From: dplatt@ntg.com (Dave Platt)
Subject: Re: 2fx SCSI
Organization: New Technologies Group, Inc.  Palo Alto CA
Date: Tue, 15 Sep 1992 17:29:55 GMT

>ive had a mac 2fx for several years now (it was one of the first delivered
>to the school).  for the first time i've added an external SCSI device -
>a tape backup unit.  i'm using the 2fx terminator, but get flakey performace
>out of the tape drive (many unexplainable errors -- things that neither 
>the tape drive manufacturer nor the backup program's manufacturer can
>explain).
>
>is there some general problem with 2fx external SCSI ports?

The IIfx SCSI port uses a special high-speed ASIC.  Its rapid
responses to state-changes on the bus, and the fast rise and fall of its
outputs, can cause problems for some devices whose SCSI implementations
are a bit sub-par (either at the chip level, the firmware level, or the
cabinet-wiring level).  It can also show up problems in your SCSI cables.

Here are some things to try:

- Try a different Mac-to-peripheral SCSI cable.  The Mac's 25-pin
  connector omits many of the ground pins used in the normal SCSI-1
  configuration.  A _good_ Mac-to-peripheral cable runs the signal and
  ground wires from the 50-pin connector all the way to the 25-pin
  connector (ideally, twisted together), and ties the ground wires
  together at the 25-pin connector.  A _bad_ cable ties the 50-pin-
  connector grounds together in that connector, and runs only a few
  ground wires to the 25-pin connector.  Bad cables may be subject to
  crosstalk between signals;  this problem is worse on the IIfx due to
  the fast rise- and fall-time of its signals.

- Try a longer or a shorter Mac-to-peripheral cable.  Adding extra wire
  can damp out reflections on the bus, and may allow your IIfx to work
  with a less-than-perfectly-conformant SCSI peripheral.

- Check the wiring inside the tape drive cabinet.  A properly-wired
  cabinet will run a ribbon cable from the SCSI-in jack, over to the
  drive controller, and then back to the SCSI-out jack (a "horseshoe"
  arrangement).  A poorly-wired cabinet connects the two jacks together
  directly and then runs a single ribbon cable over to the drive... this
  creates what's known as an "unterminated stub".  Unterminated stubs
  are bad news... they're permitted by SCSI-1 as long as they're less
  than 10 centimeters long and aren't located too near one another on
  the bus, but they are discouraged and I believe they should be
  entirely avoided.  I've noticed that the IIfx tends to be intolerant
  of out-of-spec stubs.  Unfortunately, the only cure for a stubbed
  cabinet is to rewire it.

- Make sure that your IIfx terminator is at the physical end of the bus.
  If your tape cabinet has two jacks, plug the SCSI cable into one and
  the terminator into the other... don't use the terminator in its "flow
  through" configuration between the cable and the cabinet (this creates
  a stub, even if the cabinet is properly wired).




-- 
Dave Platt                                                VOICE: (415) 813-8917
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