SCSI-2 termination vs SPI

dean wallace 75671.3443 at compuserve.com
Thu Sep 26 07:18:31 PDT 1996


* From the SCSI Reflector (scsi at symbios.com), posted by:
* dean wallace <75671.3443 at CompuServe.COM>
*

     Gerry if there were no leakage currents then
we probably could use 24mA (@0.5V), but we are allowing
200uA per device which is how (I think) we came up with
the 22.4mA number for each terminator. I think this is a
case where rounding up would not be acceptable, unless 
controller folks think this is too much leakage per device.
The 25.4mA number is then just an extension of the 22.4mA
at 0.2V using the minimum alt-2 impedance of 100 ohms. 
The allowable range is already pretty large, the lower 
limit of output current is 17.4mA (@0.2V, using 132 ohms
and 2.5V output).

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
>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.

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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