Re: Mysterious pins on R8 lensmount

[pgs@thillana.lcs.mit.edu (Patrick Sobalvarro)]

   From: James J Dempsey <jjd@k12-nis-2.bbn.com>
   Date: Fri, 06 Dec 1996 18:31:47 -0500

   ...
   The latest Leica Views which I received only a few days ago says that
   one of the functions of the contacts is to communicate information
   from a ROM in the lens to the processor in the camera.  The only
   example of information to be communicated that it mentioned, I think,
   was focal length.  It suggested that the camera could direct a smart
   flash to change it's angle of dispersal since the camera would know the
   focal length of the lens from the ROM.

   It also mentioned that older Leica R lenses could be retrofitted with
   the ROM for a small charge.

   Seems like a weak explanation, though.  They have a bunch of contacts
   there.  Leica M seems to communicate the focal length of the lens to
   the camera with no contacts.  There must be a better explanation.

My guess is that there are so many contacts there because the "ROM" in
question is quite a simple one.  I'd bet that the modified lenses
don't get any semiconductors put in them.  In the cutaway drawing in
the brochure I have, there are 8 pins.  Maybe I'm belaboring the
obvious here, but it you use the mount for ground, you get 256 values
just by selecting pins to wire to ground.  That would make the
conversion of old lenses pretty cheap, so long as all you want to do
is communicate a static value like focal length, because it wouldn't
be hard to install a little connector that has some pins wired to
ground -- probably as easy as adding a third cam to a 2-cam lens.

Because there are at most 32 or so focal lengths to distinguish,
probably they have an encoding scheme in which 3 or so pins are used
to communicate a value that tells what the other five pins mean.  For
old lenses, it says that all there is here is a focal length value.
In future lenses, they could use serial encodings or what-have-you
over those five pins, and communicate anything they wanted.