[Coco] Multipak redesign/replacement

[Richard E Crislip]

On Tue, 24 Feb 2015 22:33:17 -0500
Gene Heskett <gheskett at wdtv.com> wrote:

> 
> 
> On Tuesday 24 February 2015 21:02:12 RETRO Innovations wrote:
> > On 2/24/2015 12:19 AM, Al Hartman wrote:
> > > If you check the May 1985 Issue of Rainbow Magazine, page 222 has
> > > a review of the Spectrum Projects Multi-Pak Extender Cable.
> > >
> > > I can't believe Bob Rosen could make something in 1985 that can't
> > > be made today. I have one in a box somewhere, and it worked well.
> > >
> > > 24 inches long, and shielded cable.
> > >
> > > -[ Al ]-
> >
> > I can't speak to his product offering, but I am not prepared to
> > create something that causes people issues.  I appreciate that
> > certain lengths or specific brands of flat cable might do OK on
> > 1.77MHz signals, but such cable is not designed for such speeds and
> > should not be used for such speeds.
> >
> > Jim
> 
> Au contaire;
> 
> Transmission line class 101 is in session:
> 
> Such cable is 100% useful at mind boggling speeds.  All we have to do
> is follow best practices for a scsi interface.  That means the cable
> is driven at the end sourcing the signal by a driver whose source
> impedance matches the cable when that cable is looked at as a
> transmission line, AND that it be terminated at the other end with a
> load impedance that matches this transmisson lines working impedance.
> 
> For the typical flat ribbon cable, the interwire impedance is
> approximately 120 ohms according to the cable pedlars.
> 
> So how is this done in the real world for a scsi-II cable, which the
> specs as published say can be run as far as 39 meters?  The simplest
> way to visualize it is with an "active termination" chip on both
> ends, which is powered by the 5 volt buss and gives a very low
> impedance point at its output, holding that output at 3.3 volts, at
> both ends of the cable, which is then connected to the individual
> wires of the cable with a 120 ohm resistor.  This is done at both
> ends of the cable, and only at the ends.  So a signal can be injected
> at any point on that 39 meters of cable, with a 5 nanosecond fall
> time by an open collector transistor stage pulling  that point of the
> cable to ground.
> 
> This "falling edge" then propagates from where ever it originates, to
> the ends of the cable, where it sees this 120 ohm termination
> resistor.  The resistor absorbs the signal by std ohms law theory,
> and since the cable impedance and the terminating resistor are a
> close match, very little signal is reflected from the ends of the
> cable which could add or subtract from the signal at any point on the
> cable.  If they do not match, then there will be a reflection which
> will bounce back and forth on the cable until it is all absorbed.
> 
> You can hook the open collector driver transistor to the cable at any 
> arbitrary point, and it will work flawlessly at scsi-II speeds,
> possibly as high as 80 megabytes a second flowing on this cable with
> no errors.
> 
> The secret of the no error operation is the closeness of the
> termination vs the cables impedance.
> 
> When it was first brought forth for use as a storage media interface,
> the bean counters didn't like the active termination because it costs
> money, so only the top end drives and interfaces entertained it.  And
> you paid a premium for such card s and drives, typically another 50
> to 100 dollars.
> 
> What saw the production lines & was stuffed into the boxes to us used
> a resistor pack, usually setup to handle 7 lines per pack, hence 3
> packs were used to cover the active lines of the scsi-II interface.
> These resistor packs had a ground pin, and a pin intended to be
> connected to the 5 volt bus, with a 220 ohm resistor in series from
> the 5 volt connection, with a 330 ohm resistor tied to the ground
> pin, so the junction of the two resistors was then held at 3.0
> volts.  It took 3 of them and the power draw to power them was, and I
> need to do some math here since we have the equivalent of 21 each 550
> ohm resistors in parallel, each such pair across the power rails then
> drew 5/550, *21, *5=0.954545454545 per pack. Or nearly 3 watts of
> power as heat at both  ends.
> 
> Termination wise that resistor combo looked like (using the classic
> formula for paralleled resistors, was a 132 ohm resistor, not a
> perfect match, but close enough that it did work at reasonable cable
> lengths.
> 
> Then it was found that in order to prevent everything on the buss
> from being powered by the 5 volt supply with the highest voltage,
> even with the device on the far end turned off, that there needs to
> be a very low drop diode to feed this 5 volts to the terminators,
> while preventing that same 5 volts from re-entering a device that was
> powered down, so the designers then specified a high current shotkey
> diode, with a voltage drop in the forward direction that was in
> the .125 volt range.  But between the designers and the production
> line, some damned bean counter looked at the cost of that shotkey
> diode, and bought a 10 cent std SI power diodes for production to
> stick in those holes in the board.
> 
> Bam, thats .75 volts of effective voltage drop for the SI diode, so
> the logic one resting voltage, designed for 3 volts, is now only
> about 2.65 volts, assuming the psu is good.  If its starting to sag
> in its old age, and only making 4.90 volts the bus itself is down to
> 2.5 and change.  This is TTL logic levels, where a solid logic one
> must be at least 2.4 volts, no guarantees are made betwen about .8
> volts and 2.4 volts, its officially declared indeterminate.  Check
> any chip book.
> 
> So in the real world occupied by us, at the mercy of those bean
> counters who don't know squat about semiconductors, scsi-II got a
> reputation for being a cast iron bitch that only worked when it
> wanted to.  But it wasn't the bus design, it was the bean counters
> that made it that way.  I long since lost track of the number of scsi
> cards in amiga's that died with bus errors, but that between my
> replacing the 10 cent Si diodes with $2 shotkeys, and a tired psu
> with a good one ,aking a 5 volt buss at 5.05 or better, and it became
> bulletproof.  No more errors until the drive died.
> 
> All because its logic one noise margins of about .6 volts had been
> given back to it.
> 
> So what I am saying is that it can be done, and done very dependably,
> if its done right.  The IDE cable had an 18" max length, and it was
> so sloppily terminated that I have also cured several PC's making
> data mistakes by the simple expedient of cutting off the end
> connectors and cutting off whatever amount of cable I could that
> still allowed it to reach the drives and reinstalling the connectors.
> 
> Because the master/slave jumper also turns the terminations off when
> in the slave position, it is imperative that the drive called master
> is the drive on the far end of the cable, and the slave is then
> attached somewhere in the middle.
> 
> Class dismissed, says great grandpa Gene.
> 
> Cheers, Gene Heskett

I know you've been showered with flowers already, I am floored that
this is coming from one w/o a silly degree. All self taught. I'm not
worthy.