13 Oct
2015
13 Oct
'15
8:16 a.m.
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>
wrote:
--
Bill
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