RE 3: Re 2: Revised proposal on MSE high-impedance spec

John Lohmeyer lohmeyer at
Tue Jun 22 09:50:49 PDT 1999

* From the T10 Reflector (t10 at, posted by:
* John Lohmeyer <lohmeyer at>
At 6/22/99 09:43 AM , Gene_Milligan at wrote:
><<With multiple high-Z MSE receivers I believe this intersection could be
>on the 20 mA segment of Figure 26. However, I don't think the 160 mA in
>Gene's original comment is possible due to the limit in Figure 26.>>
>     I guestimated the upper limit which did not quite make it into the
>figure was at about 10 ma per receiver. I presumed we did not need to worry
>about 32 bit wide busses which could have 32 SCSI devices and judged the
>limit to be 16 SCSI devices. Thus 160 mA.

Gene and Richard,

Richard is much closer to reality with his 7 mA number for SPI-3 buses.  20
mA might be possible theoretically with a little ground shift, but I think
it is unlikely that we will even  see 7 mA with modern active negation

Gene's 160 mA is an extreme worst-case number that could only happen with
15 (not 16) 3-volt devices and one 5-volt active negation device that has a
P-channel negation transistor which clamps the signal to 5 volts.  I think
there were a few such devices designed in the early days of active negation
(circa late SCSI-2 and early Fast-20).  People quickly learned that such
strong active negation transistors caused more problems than they helped.
In fact, these devices worked better with active negation disabled -- so if
we can draw enough current to blow out these deviant transistors, we'll be
doing them a favor!  ;-)

All seriousness aside, should we handicap our ability to move forward with
new processes just to avoid a potential problem with very old devices?  How
likely is it that we'll encounter any of these old devices?


John Lohmeyer                  Email: lohmeyer at
LSI Logic Corp.                Voice: +1-719-533-7560
4420 ArrowsWest Dr.              Fax: +1-719-533-7036
Colo Spgs, CO 80907              BBS: +1-719-533-7950

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