7-track tape question

[Paul Koning]

>>>>> "Chris" == Chris Muller <chris.muller at mullermedia.com> writes:

 Chris> Hi everybody.  This is something a computer geezer like me
 Chris> should know. But I don't.

 Chris> I've read some things that seem to imply that *some* of the
 Chris> oldest drive/formatters did NOT write a longitudinal parity
 Chris> frame at the end of a block. (They did have horizontal parity,
 Chris> of course.)

 Chris> If it's true then a drive reading such tapes would not have an
 Chris> easy way of telling whether frames had been dropped during a
 Chris> read. Does this ring true to anybody out there?  Any
 Chris> observations welcome.

It sounded odd to me, so I did some reading.

The IBM 650 (1955) says that it has even longitudinal parity.  

On the other hand, the Univac Model 1 (1950 or thereabouts) says that
it has even horizontal parity but not longitudinal parity.  But that's
really rather prehistoric and doesn't use regular 1/2 inch mylar tape.

Given that even the 650 has it (as do several CDC machines I checked)
I'm somewhat doubtful about the rumor.

     paul