How can AIX tell 1/4" cartridges apart?

[rudy at chukran.austin.ibm.com]

The way most that the IBM 1/4" tape unit tells what the density of
tape is via holes punched in the tape.  I suspect that all other units
do the same, but they are less "picky" about what sort of tape they write on.
For example , I took a quick look at a 3M DC600A tape just now, and as I moved
the drive wheel by hand to move the tape forward, I saw 2 punched holes
go by.  These will
pass by the angled mirror in the cartridge and be detected by the tape drive.
This is also how end of tape is detected.  If you examine a 6150 tape, you will
find the pattern is different, but I havent taken the time to verify what the
pattern is.  The tape unit is not smart enough to know what the oxide
coating is.
In fact, I would guess the the 3M DC600A and DC6150 tapes come off the
same manufacturing line
since they are both rated for the same FTPI (12500 flux transitions per
inch).  
The difference is the number of tracks written.  QIC120 is 15 tracks,
which can be written on
either the 600A or the 6150.  QIC 150 is 18 tracks and can only be
written on a 6150.
By the way, QIC 24 is 9 tracks at 10000 tpi, which can be read by this
drive but not written.
Also be aware that this drive is strange in that if you had written your
QIC24 tape on a 600A tape,
the drive will not read it, but if you had written your QIC24 tape on a
300XLP tape, then it will
read it.

This tape drive was manufactured for IBM by Tandberg.  It has its own
very peculiar personality
that make it most unlike other popular QIC120/150 drives (like Archive
and Cipher).  THe default
setting is to enable the ECC (error correcting code) when writing tapes.
 Tapes written with
ECC on will not interchange with any other QIC 120/150 drive, so IBM
recommends that you turn
off ECC if you wish to interchange.  In fact, IBM is recommending that
you turn off ECC for good.
A future software update will change the default setting to OFF.  This
feature has not
worked well.  ALso please be aware that this setting only applies to
writing tapes.  If you
do have tapes that were written with ECC on, then the drive should read
them, regardless of the
setting.
 
Trivia exercise for the reader: Report to the class what the sensing
hole patterns are for
300XLP, 600A and 6150 tapes.  

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