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14509RE: [cosmacelf] Olduino/Membership Card Shield Adapter

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  • bill rowe
    Nov 3, 2013
      Thanks for the encouraging words Lee.

      I have many spare boards and you`re more than welcome to one.

      The challenge of even dry-stacking the 3 boards is making me realize that the shield adapter is probably impractical.  If I can make even one set work though it will be worth it as a proof of concept.  There is not that much to the circuitry but it was just beyond my breadboarding skills.  If I can get the SPI setup to work or learn enough I can rework the olduino board to include it or it could go into a variant of the MC which would simplify the whole thing.  That's what the Way Too Many Wires Olduino II was but again, beyond my ability to breadboard.

      The 7411AC238 is functionally the same as a 74238 which was what I thought I had!  It has active-high outputs and three enables which I tie active.  I have the 238 chip and it will go neatly in a spare footprint.  I had originally looked at the 4028 but didn't use it because it's quite slow - propagation delay up to 480 ns.  The 238 is much much faster.

      I hope to get this thing at least partly assembled today and judge whether it's got any sort of shot at being useful.

      In the pictures you can see the whole unholy bodge with an ethernet shield on top.  You can also see where I've started to tie the 74ac11238 footprint to the spare I'm using for the 74238.  The membership card is crouched there at the bottom of the stack gritting its teeth.

      To: cosmacelf@yahoogroups.com
      From: leeahart@...
      Date: Sat, 2 Nov 2013 10:29:51 -0600
      Subject: Re: [cosmacelf] Olduino/Membership Card Shield Adapter

      bill rowe wrote:
      > I got back my PC boards today for the shield adapter. This is to let me
      > try attaching arduino shields to the olduino. I expect to have 100kb/sec
      > SPI to let me run things like the Ethernet shield with better
      > throughput. It's interesting to note that the adapter sits on top of the
      > olduino which is on top of the MC but it only uses MC signals. The
      > olduino is handy for getting code into the MC but it won't be driving
      > the shields.
      > In the picture below the long horizontal row of headers near the top
      > connects to the membership card. The shorter row above brings up the N
      > lines from the MC. The two blocks of 6 female headers and the two blocks
      > of 8 headers will attach the shield. The 74595 and 4021 are input and
      > output shift registers for SPI and the 74373 is a parallel output port.
      > The 74238 is decoding the N lines and the other two chips are glue. The
      > chips will be in sockets but I'm just loose fitting them to make sure
      > I've got what I need.

      Bill, this really looks great! This board, with your work on the c
      compiler, should allow access to a lot of the Arduino shield boards!

      Are you planning to sell boards? If so, I'll buy one! :-)

      > when I was laying out the PCB for the olduino shield adapter I
      > grabbed the symbol for the 74AC11238 from the eagle library not
      > realizing it was different from the 74HC238.

      The 74AC11238 is a rare part. A quick search doesn't show any available
      from anyone. Could you re-lay out the board for a different part that is
      easier to get? It is functionally the same as the much more common '138,
      except that the outputs are inverted. Can you redesign the board to use
      the inverted outputs of the '138?

      Or, the CMOS 4028 is very similar to the '238 and has active-high
      outputs. The 4028 has just one active-low enable (the D input), while
      the '138 and '238 have three enable inputs (two active low and one
      active high).
      If you're not stubborn, you'll give up on experiments too soon.
      And if you're not flexible, you'll pound your head against the wall
      and not see a different solution to a problem you're trying to solve.
      -- Jeff Bezos (founder of Amazon.com)
      Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm

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