Brad Hards wrote:
> As a secondary issue, would / should this count as two hubs in the
> maximum topology depth? And can anyone provide a reference for why five
> is the maximum depth - a quick scan of the USB spec only came up with
> section 6.4.4 which talks about excessive depth, but doesn't say what is
> excessive.
Well, the number 5 is in there somewhere.
> I thought that it was driven by timing and the 5 metre
> cables, but that wouldn't seem to apply in this case.
It's mostly driven by the hub timing. With maximum length cables and maximum
hub depth there are only a few nanoseconds to spare in the worst case.
A design where two hubs are on the same PCB would safe the cable propagation
delay, but this would not be enough for another hub (I've not checked the figures,
I must admit).
I just had a similar problem with the D-link serial adapter. It has a hub in it, with
just one of the ports used for the actual serial adapter. The design is a mystery to
me
since the other ports are unused.
--
-- Lennart
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