Re 2: 97-199r9

Jim McGrath Jim.McGrath at
Wed Oct 14 12:55:01 PDT 1998

* From the T10 (formerly SCSI) Reflector (t10 at, posted by:
* Jim McGrath <Jim.McGrath at>

The origin of the 200 ns should be in the original decomposition of the
timings for the arbitration protocol itself .  The assumption in that
protocol on the total round trip delay on the bus is key to make arbitration
work, and so should be the source of the time.  Cables, expanders, etc are
constrained to fit within this budget - I don't think it works to assume
cable and expander topologies and work from that direction forward.


> -----Original Message-----
> From:	Gene_Milligan at
> [SMTP:Gene_Milligan at]
> Sent:	Wednesday, October 14, 1998 6:16 AM
> To:	t10 at Symbios.COM
> Subject:	Re 2: 97-199r9
> * From the T10 (formerly SCSI) Reflector (t10 at, posted by:
> * Gene_Milligan at
> *
> Richard wrote: <<SPI-2 never defines "bus propagation delay", does it? It
> mentions the propagation delay of a cable but does it ever define or use
> the phrase "bus propagation delay"?>>
>      It does not define the term but it states "The maximum propagation
> delay of any signal on SCSI cables shall be 5,4 ns/m or a maximum
> propagation delay of 135 ns for the entire bus." Propagation delay for the
> entire bus sounds too much like bus propagation delay to me.
> Thinking more about what Moore wrote earlier: <<The 200 ns is from EPI.
> The
> 135 ns from SPI-2 is a cable specification, and doesn't allow for
> expanders.>>
>      This explains why it is larger, but it does not explain why it is not
> a skosch larger that 270 ns. In addition the requirement in 97-199r9 is <<
> At least two deskew delays, but within a bus propagation delay,>>. I have
> assumed that "deskew delay" is intended to be and will be changed by the
> editor to "system deskew delay" and that the value is 45 ns or 90 ns for
> two of them. Consequently my implementation can comply by choosing 100 ns
> which is 35 ns less than the maximum propagation time on a standard bus
> and
> >170 ns less than two maximum buses connected through an expander to
> become
> a single SCSI domain.
>      So I reach the conclusion that 200 ns is just a convenient number to
> put a lower bound on performance with QAS since I am not required to wait
> that long - just less than. I mulled over the idea that bus propagation
> delay was of interest to avoid a reflection coming back but that could
> only
> be dealt with by using a maximum time of the propagation between the
> shortest allowed distances between stubs - an inferred minimum bus length.
> Initially I presumed Bus Propagation delay would be used to ensure driving
> a signal until it reached the far end of the bus but that would require a
> minimum of a bus propagation delay not a maximum.
>      All of this is not to argue against 200 ns but merely to point out
> that I have lost the reason for it and have concluded that the name
> generates confusion.
> Gene
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