into the host for the host to locate and recognize the attached devices. engineering. device addresses are set using several different methods. The link vs. This allows a centralized device management host to configure PA devices through a coupler using device descriptions. and testing. A coupler converts the physical layer between Profibus PA and Profibus DP. The coupler. . host systems. The fastest is about 92 Kbits/sec. 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. coupler tradeoff. the entire Profibus DP network is limited to the converted speed. including setting DIP switches or user-entered software addresses. The linking device may require configuration to map the device profile parameters from the PA segment to the DP bus. and applications. Information integration: FOUNDATION fieldbus approach FOUNDATION fieldbus uses device description technology to make all the information available to other devices. 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. The centralized device management host still must implement 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. However. and drag-and-drop commissioning. It has no knowledge of the message content. Address assignment In Profibus PA.

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