[SI-LIST] : Re: (Differential SCSI)

D. C. Sessions (dc.sessions@tempe.vlsi.com)
Thu, 05 Feb 1998 13:18:56 -0700

Mark Nass wrote:
>
> We are laying out a board that has differential a SCSI bus on it.
> The specs say the that the differential impedance should be 110-135 ohms.
> What is differential impedance? Is this the impedance between
> the 2 traces, or the impedance with respect to the plane?

The two tracks form a delta network. The effective differential
impedance is the parallel of (a) the trace-to-trace impedance
and (b) the series of the two traces' impedances to ground.
If the traces are equal and well-separated this is just twice
the individual line impedance, which would mean that your PWB
traces should be in the 55-67 ohm range (call it 61 ohms plus
or minus 10%). If your native impedances are higher, you can
get into the right neighborhood by running the pairs closer
together so that the coupling capacitance lowers the total.

-- 
D. C. Sessions
dc.sessions@tempe.vlsi.com