PDP-12 Restoration at the RICM

Jay Jaeger cube1 at charter.net
Tue Aug 18 08:08:42 CDT 2015

On 8/17/2015 1:51 PM, Rich Alderson wrote:

> From: Jay Jaeger
> Sent: Sunday, August 16, 2015 3:18 PM
> On 8/16/2015 9:00 AM, Michael Thompson wrote:
>>> We did a lot more debugging on the TC12 LINCtape controller.
>> Second gut hunch is that it would be hard to see how the drive could
>> cause this UNLESS the TC12 uses one side of the redundant tape channels
>> in one direction, and the other side in the other direction.
> Not possible.  The *drive* wire-ORs the redundant channels, before the
> data goes into (or after it comes out of) the controller.  *That's* the
> redundancy.  When reading in the reverse direction, only the mark tracks
> on a DECtape read the same way backwards as forwards; LINCtape uses a
> different set of markers.

Well, not exactly a wired or, per my TU56 manual, but yeah point taken.
 At least on the TU56 the head windings are connected in series,
resulting in the *analog* sum.  (A wired or didn't quite make sense to
me - it wouldn't really be redundant, so I looked it up).

But it brings me back to my original point - I don't see how this could
be the drive unless the block number in question always involves a
particular channel on the tape.


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