PDP-12 Restoration at the RICM
paulkoning at comcast.net
Tue Oct 13 09:54:46 CDT 2015
> On Oct 12, 2015, at 9:16 PM, Rich Alderson <RichA at LivingComputerMuseum.org> wrote:
> The M tracks are longitudinally encoded (6-bit values chosen such that they
> read the same as NRZ backwards and forwards for DECtape, 4-bit values for
> LINCtape) to predefine blocks (cf. disk sectors) for data.
More precisely: it's Manchester encoding, not NRZ. The result is that mark track codes are complemented and reversed end for end if you read them in the opposite order.
The code choices are such that this process (obverse complement) produces another code word with the right meaning for this spot of the tape in that direction. So "in the data field of the block" reads the same in both directions. But "block start" in one direction reads as "block end" in the other, which is just the result you want.
The DECtape patent (3,387,293 -- on bitsavers among other places) describes this very nicely.
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