Re: VXS & Signal Distribution

[Gerard Visser <gvisser@indiana.edu>]

Hall D Electronics:

Hi Chris,
	Really - I thought 41.0 claimed to be "the" physical layer spec? How 
can they possibly not specify skew!?
	Anyway, I agree you can best get it from the manufacturer, but then you 
have to be careful if they "change their mind" in future.
	On further thinking and a 2nd look at the table Ben provided, I think 
the 22 ns skew spec there simply means that the RapidIO receiver can 
_tolerate_ 22 ns of skew between lanes of a multilane channel 
(within-slot skew). I.e. they have a problem if backplane skew is 
greater than 22 ns. We have a "problem" (actually just a need to settle 
our differences) if backplane skew (slot-slot or within-slot) is greater 
than about 3-4 ns.

	Gerard

C. Cuevas wrote:
> Hi,
> 
> VITA 41.0 does not have backplane Payload to switch skew
> specifications.  Remember that the switch slot can be located in any
> physical slot in a star configuration, and any two physical slots for a
> dual star implementation.  For our JLAB 20 slot VXS Wiener/Elma crates,
> I would wager that the largest skew variation is between Payload Port
> 17(slot 1) and PPort1(slot 9) for the 'left' side of the crate.  Since
> the backplane is symmetrical, pair trace lengths for Port 2 vs PPort 18
> *should* be the same.  The pair to pair skew is most likely in the Elma
> backplane specification and the folks from Wiener owe me that Elma spec.
> (Elma knows the copper trace lengths)
> 
> The protocol specifications:
> 41.1  InFiniBand
> 41.2 RapidIO
> 41.3 Gig Ethernet
> 41.4 PCI Xpress
> 41.5 Aurora
> will most likely have different skew (max) specifications listed, and I
> believe the 22ns spec listing , but must admit that I do not know how
> they derive that spec.
> 
> Talk to you tomorrow,
> Chris