Fast-20 transfer period factor.

Lohmeyer, John JLOHMEYE at cosmpdaero.ftcollinsco.ncr.com
Tue Nov 21 15:02:00 PST 1995


Ernie Rael wrote:
 ----------
>The scsi spec says:
>    The transfer period factor times four is the value of the
>    transfer period.
>Value is ns.  For 20 mega-transfers/second the transfer period is 50ns.
>Which implies the transfer period factor is 12.5.  But transfer period
>factor is an int.
>
>So, is there any special convention here?

Yes, there is.  A transfer period factor of 12 means a transfer period of 
50.  From the SIP draft standard, revision 8:

"The transfer period factor times four is the value of the transfer period, 
with one exception. A value of 12 (0Ch) denotes a transfer period of 50 
nsec. This is to accommodate the fact that 50 is not evenly divisible by 4, 
yet is a value that must be supported since it represents the maximum 
allowable speed within the Fast-20 timings specified for the SCSI-3 parallel 
interface."

Regards,
John
 --
John Lohmeyer             E-Mail:  john.lohmeyer at symbios.com
Symbios Logic Inc.         Voice:  719-573-3362
1635 Aeroplaza Dr.           Fax:  719-573-3037
Colo Spgs, CO 80916     SCSI BBS:  719-574-0424 300--14400 baud




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