Re 2: 97-199r9

Gene_Milligan at notes.seagate.com Gene_Milligan at notes.seagate.com
Wed Oct 14 06:15:40 PDT 1998


* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* Gene_Milligan at notes.seagate.com
*
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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