Roy Shenfield x2473
roys at adaptec.com
Thu Jun 2 10:04:13 PDT 1994
I'm not sure how negotating for a slightly lower speed as suggested by Jim McGrath
would solve the crystal tolerance problem. As I understand it negotation implies
you are informing a device of the maximum transfer rate (minimum period) at which
you can RECEIVE data (or conversely the maximim transfer rate (minimum period) at
which a device can SEND you data).
Thus, when I negotiate with another device do I take into account the tolerance of my
crystal, the tolerance of the other device's crystal, or both? If the answer is either
the second or third choice, then there must be a tolerance stated in the spec
Furthermore, since according to the SCSI Specification the negotiated transfer period
is the transfer period factor times four (Section 6.6.21), I can not negotiate for only
a slightly lower speed, I have to negotiate for a slightly times 4 lower speed.
In other words the next transfer rate available below 10 MB/s (100 ns period)
is ~9.6 MB/s (104 ns period). Thus in the nominal case we have given up ~ 4% performance
to solve an ~.250% crystal tolerance problem. Is this reasonable?
By the way does the definition of transfer period present a minor problem with Fast-20?
Since 20 MB/s is a period of 50 ns, the transfer factor period would have to be 50(ns)/4
which is 12.5, which is not an integer. Thus the value of the transfer factor period
would have to be set to 13, which gives a transfer period of 52 ns, which is a transfer
rate of 19.23 MB/s. Am I nit-picking?
The views stated above are my own and do not reflect any company position.
Senior Staff Engineer
Email: roys at adaptec.com
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