gene - thanks for that wonderful set of thoughts! john On Wed, 2005-03-16 at 13:56 -0500, Gene Heskett wrote: > On Wednesday 16 March 2005 10:25, Eric Shibata wrote: > >Hi Gene, > >Do you mean interrupts? > >When I look at my /proc/interrupts I know I have other things there > > with my scsi adapter from BusLogic. > > > >-------------------------------------------------------------------- > >---------------------------------------------- CPU0 > > 0: 70937522 XT-PIC timer > > 1: 200 XT-PIC i8042 > > 2: 0 XT-PIC cascade > > 8: 1 XT-PIC rtc > > 11: 368831 XT-PIC BusLogic BT-930, uhci_hcd, eth0 > > 12: 4778 XT-PIC i8042 > > 14: 90206 XT-PIC ide0 > > 15: 212674 XT-PIC ide1 > >NMI: 0 > >ERR: 0 > >----------------------------------------------------------- > >Let's just say it was a cabling problem. When I insert the tape, it > > makes sound like it's rewind the tape. Would it be able to do that? > > Yes of course it would. The drive is fairly smart, and the first > thing its going to do when it senses that a tape has been inserted is > to rewind it, then inspect the header to determine what kind of a > tape it is, and if it has internal options to drive that legacy tape > if its say, a dds3 tape being loaded into a dds4 drive. So it will > make some noises all by itself while doing this. > > Now, I hate re-teaching how to re-invent a wheel here, the wheel thats > called 'scsi cableing, care and feeding' or some such silly attempt > at being a smart-ass on my part. I have done a nomogram like this > before, on this list, but not recently. I am a broadcast engineer, > where transmission line termination errors can easily cost us $50,000 > or more in stuff burned up. But here goes anyway. Bear with me > folks. > > lecture mode on > > 1. A scsi cable is a transmission line, and as such absolutely must be > properly 'terminated' at both ends of the cable, and only at the > ends. As little as 6" of unused cable hanging from one end or the > other, past the connectors that are in actual use, is enough to setup > some echo conditions in the signal transmission that will wreck any > chance of data integrity being attempted to be sent over the cable. > > Therefore, the card must have its terms enabled unless there are > cables on both the internal connector and the external connector. > Said another way, if the card has an external connector, but it is > not in use, then the cards terms must be turned on. This can be in > older cards, done by plugging in the 3 termination resistor packs in > the SIL sockets provided on the card, or in later cards, possibly by > a software option accessable via the cards own bios extensions that > you see flash up on screen for a few seconds during the machines > post. > > 2. Likewise, the far end termination must be done at the physical end > of the cable, not at some socket in the middle of the cable. No > unused cable is allowed to be hanging off the last used connection. > > Drives also vary these days in how they handle the termination, with > far more drives requireing the resistor packs to be installed than > not, although there is motion toward the use of what we call active > terminations in this field too. If the drive isn't terminated, its > not going to work UNLESS its not the last drive on the cable, in > which case its perfectly legal for it not to be terminated. Its the > last device that must be terminated. > > 2a. These terminations are the only 'loads' on the cable. The drive > itself, and the cable, are all designed in a wired OR configuration, > where the terms provide the logic one pullup voltage, and any device > on the bus can turn on a transistor and pull that data line to > ground. When they are 'off', the devices present a very small > loading on the cable so that many devices can share a cable. > > There is extensive collision avoidance built into the protocol, so > that when more than one device does this, its detected and dealt > with. Silently, transparently, and often with no more than a couple > of milliseconds delay in completing the data exchange the device > requested. > > 3. There are extant, many cards whose term voltage isolation diodes > are common power type silicon diodes. Some, and I have no idea if > the buslogic is among them, have used a schotkey diode for this, > cutting this voltage loss by about 2/3rds. The reason for the diode > is to prevent the case where the computer psu is turned off, but an > external drive enclosure is not, and that drive enclosure is also > supplying termination power to run these resistor packs to the 'TP' > line in the cabling. In that event, the computer would then be > powered from the drive exclosure if it weren't for this diode > preventing it. > > Where the type of the diode becomes a factor is when one is dealing > with a cable that despite so-called proper termination on both ends, > is still allowing some ringing on the edges of the signal > transitions. When the term supply is reduced from its designed value > of 5 volts because of losses in this diode, then the logic one noise > margin fades away from its designed value of having the logic one > resting voltage, wholly established by these terminators, fall from > the designed value of 600 millivolts above that voltage which is > guaranteed to be a logic one (2.4 volts), to as little as 100 > millivolts, at which point it doesn't take much ringing to get a dip > down into the voltage range that is officially defined as > 'indeterminant', and you have a detected logic error, not often > correctly reported in the logs, just a coverall, sometimes > meaningless, error that its not working. > > This has become a much smaller consideration where the so-called > active terms are in use. > > But lets talk 'transmission lines' a bit, at the physical level of the > cable you've had in your hands already. > > That cable, when a length of it is inspected with the measureing tools > available, and considering that every alternate wire in the cable is > a ground wire (unless the cable is being used in an hvd or lvd > system), will have a characteristic impedance of about 122 ohms, give > or take 10 or so due to tolerances in the ribbon cables manufacture. > Now, if that cable has a 122 ohm load on both ends, a signal > traveling down its length will be absorbed in these resistors, and > nothing will come back like an echo, but then superimposed on the new > voltage level this signal represents. The signals ring in other > words, if looked at with a sufficiently broadband oscilloscope. A > 100mhz scope is barely adequate. > > To achieve this termination way back in time 35 years ago, and still > in use today, it was common to use these resistor packs, which > consist of a 220 ohm resistor with one end tied to the term power > supply of nominaly 5 volts, the other end connected to the data line > being terminated, and a 330 ohm resistor is also tied to this data > line with the far end of it being grounded, those values being used > because thats what the resistor makers had to offer at Orville > Redenbacher's "popcorn" prices. This establishes a termination > resistance value by the normal rules for paralleled resistances of > around 132 ohms. > > Moderately close, and given correct noise margins, a generally > workable scheme. But, what happens when that 5 volts is reduced by > the nominally .6 to .75 volt drop of the silicon diode in series with > that voltage? Well, the expected 3 volts for a logic one can drop, > often to the point that this resistive scheme so carefully worked out > 35 years ago, actually presents only a 2.58 volts logic one level, > and only 180 millivolts of the 600 millivolt noise margin is left. > Then couple that with the fact that the psu itself may be sagging and > the 5 volt line is only 4.85 volts at the connectors where the drive > is drawing power from. Then you have a logic one voltage of only 2.4 > volts, the noise margin has all been used up and no amount of virgins > sacrificed is going to make it work. Ever. > > When the terminations are made 'active', this is marketing speak for > having a seperate 3 volt regulator set up on the card, and possibly > the drive, with enough 120 ohm resistors coming off of it to feed > every data line in the cable by its designed 3 volts, and it comes > closer to actually matching the impedance of the typical ribbon cable > to boot. And it has the added advantage of only drawing power when > the bus is active, unlike the resistor packs, which drew 320 > milliamps from a sometimes scarce power resource when resting, and > proportionatly more when active, in portable applications this became > a major source of power loss to be gotten rid of. > > In short, active terminations beat passive any day of the week when > the variations of the real world are plugged into the formula. > > In your case, I suspect the drive is not terminated at all, and the > cable is, electrically speaking, ringing like a bell. Or that you > have the drive connected at a cable plug that is not the last one on > the cable. The quality of the terminations can only be assessed with > high bandwidth oscilloscopes, but a very very good idea can often be > had just by measuring the resting voltage of a data line with > everything powered up and that can be done with a $20 dvm from radio > shack. > > If its above 2.9 volts on the cable with first one end unplugged, then > plug that one back in and unplug the other and recheck, then the > chances are you are pretty good and have other hardware problems. > No, or very little voltage when one end of the cable is unplugged > means the device the cable is still plugged into isn't terminated and > this must be fixed. > > If its only 2.65 to 2.75, its going to be a problem child > occasionally. Below 2.5 and its likely its not going to work until > the proper voltage is re-established. > > I haven't dealt with the logic zero noise margin considerations > because its a zero if the line is pulled down to less than .6 volts, > and I've not seen the scsi bus driver yet that couldn't pull the line > to well below 25 millivolts, hence its generally not a problem. If > it can't, the driver chip is toasted, go get someone who knows which > end of a soldering iron gets hot. I do, but I've yet to have to > replace one because of that. > > Now, I hope I've helped to clarify this thing called a 'scsi bus'. > > lecture mode off > > > This is my first time setting up a tape drive, it didn't sound like > > it should be this difficult. > >Thanks, > >ERS > > > >On Tuesday 15 March 2005 12:14, Eric Shibata wrote: > > >HELP! > > >I can't even get the status. No matter what I do I get the > > > following error. > > > > > >/dev/tape: Input/output error > > >/dev/st0: Input/output error > > > > > >Tape drive: HP SureStore T4i > > > > > >ERS > > > >That sounds a bit like cabling and or termination problems. >
