AHA2790UW has speed-limit problems ?

[Ross Harvey]

> From: mikebw at bilow.bilow.uu.ids.net (Mike Bilow)
>
> At the risk of getting into an argument which could be as unending and
> controversial as whether toilet paper should feed from under or over the roll,
> it is worth pointing out that damping per se is sufficient to solve this
> problem, and the easiest way to achieve that -- regardless of what the standard
> says and whatever theoretical justifications may be advanced -- is simply to
> overterminate the bus.  If there is a third terminator added in the middle of
> the bus somewhere, it will usually correct superimposition problems.

I would advise against doing this!  Three reasons:

  1. The terminators have a DC load. The load from the first pair of
     terminators is already at the specified max for the drivers. Adding
     another will run the drivers in each disk and at the host adapter over
     their sink and source specs, possibly burning them out, and certainly
     raising their junction temps, degrading their performance, cutting
     into noise margins, and possibly causing heat-related failures elsewhere.
     I believe forced perfect terminators also include a standard active
     terminator with a DC resistive load, so I don't think using FPT works
     around this problem.

  2. At the point where the incident wave hits the middle `terminator',
     the bus Z becomes Z/2. A terminator of resistance R is equivalent to
     an infinite length transmission line of impedance Z==R. The middle
     `terminator' then appears as a fork, with an infinite length line one
     way and the remainder of your SCSI `bus' (it's not really a bus any
     more!) going the other way.  For an incident wave with voltage step
     V, you get an inverted reflection of V/2 heading back to the
     src and a truncated forward wave of V/2 heading towards the original
     dst. At the source, the driver will put out another V/2, (if it can,
     it's now being asked to sink or source more mA than its data sheet 
     allows) and this will reflect forward and back as V/4 at the middle
     `terminator'.

     The V/2 signal levels produced by this effect are particularly nasty,
     because they hover at the switching thresholds for a while and so any
     noise (and we have lots, right? that's the original problem) will be
     superimposed right at the switching level and may be seen as transitions
     by the receivers.

     You might observe this to work, anyway, in some specific example case.
     This might be due to a combination of slow drivers and a short bus,
     so that it is partly a lumped load and not a transmission line, and
     because the drivers fortunately had extra capacity, they didn't burn
     out, and you didn't notice the possibly shorter drive lifetimes. Or,
     perhaps, because an end terminator was defective or not really enabled
     as expected. But see #3.

  3. The correct solution is to recycle the SE HW and go differential,
     LVD, or FC. Even if you can tweak it into working by upgrading
     terminators or cables or by --oh dear-- *refolding and untwisting the
     cables*, whatever, you can bet money that you have no noise margin.


  --Ross Harvey

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