tape drive error

[john bray]

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.
>