PDP-12 Restoration at the RICM

william degnan billdegnan at gmail.com
Tue Oct 13 10:16:54 CDT 2015

Did you ever speak with anyone at System Source (Bob) about their PDP 12?
Maybe they'd be interested in collaborating.  http://museum.syssrc.com/

On Tue, Oct 13, 2015 at 10:54 AM, Paul Koning <paulkoning at comcast.net>

> > 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.
>         paul


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