Fast 20 Line Impedance Specs-Comments Please

Norm Harris x2230 nharris at
Fri May 6 10:33:49 PDT 1994

  At the FAST-20 Adhoc held May 5 in Milpitas, the group consensus was to specify    line impedance for FAST-20 as follows:

   90 ohms +/- 6 ohms (REQ and ACK Pair)
   90 ohms +/- 10 ohms (all other signals)

Unfortunately, we did not have any of the cable folks present to comment on these.

It was stated that: 

(a) impedance tolerances are usually speced at +/- 7 ohms by cable manufacturers
(b) The impedance of the center conductor pair in a SCSI cable is typically 5 ohms   above  that of outer pairs.  Therefore a cable cable construction requiring REQ/ACK  as the center pair typically raises the impedance of the REQ/ACK lines about 5 ohms.

Concern has been expressed about cost and manufacturability of cables speced in the  above manner. The 90+/-6 ohm spec for REQ/ACk would typically be achieved with a     cable having 90+/- 10 ohm specification for all conductors.  In light of this fact,  is the above spec unnecessarily restrictive?  

Further concern was stated about impedance specs for backplane and flat cable        implementations which tend to have uniform impedances across traces.  Any thoughts onon the wording for these implementations?

Interested parties should respond via Email and/or come prepared with presentations  on the above subjects at Harrisburg on Wednesday of the Plenary Week.  

Norm Harris
Adaptec Inc.
Enterprise Computing Group
(408) 945-8600

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