PROFINEWS North America, Issue 15 July 2007

DEBUNKING THE MYTHS

A busy month, proving that misunderstandings still abound! We found these points in an FF newsletter that deserve deeper analysis.

Myth 1: FF has better diagnostics than PROFIBUS:

Response 1: Both FF and PROFIBUS provide two-way communications which enables instrument manufacturers to develop advanced diagnostics. Therefore it is up to the manufacturer. , Typically they are same. If they are not the same, then it is the manufacture who has decided not to include the feature - it is not related to the communication system.

Myth 2: Because FF is a publisher-subscriber it can alert the user to problems unlike a Master-Slave protocol (e.g. like PROFIBUS).

Response 2: In a publisher-subscriber protocol, you still need a 'traffic cop' to direct who can talk when - in the case of FF it is the so-called Active Link Scheduler. In PROFIBUS, the traffic cop is the master. The real difference between master-slave and publisher-subscriber is hidden in protocol design theory and really does not affect the end user very much. As far as alerts go, when a slave wants to alert a PROFIBUS master it sets a bit in the return message in data exchange, then the master knows to issue a 'diagnostic request' during the next cycle. In a typical system, this would take no more than 200ms - plenty fast enough for any end user!

Myth 3: PROFIBUS PA is a master-slave system, but it has no formal user layer like FOUNDATION Fieldbus, so host system control schemes may not be equal in how the diagnostic information is used.

Response 3: In communication theory, what belongs in the 'application layer' and what belongs in the 'user layer' is open to interpretation. PROFIBUS tends to lump everything into the application layer, whereas FF puts it in the user layer. In reality they are dealing with the same stuff. As far as diagnostic information goes, as long as the master conforms to the PROFIBUS profile specification, it will always handle the diagnostics the same way. FF exhibits variations between masters on how they handle communications and diagnostics, which leads to each FF host having a 'preferred vendors' list. PROFIBUS does not require a 'preferred vendors' list and therefore its devices are always truly 'open'.

Elsewhere, Walt Boyes complains in his blog about fieldbus numbers: "Why don't you all want to include HART in the numbers game?" he pleads. " ... there are roughly 22 million installed HART devices in process automation. This is a number that absolutely dwarfs PROFIBUS PA and Foundation Fieldbus, both of whom have roughly a half million ... "

Whoa, slow down Walt! No-one questions the massive success of HART. PROFIBUS and PROFINET can both easily embrace HART devices - what more do you want by way of recognition? But the fact is that HART depends on good old-fashioned 4 - 20mA and connects but one device per cable. It's not a bus, never has been and never will be (although it had a digital multi-drop spec in its early days that mysteriously disappeared!). Fieldbuses typically connect 10 devices per cable (it can be more) and can transmit far more data. That puts HART in a different context. Yes, with wireless added, it moves towards the networking world, but it reigns supreme in its own marketplace otherwise.

Finally, Polly's fame is spreading. We hear - on jungle drums of course - that she will make a PROFIBUS PA (personal appearance) at the PTO's General Assembly Meeting in August. If you're a PTO member this is a meeting NOT to miss. Register today.

Have you any PROFIBUS or PROFINET myths to report. If so, let us know.

Download Polly's pdf to read more PROFINET myths, especially about SUE ... and learn the facts before you become a fieldbus parrot!