SCSI-2 termination vs SPI

dean wallace 75671.3443 at compuserve.com
Tue Sep 24 09:25:13 PDT 1996


* From the SCSI Reflector (scsi at symbios.com), posted by:
* dean wallace <75671.3443 at CompuServe.COM>
*
     At the last spi-2 working group the differences between the scsi-2 alt-2
terminator and the spi alt-2 terminator were discussed. John asked me to
put a message on the reflector for discussion. The foil that was presented
at the last working group meeting is:

* SCSI-2 alt-2 terminator allowed to source 22.4mA (@0.5V). The current
at the driver could not exceed 48mA (@0.5V).

* SPI specifies a maximum terminator current of 24mA (@0.2V).

* The SCSI-2 terminator is not compatible with the SPI terminator. The
SCSI-2 alt-2 terminator exceeds 24mA (@0.2V).

* If the alt-2 terminator has a maximum current of 22.4mA (@0.5V) and a
minimum impedance of 100 ohms, then the output current is 25.4mA
(@0.2V). This means a SCSI-2 compliant terminator can be incompatable
with a spi terminator.

What I had proposed was;

1) An alt-2 terminator has a maximum current of 22.4mA (@0.5V) and
25.4mA (@0.2V). The termination impedance range is 100 ohms to 132
ohms.

2) Non alt-2 terminators have a maximum current of 24mA (@0.2V).

Bill Ham suggested that the current for non alt-2 terminators also be
increased to 25.4mA (@0.2V). I think that the intent of reducing the Vol
to 0.2V (from 0.5V) was to prevent terminators with clamps or non linear
terminators from supply large currents below 0.5V that could cause 
reliablity problems for the driver folks, not to make SCSI-2 terminators
incompatable with spi terminators.  I would appreciate comments
since I will bring in a proposal to the next scsi working group.

REGARDS DEAN WALLACE LINFINITY MICRO

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