SCSI-2 termination vs SPI

Gerry Houlder Gerry_Houlder at
Tue Sep 24 12:56:13 PDT 1996

* From the SCSI Reflector (scsi at, posted by:
* Gerry Houlder <Gerry_Houlder at>
Dean Wallace's messages read, in part:

>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
>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.

Your intent is correct. How tight are we on the "22.4" and "25.4" mA limits?
Are these absolute maximums for SCSI-2 terminators or could they be a
little more? If we specified 23 and 26 mA, respectively, are we still OK
(so we don't damage anyone's driver)? These numbers sound more like
carefully designed numbers instead of numbers that are tweaked to a
specific vendor's part.
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