In this article (and next month’s) I am going to shed the light on questions that some of our novice PROFIBUS PA users have asked us recently. There are specialists all over the world who are on the eve of kicking off an implementation of PROFIBUS PA for the first time in their automation projects. This is the legacy of the conservative nature of our industry. I was about to write that I am feeling old after ‘looking’ at PA installations for almost 20 years.
The good thing for the novice users, is the mature status of PROFIBUS PA with the availability of many products, services, and a widely supported standard. When I got the assignment to be chairman of the team to standardize the certification guidelines for profile 3.0 in the beginning of 2000, we had many obstacles to cross. Already a stack of submitted exceptions coming from instrument manufacturers was piling up and testing methods in the current test labs had to be redefined. After a long stretch I was proud to be part of a group of PROFIBUS ‘heroes’ who could submit the PROFIBUS PA certification guidelines, in order to make the document official by the advisory board. Let’s look at the questions regarding PA Profiles:
What is a Profile within PROFIBUS PA?
PROFIBUS PA devices are relatively intelligent and can execute part of the information processing in automation systems. This was previously done by the PLCs or DCS systems (translating 4-20 mA to digital). The process industry demands among other criteria more data reliability and accuracy compared to discrete automation. The PA profile was designed in close cooperation with the process industry and defines all functions and parameters for different classes of instruments to ensure data reliability and pre-processing of data.
The PA Profile is based on internationally accepted function block technology. The profile contains descriptions for the following instrument types: pressure, level, temperature, flow, analog/digital I/O, analyzers, valves, and actuators. It all comes down to pre-processing of the cyclical data within the instrument according to provided settings. In parallel attached status values and alarms are generated to inform the host system about the reliability of the data.
The profile is divided into 2 classes (A and B):
- Class A of the profile describes common parameters of simple devices. The scope is limited to the basic functions. This set consists of the process variables (e.g. temperature, pressure, level) added by measured value state, the tag name, and the engineering unit.
- Class B is an extension to class A and covers more complex application functions for identification, commissioning, maintenance and diagnosis. The relationship of the parameters to the classes is visible within the parameter definitions and in the conformance statements.
Personally, I am really an enthusiast of the Simulation Value. This overrides the process value which the instrument has to cyclically transfer to the host system. By adjusting the simulation value, the user can test the behavior of the entire algorithm in the host system. Also, it’s helpful when adjustments to the installation have to be made and unnecessary alarms want to be avoided. The quick change to another unit is also awesome – just set a temperature transmitter to Kelvin and immediately the digital process value has adapted. In my opinion, the PROFIBUS organization is offering the process automation industry a technical platform that is able to cover most applications and create value for the end-user.
Next month: What is a Profile Ident Number?
Dennis van Booma
PROCENTEC
For more background on profiles, see these past PROFINEWS articles: