[SI-LIST] : Re: Differential SCSI

Fred Townsend (fred@luxtron.com)
Thu, 05 Feb 1998 14:49:00 -0800

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?

Large error buzzer! We are throwing out the baby with the bath water
again.

110 ohms is the surge impedance of the cable. This is also known as the
driving point impedance or the instantaneous impedance. This is not an
impedance that is carefully crafted for the circuit. Rather it is just
about the only thing that can be manufactured in the high density
cables. Note there is no ground plane on the cable so the impedance is
pretty high.

This sounds like the cart driving the horse and cart loses a wheel when
it reaches the PC boards. Without knowing the epsilon for your laminate
I think you will find it very difficult to make a 95 ohm trace (ground
plane and trace to trace in parallel)!

If you can't make a 110 ohm bus trace ( I couldn't) then you have two
choices:

1) Make a impedance transformation
2) Keep the traces very short.

Since you must source terminate this bus (YOU DO HAVE A SOURCE
TERMINATOR DON'T YOU?) option 1 is complicated by finding a terminator
such as a Dallas part that matches the transformed impedance.

You will probably find option 2 much easier. Simply place the SCSI chip
(Adaptec, NCR, or Fujitsu?) next to the cable connector. Sneak in the
"SO" case terminator packages wherever you can as long as they are in
between. NO TEE stubs! I put mine on the solder side. The trace
impedance doesn't matter that much if you can keep those traces to under
an inch in length.

Been there, done that!

Fred Townsend
DC to Light Consulting Services
Silicon Valley