>>>>> "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
Received on Wed Sep 29 2004 - 12:51:58 BST
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