How many devices can you have on an SMBUS segment?

[buenos]

Hi

This a practical question!
For 100khz SMBUS, how many devices would result in too much bus capacitance normally?
Without simulating a PCB for capacitance, just give a practical number that you used and still worked fine.

 

[shaiko]

The total capacitance will depend on the trace length and on the trasistors technology of the bus occupants.
On the other hand, you can nagate the capacitance side effects with using a stronger pull up resistor - so there isn't a defininate answer to your question.

 

[buenos]

With my original question I wanted to avoid receiving "it depends" type of answers, as they are not very helpful. I know trace width, stackup and pullup can effect RC time constants qualitatively.
Thrown me practical numbers from your design/prototyping experience. For example: "I had n devices on my proto board and it sometimes failed, then I cut the trace of 2 devices then it passed the tests.".

 

[shaiko]

I know trace width, stackup and pullup can effect RC time constants qualitatively

I once tried interfacing 2 devices over an SMbus and it didn't work well.
At first I thought the reason was that one device was in Toyko and the other in Manhattan...but then I figured out that the use of pull up resistors higher then 11.5 Megaohms may have contributed to the problem.

Anyways, I heard that in bad weather, you shouldn't use more then 57 devices over an intercontinental SMbus. Hope this helps more...