[Simh] Byte-swapped tar tape circa 1987

[Larry Baker]

Good point.

About 15-20 years ago we transferred our library (100's, maybe over  
1000) of 9-track tapes to a disk archive.  I think we sent the tapes  
out to be cooked, which was to make sure the adhesive bonding the  
oxide coating to the mylar tape was good.  I also remember the person  
who played back the tapes running them through a tape cleaning machine  
before playing them back.

I neglected to mention that, out of the eight tapes I tried to read  
last week, one kept getting stuck on the read head.  We have a  
streaming tape drive now, and it is good at stopping right away when  
that happens.  I have never had this happen before.  I cleaned the  
tape surface with alcohol, but that did not help.  There must also be  
some sort of lubrication in the oxide on 9-track tapes that is no  
longer effective on this tape.  It was a different brand than the  
other tapes, perhaps inferior.  I remember in the old days there were  
tape head cleaning fluids.  I assumed they were just alcohol.  Now I  
wonder if perhaps they also included a lubricant.

Larry Baker
US Geological Survey
650-329-5608
baker at usgs.gov

On 31 Jul 2011, at 5:35 AM, Göran Åhling wrote:

> Hi!
>
> Great explanation!
>
> But, a slightly touching question: did you ever have any deeper  
> thought upon mechanically reading this old tape?
>
> I've just been asked to advice someone that has a unique 1/2"-tape  
> with measurements data that now suddenly has been of interest to  
> read out and compare to modern measurements. Format etc would be of  
> no problem (VMS / RMS is part of the writing...).
>
> The one who asked had heard about old tapes loosing the magnetic  
> part from the tape film during first unwinding to let it pass the  
> reading-head of a tape-station. He was speaking from that some  
> people are said to be talking about "steam-treating" the tapes  
> before letting the tape out, for example.
>
> Any advice or still better, knowledge, would be greatly appreciated.
>
> /Göran Åhling.
>
> On 2011-07-29 01:44, Larry Baker wrote:
>> I thought I would pass along something I just discovered about an  
>> old tar tape from 1987.
>>
>> One of our scientists brought me a stack of old 9-track tapes  
>> (circa 1987-1990) to read this afternoon.  All but one were ANSI- 
>> labelled "VAX COPY" tapes.  The one that was not said it was a Unix  
>> tar tape.  I have a tape scanning program that told me it had a  
>> single file on the tape with 10240 byte blocks, which matches what  
>> you should see on a tar tape.  I read it on to our MicroVAX as a  
>> foreign tape with RECORDSIZE=BLOCKSIZE=10240.  But, when  I tried  
>> to use tar to read it (VMSTAR on our Alpha, tar on my Mac, tar on a  
>> Solaris SPARC workstation), it kept saying it was not a tar  
>> archive.  I opened the file on my Mac using TextWrangler.  There  
>> were lines that looked like data, and text that looked like it  
>> could be a file name, but I didn't recognize any of the text.  I  
>> figured at least it was an uncompressed/unencrypted archive of some  
>> sort (thank goodness).  I thought it might be the old Unix dump  
>> tape format, but I could not find out what that format was like (I  
>> didn't spend much time Google searching).  I looked once more at  
>> the ASCII in the archive, and mentally byte-swapped some of the  
>> strings that looked like they should have been file names.  BINGO!   
>> What looked like gibberish became words.  I used dd conv=swab on my  
>> Mac to byte-swap the file and, lo and behold, tar -tf worked.
>>
>> About the time the tape was written, we had a pair of VAX-11/750's  
>> networked to a VAX-11/785.  One of the VAX-11/750's ran BSD Unix.   
>> I don't know whether there was a device name option to select byte- 
>> swapping (in those days, at least, Unix used different device names  
>> for the same tape drive to select among the capabilities of the  
>> device, such as to select the density) or if BSD always byte- 
>> swapped when writing tapes on a VAX (I doubt it -- this is the  
>> first time I have ever encountered a byte-swapped 9-track tape).   
>> Whatever the reason, I thought it might be useful for this group to  
>> know that such tapes exist.  Thus, if you get a real tape of a tape  
>> image that purports to be in tar format, but is not, don't give up  
>> -- it could be byte-swapped!
>>
>> Larry Baker
>> US Geological Survey
>> 650-329-5608
>> baker at usgs.gov
>>
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