On Wednesday 12 January 2005 14:55, Gene Heskett wrote:
>On Wednesday 12 January 2005 12:04, Jon LaBadie wrote:
>>On Wed, Jan 12, 2005 at 11:18:00AM -0500, Eric Siegerman wrote:
>>> On Tuesday 11 January 2005 16:40, Jon LaBadie wrote:
>>> >Also, I think that if both types of devices exist on the same
>>> > bus, the lower performance one determines the performance of
>>> > the entire bus.
>>>
>>> In theory, this is *not* the case. ...
>>>
>>> On Tue, Jan 11, 2005 at 10:48:17PM -0500, Gene Heskett wrote:
>>>
>>> As to the whole chain being restricted to the speed of the
>>> slowest device, I've heard that, but it doesn't, on the face of
>>> it, make a lot of sense to me
>>
>>Gene, Eric,
>>
>>Thanks for the correction.
>>I'm glad I remembered to say I wasn't a scsi expert.
>>My comments provide the proof.
>>
>>> <off-topic>
>>>
>>> > To many folks forget that a a scsi bus is indeed
>>> > an rf transmission line, subject to the usual rules about vswr.
>>> >
>>> >From the context it's pretty clear what "vswr" means, but what
>>>
>>> does it stand for?
>>> </off-topic>
>>
>>Wondered too, as I'm posting anyway, I'll guess: v??? standing wave
>> reflections
>
>VSWR=Voltage Standing Wave Reflections. Back in rf days, because
> the signal is repetitive, very high voltages could develop at
> certain points in a transmission line driving an antenna that
> wasn't an ideal load, so they came to be known as standing waves.
> And it doesn't take a lot of miss-match to burn a 6 & 1/8" 75 ohm
> rigid copper coax with 30kw peak power going into it at channel 19
> (about 507mhz), making you call in a tower crew and replace the
> burnt section and clean the teflon carbon out of the other 1000
> feet of it. Lets just say thats expen$ive... But now lets get
> down to the much profaned scsi buss, where these same physical laws
> apply even if they don't make junk and smoke out of expensive
> copper and teflon parts.
>
>Understand we are dealing with a bus capable of responding to a 10
>nanosecond or less signal for starters, often lots less.
>
>Some ascii art if you'll all bear with me & use a monospaced font.
>The vertical scale:
>3.0 Volts a solid logic 1
>2.4 Volts unk upper bound, usually a 1 99% of the time
>1.8 Volts unk logic state
>1.2 Volts unk logic state
>0.6 Volts unk lower bound, usually a 0 99% of the time
>0.0 Volts a solid logic zero
>
>Now assume a worst case condition of no termination just to make it
>simple.
>
>Input pulse from typical wired-or driver, active pulldown.
>
And screw it, the (insert 15 minute non-repetitive monologue of
swearing) list server ate my tabs.
Sorry folks, I tried.
>t0 t+5ns t+10ns t+15ns t+20ns t+25ns t+30ns
>1-------| |----------------------------
>? | |
>? | |
>? | |
>? | |
>0 ----------------|
>
>Same pulse 18" down the cable after the pulse has reached the end of
>the 24" cable and bounced because its not terminated.
>
> |-----| <- this could be clipped
> |
> | | |-----|
> | | |
> | | | | |-----
> | | | | |etc
>
>------------------------| |-----| | | | |etc
>
> | | | | | |etc
> | | | | | |etc
> | | | | |
> | | | | |-----|etc
> | | |
> | | |-----|
> |
> |---------------|
>
>As you can see, after the main data pulse, there are several forays
>into unknown territory as the rest of the circuit slowly absorbs the
>unwanted echo. Also, in the real world, there would be some wibbles
>in the initial logic zero time I drew as a flat line, but most of
>these are absorbed by the very solidly turned on driver transistor
> at the time. These largly disappear if the terminations are
> correct, but only active terms really match the cable that well.
>
>This loss of the upper boundary noise margin in particular is one of
>the reasons I'd love to see a special, 5.8 volt supply made
> available just for supplying term power to scsi busses, the extra
> .8 volts to make up for the losses because the average card
> designer gets his choice of a low voltage drop schotkey diode for
> the isolator overridden by some friggin bean counter who doesn't
> understand that his arbitrary replacement of a 50 cent diode with a
> 10 cent si diode, and its .7 volt loss, has just cost the product
> about 90% of its logic 1 noise margin by reducing the upoper
> resting voltage of the buss to the 2.6 volt area. And we wind up
> advertising for virgins to sacrifice to make the darned thing work.
> Hell, even a Ge diode would be a better choice if they think they
> cannot afford that 50 cent schotkey.
>
>Hopefully this poor ascii art might clarify the situation a bit. If
>I'm curious, I turn on my scope and look, but TBT, my 100 mhz dual
>trace scope is actually a bit slow to really display stuff like
> this. Stuff like this really needs a 500mhz scope to display it
> anywhere near in real time, but the chips on a scsi buss are in
> fact that fast!
--
Cheers, Gene
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
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by Gene Heskett are:
Copyright 2005 by Maurice Eugene Heskett, all rights reserved.
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