In this article (like last 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.
What is a Profile Ident Number?
The PA Profile describes a method to exchange an instrument of manufacturer-A with an equivalent instrument of manufacturer-B. Instead of using the Ident Number of the instrument which is connected to a specific GSD file, the user can select a Profile Ident Number by means of an asset management tool.
Normally every instrument type has its own fixed Ident Number (PROFIBUS DP legacy). The main reason is not just to check which instruments are available on the bus, but also to verify that the correct GSD file was used. The GSD file also contains an Ident Number. If there is a mismatch ‘in the field’ in contrast of what has been configured, the instrument will never go into data-exchange with the host system. If the network has been properly configured (Ident Number instrument equals Ident Number GSD file), the instrument can in default conditions only be replaced by an identical instrument. An alternative instrument requires a configuration change (new GSD file) and the upload of the new configuration could lead to a host system stop.
The PA Profile supports Profile Ident Numbers to avoid a configuration or software change. The user can select by which Ident Number the instrument can identify itself. If the user selects the Profile Ident Number and uses the Profile GSD file in the configuration tool. The instrument can always be exchanged by another instrument that uses the same Ident Number. Some instruments support multiple Ident Numbers to be even compatible with previous product versions.
Some examples ranges of Ident Number that have been standardized in the PA Profile:
- Transmitter 0x9700 to 0x970F
- Actuator 0x9710 to 0x971F
- Discrete input 0x9720 to 0x972F
- Discrete output 0x9730 to 0x973F
There are currently instruments available which are able to switch Ident Number independently. They always adjust themselves to the Ident number that has been programmed in the host system. As long as it is their Ident Number or the designated Profile Ident Number.
My personal experience is that many users just use instruments with the original Ident Number that was set when they unpacked it. They are unaware that this feature is available. Another group claims they will always replace it with the same instrument because the process has been designed like that (sensor quality and behavior). In any case, I recommend users re-evaluate their strategy regarding device exchange now that this feature has been clarified.
For more background on profiles, see past PROFINEWS articles: