Stephen Mitschke
August 12th, 2003, 01:41 PM
Dear Mr. Raynor,FOUNDATION(tm) Fieldbus was designed for interoperability, notinterchangeability. Interoperability means that devices from differentsuppliers can work together, interchangeability means that one can replaceanother. Interchangeability would of course mean that not only would theFieldbus aspects have to be the same, all other aspects such as responsetime, ambient condition effects, accuracy, mechanical connections andcertifications etc. would also have to be the same or at least "good enoughfor the application". I guess the same applies to Fieldbus. Although theFieldbus aspects, like other aspects, are not identical, they may be "goodenough for the application". Thus, interoperability does make interchanginga bit easier.There are a number of steps you can take to make future interchangeabilityeasier. Be aware that there are "standard" blocks that are defined by theFieldbus specification. There are "enhanced" blocks that have all theparameters and functionality of a standard block, plus a little bit moreadded by a particular manufacturer's device. And there are "custom" blocksthat are completely unique. When you design your control strategy use"standard" blocks as far as possible. This will make it easier to reassignthe block to another device type in a future replacement scenario. Use"enhanced" and "custom" blocks only when required. Make sure to design yourcontrol strategy template library as far as possibly using "standard"blocks. Some configuration tools may come with templates that usemanufacturer specific blocks, but it may be better to create your own tomake them more universally adaptable. Devices that have "dynamicallyinstantiable" function blocks are well suited here because they typicallyhave a larger library of function block types including both "standard"blocks as well as "enhanced" blocks allowing you to chose what you need forthe application. Obviously this is also a good feature in a new device atthe time of replacement since a device with dynamically instantiablefunction blocks are more likely to have a block you need. Some manufacturerscharge extra for releasing some of the blocks so if you need advancedfunctionality make sure to specify it when tendering a job. All functionblocks are identified by a Profile and a Profile Revision. We (Smar) make aconfiguration tool that makes use of this configuration to tell you if youare working with a "standard", "enhanced" or "custom" block. The tool checksthat the device supports not only the type of block, but also the particularprofile and revision, before it is assigned and downloaded to the device.When you pick a new physical device type for use at the particular plantlocation you will be notified that some blocks don't fit. At the moment thiswill happen for any transducer block because all transducer blocks are"custom". In the near future several transducer blocks will be standardized.It's unlikely that any transducer block can be implemented only as"standard", but at least they will be "enhanced" instead of "custom" makingdevice change easier. Anyway, changing to a different transducer block isnot difficult. Many times no parameter other than MODE_BLK has to be set,sometimes perhaps 2 or 3 other parameters. Be careful to check what theCHANNEL parameter in the corresponding input/output function block should bethough.4-20 mA is often seen as completely interchangeable and the aim forFieldbus, but how interchangeable are they? When changing 4-20 mA device youneed to set the range and maybe configure sensor type using option boards orDIP-switches. It is not simply changing one for the other. Reconfiguring anew transducer block in a Fieldbus device is just as simple.Jonas Berge, Smar Singapore