How many devices on a PROFIBUS DP network in Grand Rapids?

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There was a good crowd at the Grand Rapids PROFIBUS one-day training class last week.  Manny and Torsten were there while I was in Vancouver with Hunter for a PROFINET one-day training class.  One of the questions that came up in Grand Rapids concerned how many PROFIBUS DP devices are allowed on a network.  We have a slide that addresses that:

(click for larger image)

The questioner thought that the four devices with no address still counted against the 126 allowable addresses on a network.  They don’t.  They do count against the number of devices on a segment and that probably accounts for the confusion.  The address space is independent of the network segmentation.  (Segmentation is provided by the repeaters.)  So while the repeaters count against the physical number of devices that are allowed on a segment, they do not have an address.  Why the limit of 32 physical devices on a segment?  That’s a limit of RS485 that PROFIBUS DP is based on.  For more details see the Wikipedia article and especially the B&B Electronics link at the bottom: “Technical library of RS-485 articles and application notes.”

Bottom line: we’re talking about two different domains – physical and virtual.  The physical with limits imposed by the RS485 standard and the virtual address space independent of the physical with limits from the PROFIBUS specification.  Or, more basically, the devices with no address don’t subtract from the total number of addresses available.

As always there were suggestions on topics to increase and topics to decrease.  As always some wanted to increase and others to decrease the same topic!  We continue to try to balance that.

Other comments from the Course Evaluations:

“Great course.  Learned a lot from ‘the bottom up.’  They (correctly) assumed I knew nothing about PROFIBUS.”

“Was good to have vendors here. Was glad there wasn’t a sales pitch.”

“The Canadian had an awesome sense of humor.”  [Yes, Manny, the Canadian, went to Michigan and Carl, the American, went to Canada.  I do have to note that I lived in Canada for a couple years and speak nearly-fluent Canadian.]

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