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Process Control Communication: More Than Just APL

Ethernet-APL receives a lot of attention for its ability to bring Ethernet-based communications and power to devices in hazardous zones. But a physical layer and some data do not make a complete solution.

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Compared to factory automation, process control places additional demands on communication technology. Process plants, which extend over wide areas, have a lifespan of 20+ years. They invariably run continuous production processes, where unplanned breakdown or disturbance would pose a serious hazard to people and the environment, in addition to substantial financial loss. Therefore, operators want continuous access to horizontal and vertical data and information flows while making it easy to maintain their devices.

For Profibus & Profinet International (PI), the migration from fieldbus-based communications (Profibus) to Ethernet-based communications (Profinet) is in full swing. This is particularly true in factory automation with proven benefits for end-users. Similar trends are also becoming evident in process control, as the Profinet features that fulfill the needs of process control industries now have a physical layer (Ethernet-APL) to make them available at the field level.

Let’s look more closely at these features and their importance:

Dynamic Reconfiguration
No matter whether they occur in factory automation or process control, changes sometimes need to be made to the communication between a controller and a device. Examples include changes to device configuration or parameterization. Since process plants often run continuously, these changes must occur during ongoing operations. The dynamic reconfiguration feature allows for a Profinet device to establish more than one communication relation with a controller. A new relation with the new configuration/ parameterization is established in addition to the existing relation, and then swapped over seamlessly. This occurs without interruption to either the device, the controller program, or the network.

Maintenance and diagnostics
Status-oriented maintenance is important for the operation of all factories and plants. It is based on the capability of devices and components to determine their status and communicate using standardized mechanisms. To this end, Profinet provides a system for the reliable signaling of alarms and status messages from devices to controller. This diagnostic model covers system-defined events such as the removal or insertion of I/O modules and the signaling of malfunctions, such as a wire break, that are detected by the control mechanism. Besides "good" and "faulty" status messages, the underlying status model also knows the optional levels for "maintenance required" (e.g., when media redundancy is lost) and "maintenance demanded." An I/O module can also distinguish between diagnostic alarms (errors within a device or component) and process alarms (problems with the process, such as exceeding a temperature limit). To ensure a uniform display of different diagnostic messages, the Profinet diagnostic model has been assigned to match NAMUR NE 107.

Device replacement and PA profile 
The replacement of Profinet field devices can be performed with minimal manual intervention because of the constant exchange of information between devices on the network. If a device fails, a spare device can simply be installed in its place. Using the network neighborhood information, the controller recognizes the replacement and reassigns the same name, address, and parameter set as the failed device. Fast device replacement without an engineering tool is therefore possible. Cross-vendor interchangeability is also possible with application profiles.

An application profile is essentially an agreement among a family of devices on how to structure Profinet data. Examples include ProfiDrive for drives and motors, or ProfiSafe for functional safety components. The PA (process automation) profile for process control instruments ensures the uniform behavior of PA devices of various types, versions, and even vendors. For example, the chances that a vendor has not made enhancements to its devices during the lifespan of a process control plant are small. In the case of device replacement, the newer device might have acquired a new identity number in the interim. According to the PA profile, when it is swapped in for the failed device, it can assume the legacy device’s identity and avoid any manual setup. This capability can be true across vendors as well, where all devices in a device family (e.g., flow meters) are able to use a generic identity from the perspective of the controller.

Availability and redundancy
High system availability is ensured by Profinet redundancy solutions for media, controllers, and devices. Media redundancy creates multiple physical communication paths to Profinet devices. In case of failure of a communication path (e.g., a wire break), a second communication path is automatically used. Device and controller redundancy are also possible to maximize uptime in case of failure.

Timestamping and sequence of events
In large plants, the ability to record the actions, alarms, and status messages to a sequence of events is frequently required as alarms often arrive in waves, with secondary and tertiary alarms following only from the primary alarm. Quickly finding which alarm first created the cascade is critical. With millisecond-level timestamping, Profinet provides a standardized (IEEE 1588) solution including archiving and control.

Migration and outlook

With proxy technology, existing plant sections can be integrated into a wider Profinet infrastructure. For process control, this incorporates existing fieldbus systems like Profibus PA, Foundation Fieldbus, HART, and others. Proxies can best be thought of as gateways in that they translate data from one protocol to another. Proxies go a step further by representing device data in a standardized manner on the Profinet side. Control systems use them to access subordinate field devices both cyclically and acyclically. Properties of the legacy systems, such as diagnostics and configuration, can be used as native properties on the Profinet side.

With the release of Ethernet-APL, a robust, two-wire, powered Ethernet physical layer including intrinsic safety for operation in hazardous areas is now available. It enables a direct connection of field devices to Ethernet, so that process control industries can benefit from a convergence of their OT (operations technology) and IT (information technology) systems. It is based on the proven trunk-and-spur topology. Trunks provide high power and signal levels for long cable runs of up to 1000m. Spurs carry lower power with optional intrinsic safety for lengths of up to 200m. The validation of intrinsically safe connections is done like FISCO (Fieldbus Intrinsically Safe Concept), so that for every connection one simple validation is possible with no additional calculations required. Since Profinet is an Ethernet- based protocol, it runs over Ethernet- APL out of the box.

Summary
Ethernet-APL is an enabler of digitalization in the process control industries, but it is not a complete solution. It is just a physical layer. Protocols are still needed to format and communicate data and information. To meet the stringent requirements of process plants, simple data transfer is not enough. Modern plants can gain a sustainable advantage by installing a Profinet network that eases maintenance tasks with robust diagnostics and timestamping, allows for flexible topologies, ensures maximum uptime through high availability, coexists plainly with other Ethernet-based protocols, and integrates legacy systems where necessary.

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