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." -Ed Howdershelt (Author) 99.31% setiathome rank, not too shabby for a WV hillbilly Yahoo.com attorneys please note, additions to this message by Gene Heskett are: Copyright 2005 by Maurice Eugene Heskett, all rights reserved.
