Handling Profibus PA field devices made easy

Easier to use

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Profibus fieldbus technology has been an established communication standard in manufacturing plants for many years. However, it is still the subject of a heated debate in the process automation sector compared to the seemingly simpler 4…20 mA. The benefits of Profibus are greater the longer a Profibus-equipped plant is in operation and the more familiar employees become with its features.

Authors Albert Justus Product Manager for Communication and Device Integration in the Field of Process Instrumentation, Siemens Holger Rachut Product Manager Simatic PCS7 for Field Device Integration, Siemens

The desire to generate more performance from a new technology implies a willingness and an ability to adjust to different operating sequences and tools. One of the most frequently voiced arguments is that “Profibus PA field devices are more complex and time-consuming to handle than conventional devices with 4…20 mA technology”. This statement is true if a modern field device only needs to provide a single, analogue measured value without any accompanying status information, if parameter reassignment and troubleshooting are to take place in the field, if the main troubleshooting tool will be a simple multimeter or if preventive maintenance and asset management are never going to be an issue – in other words, if the device will be used like a traditional 4…20 mA device.
It no longer applies, however, if the owner views modern field devices (Figure 1) as valuable components (assets) of an automation solution – valuable, for example, with regard to measurement data already available in digital form or to the option of assigning device parameters directly from a central location or valuable owing to a field device’s ability to provide current information about its own status for condition based or preventive maintenance.
It also ceases to be valid if the tools and meth-ods provided for operating and maintaining Profibus PA systems are properly used, that is to say if the persons responsible are appropriately trained. The two examples described in the following demonstrate how plant performance and availability are increased with Profibus PA. They also show how handling remains easy – or in some cases becomes even easier, for instance when existing measuring devices are replaced.
Neutral configuration
When a plant is configured, the final choice of devices (type and manufacturer) is often still unknown. A neutral configuration can be employed in this basically unsatisfactory situation. This approach uses interfaces that are identical for all potentially suitable products, so that the plant can be configured regardless of the actual devices selected subsequently.
With conventional field devices (nowadays almost always with a Hart interface), this neutrality is based on the 4…20 mA interface – something that all devices of this type have in common (Figure 2, left). This interface is followed by a conventional configuration with one cable per device or measured quantity, one communication direction, two signal transformations and finally a parameter description.
With Profibus PA field devices, equivalent neutrality is ensured by the profile GSD, which specifies identical functionality corresponding to the process values to be transmitted for all PA devices and is therefore also device-neutral (Figure 2, right). This interface is followed by the familiar PA configuration with only one cable for all devices or measured quantities, two communication directions, only one signal transformation and the final parameter description.
Both concepts thus offer equivalent configuring neutrality and ease of handling. However, Profibus PA has the added benefit of diagnostic information (status byte) during operation and ease of access to instrumentation over the network.
Straightforward addressing
Profibus allows segments with up to 32 nodes to be created in networks with up to 126 nodes; with 4…20 mA technology, on the other hand, each device constitutes a separate unit. Profibus devices are all addressed uniquely and represented centrally in tools; they are thus at least as visible to users as the same number of single 4…20 mA devices. Figure 3 shows the addressing concept with consecutive numbering of the nodes in the network, in which only node 1 is reserved for service tools and node 126 for newly supplied devices (default setting). In practice, it has proven worthwhile to set up a separate service range with separate addresses below 126 to enable actions such as repairs or replacements to be performed with greater flexibility and ease.
The example in Figure 3 shows that it is very easy to replace a defective device with a new one. Only a few manipulations in the plant and a few clicks in the tool are needed to re-position the replacement device delivered with station address 126 (as default) to the correct address (20) and to read the predecessor device’s parameter set into it.
Replacement in a few mouse clicks
In a fieldbus system, the replacement of existing devices and the installation of additional devices are actions that should be routine, easy and reliable. With PA devices, a special rule applies. Aside from installing the device in the field, all that needs to be done is to assign the correct address to the new device. A simple mouse click sequence that is identical for all actions and can be executed centrally has been created for this purpose. The specification of 126 as a special service address prevents pos-sible disturbances on the bus segment when replacement devices are connected.
The device-neutral profile GSD harmonises the basic functions of all PA field devices and is thus fully compliant with the standardisation aspect of 4…20 mA technology that is at the forefront of the debate. The profile GSD also permits significantly easier operation in the field: newly installed field devices communicate their measured values promptly – without having to make additional settings – according to the specifications of the profile GSD, while field devices with PA Profile 3.02 are adjusted automatically to the specifications of the profile GSD once they have been installed and addressed. As a result, device replacements are invariably faster and more straightforward.
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What is a GSD file?

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In the context of Profibus, a GSD (General Station Description) file is an electronic data sheet provided by the device manufacturer whose purpose is to describe the device properties. A GSD comprises a precise description of the set of properties needed to configure the cyclic data exchange between the field device and the automation station (master controller or control system). This includes information for identifying the device, for example, supported transmission rates and hardware and software versions. A profile GSD also contains details of all parameters specified in a device profile (such as the PA profile) for all manufacturers and mandatory for “profile devices”. The content of a profile GSD is thus identical for all profile devices and accordingly device-neutral.
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