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From: jerry@gumby.Altos.COM (Jerry Gardner)
Newsgroups: comp.periphs.scsi
Subject: Re: <13400@unixland.uucp>
Message-ID: <4705@gumby.Altos.COM>
Date: 28 Feb 91 18:25:09 GMT
References: <13400@unixland.uucp> <283400055@adaptx1> <1942@public.BTR.COM>
Reply-To: jerry@altos.COM (Jerry Gardner)
Organization: Altos Computer Systems, San Jose, CA
Lines: 31

In article <1942@public.BTR.COM> thad@public.BTR.COM (Thaddeus P. Floryan) writes:

>Uh, Roy, I beg to differ with your conclusions in this regards.
>
>The delay, using your example, "should "be a CONSTANT 12nS upon the initiation
>of data transfer, not an additive delay with each new byte (unless my
>understanding of SCSI is really skewed, for which I'd appreciate clarification)

>I (and, I'm sure, others) would appreciate your comments (and, if I'm wrong,
>don't hesitate to flame! :-)


Roy is correct.  To transfer a byte over the SCSI bus requires the target
to assert REQ, after which the initiator (the controller) is free to put
a byte on the bus or remove a byte.  The initiator will then assert ACK,
telling the target (the drive) that it has read/written the data.  When
the initiator removes ACK, the target is again free to assert REQ again
to transfer the next byte.  

Using a long cable introduces delays in the REQ/ACK handshake.  It takes
longer for the initiator to see a REQ on the bus and to respond to it,
and it takes longer for the target to see the negation of the ACK and
to respond to it.  Thus, long cables introduce delays for every byte
transferred, not just one delay at the beginning of the transfer.


-- 
Jerry Gardner, NJ6A					Altos Computer Systems
UUCP: {sun|pyramid|sco|amdahl|uunet}!altos!jerry	2641 Orchard Parkway
Internet: jerry@altos.com				San Jose, CA  95134
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