tape drive error

Wed Mar 16 18:56:32 UTC 2005

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.

-- 
Cheers, Gene
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Copyright 2005 by Maurice Eugene Heskett, all rights reserved.