Fwd: FW: Period Tolerance history (fwd)

John Lohmeyer lohmeyer at ix.netcom.com
Wed Sep 9 11:30:38 PDT 1998

* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* John Lohmeyer <lohmeyer at ix.netcom.com>
Rob Elliott has had trouble posting the following message to the T10
Reflector.  I am trying to forward it on his behalf.  -- John

>Forwarded message:
>>From relliott Wed Sep  9 09:11:35 1998
>Subject: Period Tolerance history
>To: t10 at symbios.com
>Date: Wed, 9 Sep 1998 09:11:35 -0500 (CDT)
>X-Mailer: ELM [version 2.4 PL24beta]
>Content-Type: text
>Content-Length: 1133      
>I looked through the archives and found two proposals related to the 
>mysterious "period tolerance."  These should help explain the history 
>of those numbers.  
>94-142r1 (July 1994), by Bill Galloway at Compaq, introduced the concept.  
>He asked for 0.25% transmit period tolerance and 0.5% receive period 
>tolerance.  That corresponds to these values (although only Fast-10 
>existed then):
>            Period  Receive Tolerance   Transmit Tolerance
>Fast-10     100 ns      0.5 ns          0.25 ns
>Fast-20     50 ns       0.25 ns         0.125 ns
>Fast-40     25 ns       0.125 ns        0.063 ns
>97-268r1 (November 1998) by Tak Asami at Adaptec, complained that these 
>were too tight for PLL-based designs running at Fast-40.  He proposed 
>changing the transmit period tolerance to 1.0 ns and the receive period 
>tolerance to 1.1 ns.
>Apparently both proposals were accepted. 
>Rob Elliott      UNIX mailto:relliott at hobbit.eng.hou.compaq.com    
>Houston, TX        PC mailto:Robert.Elliott at compaq.com
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