Gene Heskett wrote: > On Saturday 06 March 2010, Jon LaBadie wrote: > > > Because the scsi buss is a transmission line, and demands a decent VSWR, > there are 2 things to remember when dealing with scsi. > > 1. A list of pre-requisites that must be met if it is to work: > > 1.a: termination power, it has to come from someplace, preferably the host > controller. > > 1.b: terminations can only be at the ends of the cable, one and exactly one > on each end. Leaving a 6" pigtail hanging out because you removed the drive > on the end of the cable, and which co-incidentally also was the terminating > device is a more common mistake than one would think. > > 1.c: when supplying scsi term power, there needs to be an isolation diode > between the supply and the term power line in the cable, pointed so that the > voltage can get to the cable ok, but cannot be fed back into an accessory > device after it has powered the terminators on that device. > > 1.d: Because this diode has a voltage drop, and the original idiots (yes, > that _is_ the right terminology) chose the resistor values based on std > values AND a 5 volt supply, so a common silicon diode virtually cannot be > used if the scsi bus is to be error free. The .65 to .7 volts of drop in an > Si diode reduces the logic one voltages noise margin by about 2/3rds. from > the target of 3.0 volts, but usually about 2.85 due to the resistors chosen, > throw in the .7 loss of the diode divided by the resistor ratio's (330 to > ground, 220 to the 5 volt line, and your logic 1 noise margin is whats left > after you subtract the 2.4 volts of a guaranteed logic 1. At 2.7 volts it > might work if the resistor tolerance are spot on. At 2.55 volts, you have > only a 150 millivolt noise margin and you may as well hold a seance, its > dead, Jim. Therefore this diode must be either a power germanium or a > schotkey rated for the current in order to get a decently low forward voltage > drop. Otherwise find a 6 volt supply for termination power. TBT, when the > psu voltage in the host machine is starting to sag in its old age, and is > already 200 millivolts low, I've been known to go get a 6 volt dc wall wart > from the shack and use that. It Just Works(TM). FWIW, I have yet to see a > Ge or Schotkey diode used on any of the major scsi vendors cards, so don't > assume that just because it says Adaptec on it, it can't be wrong. And it is > precisely those cards I've had to wall wart power several times. > > If you've done all that & it still doesn't work, have a tech with a 100+ mhz > scope look at it to see why the signal echos (aka VSWR) are so bad. Maybe > you've got one bad resistor in the termination packs, so check all active > lines, 21 of them in a 50 wire scsi-II cable IIRC. Make sure he understands > exactly what the term 'VSWR' means, a great many don't, somehow thinking it > only applies to continuous wave radio/tv transmitters. VSWR is VSWR whether > its digital, or CW, its still VSWR. > > 2: Alternatively, you can advertise for a virgin to be sacrificed. It won't > help of course. And its a terrible waste considering how hard it is to find > one these days. > > -- > 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) > > Reality is bad enough, why should I tell the truth? > -- Patrick Sky
Well, thanks for the long explanation. I doubt i will be able to test the setup as much as you have described but it's useful information. I've tried a couple of different cables, unplugged and plugged back in, all that jazz. These are the cables i've used previously without any trouble. I'm more inclined then to think it is the drive. but who knows.. +---------------------------------------------------------------------- |This was sent by r...@mrxfx.com via Backup Central. |Forward SPAM to ab...@backupcentral.com. +----------------------------------------------------------------------
