"Dunlap, Randy" wrote:
> I hadn't seen the 5-limit anywhere lately either,
> and I looked for it a few weeks ago unsuccessfully.
> However, I don't usually spend much time looking at
> "Chapter 7: Electrical", but that's where it is since
> it is (worst case) timing-related.
>
> On pdf page 150 or spec page 134, at the top:
>
> "Figure 7-31 depicts the configuration of six signal hops
> (cables) that results in allowable worst-case signal
> delay. The maximum propagation delay from the upstream end
> of a hub's cable to any downstream port connector is 70ns."
>
> Note that this doesn't STRICTLY say a limit of 5 hubs ("six
> signal hops"), but that's what the graphic there shows.
> It's also what all of the USB Compliance/Interoperability
> Gold-Tree test configurations show IIRC.
Thanks very much Randy. I thought I must have imagined it.
The Gold Tree tests is actually one of the reasons why I was asking.
When I write tests, I like to understand what failure of the test means.
The Linux compliancy tests try for operation behind five hubs.
My understanding from this is that providing the cable delay + hub delay
is <=70ns, there could be an arbitrary number of internal hub chips.
This means that it may not be possible to have a heiristic in the USB
code that can prevent excessive depth, since we can't tell the
difference between two hubs with a five metre cable between them, and a
hub with two TI2040 chips wired together to make seven downstream ports.
Has anyone tried excessive depth? Maybe I have to get some more 5 metre
cables...
> The USB 2.0 spec will be public soon. It clarifies
> this a bit better.
Can you say whether the maximum depth problem is easily solvable with
2.0?
Brad
---------------------------------------------------------------------
To unsubscribe, e-mail: [EMAIL PROTECTED]
For additional commands, e-mail: [EMAIL PROTECTED]
