SPI-2 Letter ballot resolution

Kevin Gingerich k-gingerich at ti.com
Mon Feb 16 14:33:22 PST 1998

* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* Kevin Gingerich <k-gingerich at ti.com>
The necessity for specifying a common-mode voltage range is for bus operation
in the presence of noise. Coupled noise is almost always time varying and not a
steady-state voltage. That is why the cable grounds are not a low impedance and

Noise is also statistical. There is a finite probabilility that whatever ground
offset voltage range you design your system for, it will be exceeded. The
probability of exceeding the LVD noise voltage tolerance of +/-355 mV is much
higher than the +/-7 V of RS-485. 

To my knowledge, there is no statistical sampling of the noise coupling to SCSI
buses that would lead to system requirements. In lieu of that, it seems
advisable to get the most you can out of the available technology.

Kevin Gingerich
Texas Instruments, Inc.

>  From: Gene_Milligan at notes.seagate.com, on 2/16/98 1:02 PM:
>  * From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
>  * Gene_Milligan at notes.seagate.com
>  *
>  Regarding 98-108r1:
>  <<65) I have repeatedly argued that the voltages specified in Table 22 for
>  HVD are component specifications and are not appropriate for the SCSI
>  environment. They merely lead to FUD about an inadvertent inclusion of
>  another version SCSI device on the HVD bus. In addition to the likely hood
>  that the cables would burn up with these values, if they did occur they
>  would also occur with LVD. Rejected: This is part of 485 and there is
>  nothing we can do about it.>>
>       I reject the rejection. It is non-sensical to have the component
>  standard dictate the system requirements. It should be the reverse. T10
>  would be derelict to bury their head in the sand on this issue. The sources
>  of the offset voltage are independent of whether HVD or LVD is being used.
>  Once again, even if it made a difference which type was being used, the
>  cable would have burned up prior to the transceivers being allowed to come
>  out of the high impedance mode to attempt signaling. The specifications for
>  the insulation are in hundreds of volts, this does not make it necessary to
>  have the devices accept hundreds of volts. The statement "This is part of
>  485 and there is nothing we can do about it." is spurious and has no
>  relationship to the application.
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