RE: [SI-LIST] : Preferred PWB impedances

Greg Edlund (
Wed, 19 Nov 1997 09:20:24 -0500


I favor the output impedance specification for a driver rather than mA.
It seems that drivers from different vendors have different output
impedances for the same current rating, so specifying mA doesn't
uniquely specify an output impedance.

I agree with you about the 50 Ohm and 65 Ohm drivers. (Actually, 60
Ohms is easier to achieve when you find yourself boxed in by PWB layup.)
In addition, I would add a 60 Ohm controlled impedance, controlled edge
rate driver for cases where you have a very large number of SSOs.

Greg Edlund, Principal Engineer
Server Product Development
Digital Equipment Corp.
129 Parker St. PKO3-1/20C
Maynard, MA 01754
(508) 493-4157 voice
(508) 493-0941 FAX

From: D. C. Sessions[]
Sent: Tuesday, November 18, 1997 11:04 AM
To: SI-List
Subject: [SI-LIST] : Preferred PWB impedances

I had an interesting meeting late last week. VLSI is
drafting the specs for our newest I/O library and rather
than wet-finger it, we're asking for a little help in
making them meet real-world needs.

Here's the question for the floor:

If you could have only four 3.3v CMOS driver types, what
would you have. (Aside from HSTL, SSTL, etc.) One obvious
candidate is a reflected-wave driver for 50-ohm nominal
lines; that comes to about 10mA at 350mV from the rails
under worst case conditions. A 65-ohm reflected-wave
driver would run about 25% lighter, or about 8mA.
Incident-wave is another matter entirely.

SO! the floor is open for nominations. Keep in mind that
excessively strong drivers are both inherently slower and
more vulnerable to SSO degradation.

D. C. Sessions