[Color Computer][Coco] Tandy Hard Disk Controller
mark at cloud9tech.com
Sun Mar 5 08:26:39 EST 2006
Well I can tell you for a fact that the TC^3 will work with a CD-ROM
or whatever at any of the SCSI addresses with a fully loaded bus.
Never had a termination problem. Though on the SuperBoard TERM power
will be selectable.
The rest of your post.... No wonder you don't have time to round tuit
and get it done. Seems like I was back in Communications design class
20+ years ago. Been there, done it. You need to get your SCSI going,
so.... GET R DONE. Convert type time to tuit time.
As far as your SCSI to IDE comparison. Yes SCSI is very elegant
compared to IDE. We had a heck of a time one whole summer
testing/debugging the SuperIDE. Most don't even follow the spec. Even
the same manufacture's drives of different IDE eras won't work in a
master slave configuration.
Well I'm going to end my part of the thread here. Normally I'm pretty
quiet here because I have very little free time. I'll do scan most of
the posts. I spend my time working on my CoCo projects, hands on,
rather than just typing about them. Much more gratifying to me.
At 3/5/2006 12:02 AM, you wrote:
>On Saturday 04 March 2006 23:25, Mark Marlette wrote:
> >The 4n1 worked, but it didn't even follow the electrical spec for
> >SCSI. I'm sure if you would have loaded the bus with all devices it
> >would have failed. Not the case with the TC^3.
>I do now that it failed when a cdrom was also attached and set for the
>next address up. I haven't tried that with your tc^3 yet though, I
>need to salvage another connector to install, & find the round tuit to
>get it done.
> >Glazing over??? Come on Gene I wanted you to teach me something I
> >didn't already know about SCSI. :)
>Doubtfull if I could have taught you anything about scsi, even a decade
>ago. The one single thing I've long since learned is that very few
>engineers building this stuff, really, truely, understand scsi's
>termination. The biggest single reason scsi has a reputation for
>needing ritual sacrifices, vestal virgins prefered to make the bus
>actually work I'll discuss.
>The first huge engineering, or bean counter mistake, I think sometimes
>the engineers get over-ridden in their parts procurements by bean
>counters that don't always understand the nuances of the difference in
>something as simple as a diode. The engineer usually knows about the
>problem, and specs a schotkey for the bus isolation diode in an attempt
>to maintain a decent noise margin in the resting logic 1 voltage. With
>that, the usual 220-330 resistor network used per line gives maybe 2.85
>volts. Thats arrived at via assuming the psu is 5 volts, the schotkey
>drop is .15 to .2 volts, leaving a 4.8 to 4.85 voltage for the
>termpacks. So the logic line on the buss is sitting at an open
>collector voltage of 2.88 volts, and the bus has a noise margin of 440
>millivolts above a guaranteed logic 1. With decent cabling, the buss
>will work error free even if it does ring a little. And it will.
>Now sub a small si diode for that, which may well drop that 4.8 volts to
>4.3 or less as the si diode will drop .75 volts at that current level
>typically. (4.3/550)*330 now equals 2.58 volts, only a 180 millivolt
>noise margin. Then due to wireing losses and old age in the psu, its
>now down to 4.88 volts on the 5 volt line at the motherboard connector,
>and by the time it gets thru the motherboard to the scsi card & the top
>of that diode, its maybe 4.6 volts. Subtract the diodes nominal drop
>of .7 volts and you have (3.9/550)*330=2.38 volts and no noise margin.
>The bus rings maybe .7 volts on a signal edge, and there's not a ttl
>gate in the world that won't see that and make a mistake. Not even
>active terms on the other end will fix that. And people wind up
>sacrificing goats, chickens (I used to use KFC's, but after they made
>me sick a few times I got smart), whatever, trying to find the magic
>incantation that makes it work.
>I don't recall how many scsi busses (7 or 8 at least) I've converted
>from flakey to dead solid with nothing more than a small schotkey power
>diode replacing the cheapo si diode that the card shipped with back
>when I was doing some amiga work. And one run of an expensive card
>actually had the termpacks installed backwards so the resting voltage
>was only about 1.9. Soldered in, it was easier to cut and jump the pcb
>to fix it than to extract 3 termpacks and turn them around. I called
>the maker and he wasn't the least bit embarrased. And that was the
>last of those cards we ever bought.
>The bus 'ringing' is just another word for VSWR, and those 220-330
>resistor packs are just the nearest handy std value that attempts to
>match the impedance of a ribbon cable, but misses the mark by at least
>10 ohms most of the time for flat cables, & I haven't the foggiest how
>far off it is for round cables, I shudder to contemplate it. And I've
>had much better luck with std ribbon cable than with rounded stuff. At
>least one can mathematicly calculate the flat ribbons impedance with a
>10% slop figure and come up with something in the 118-124 ohm range if
>I did it right. The termpacks 220 and 330 in parallel give you around
>132 if the tolerances are nominal, so you can see there is an error,
>leading to some ringing.
>Active terms are another horse entirely, generally consisting of a bias
>supply, and a power op-amp that supplies about 3 volts at a very low
>impedance. All the data lines are then fanned out from this fixed
>supply, usually by trimmed onchip r's of about 132 ohms per line, so
>both the logic 1 voltage they supply, and the terminating impedance are
>a lot closer to ideal. The development of this technology was driven
>by the portable business, where every watt you burn is that much less
>battery life. Termpack resistors draw .818 watts of power full time,
>the active terms draw it only when the lines are carrying data, and
>then only in proportion to the logic 0's on the buss at that instant.
> >All the flavors of SCSI, the spec is something to get lost in for
> >sure. Know it all? Nope, just enough for the CoCo. :)
>And thats my contribution to the knowledge pool vis-a-vis the care and
>feeding of a scsi bus.
>To a lot of folks its voodoo, to someone familiar with transmission
>lines, which both the scsi and the atapi/ide busses are, the
>explanation, and the fixes to make it work, makes sense.
>Relatively unk is that in the case of ide/atapi drives, the one jumpered
>as master also turns the cable terms on, and that means it should be on
>the end of the cable, with the slave drive in the middle. Its not
>always happy if the master is on the center connector, and the slave,
>or nothing, is on the end of the cable. Thats asking for trouble with
>a big kick me sign taped on your back.
>People having trouble with vz bouncing email to me should add the word
>'online' between the 'verizon', and the dot which bypasses vz's
>stupid bounce rules. I do use spamassassin too. :-)
>Yahoo.com and AOL/TW attorneys please note, additions to the above
>message by Gene Heskett are:
>Copyright 2006 by Maurice Eugene Heskett, all rights reserved.
>Coco mailing list
>Coco at maltedmedia.com
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