Alt-BEAM Archive

Message #12491



To: beam@corp.sgi.com
From: Justin JAF60@student.canterbury.ac.nz
Date: Sat, 25 Mar 2000 19:54:43 +1200
Subject: [alt-beam] Re: Logic puzzle - cool! Try this tougher one...


>The truth table in your latest circuit is different than the
>truth table in the preceding message. In the circuit diagram
>truth table the second output has three 'high' states. Which
>is correct?

Ok, this could get confusing... Two of those HIGH's are in the
irrelevant section of the table, which has been inverted (I was
breadboarding the outputs to HC240 enables, which invert the output),
and the outputs in state two are swapped from the original truth table,
as this doesn't affect the practical problem (which I have only touched
on so I'm probably the only one who knew that :)

>I'm having trouble with the circuit, too. Where you show the
>lower transistor connecting to the lower switch; is that also
>connected to the lower output? For the base of the upper
>transistor you've drawn a curved jumper section to show that
>it does not connect to the upper switch. I'm told that curved
>symbol has fallen out of favour! Current industry preference
>is that crossing wires do not touch, and connecting wires are
>shown by a three-way "T" intersection.

The crossing wires do connect. I'll bet that extra resistor (220K) threw
you! :-) I looked at the circuit again this morning, and thought "what
the hell is that resister doing?!?". It's residue from an earlier
incarnation of the circuit before I took a diode out and shifted a
connection. So the circuit works with 7 components. Wilf's XOR uses
five, and from memory, all it takes to make a XOR into the puzzle gate
is wire, so that would be a 5 component solution.... :-)

Speaking of the evolving language of schematics (and thanks for that
info by the way - I could do with being brought up to date :), I was
wondering about customs regarding the dircetion in which you draw
inverters (and other gates) - I've seen diagrams sacrifice some
legibility in order to have all the gates facing the same direction.
Is this just a cosmetic quirk of the drawer, or is it customary, or is
there a method behind it (eg gates in different trains or chips face
different directions or something)?



12492 Sat, 25 Mar 2000 00:30:39 -0800 [alt-beam] Newbie-Reverser circuit drawn wrong?? beam@corp.sgi.com Ken Hill Regarding the 240 reverser circuit from the beam online site, If appears
that from the data sheet of the chip that the number 2 inputs and outputs
should actually be reversed on the drawing? Those two bottom buffers run
the other direction across the chip don't they? Or does it matter? Or
should they just be moved up one pin on the chip?

I assume that the IN's are enable + and - from the core or whatever brain
circuit, and the OUT's go to + and - motor bridge IN's. When it reverses,
these switch polarities at the reversers output? Using 2 opposite facing
LEDs across each of the outputs, I can't get it to work that way on the
breadboard. (either as drawn, or reversed). I can verify that the buffers
are turning on with a low to the enables, but no reverse. (And they said
this was a "very easy" circuit..... :)

Also, I found the circuit for the 245 H bridge and the 139 reverser / H
bridge circuit. If I use the 139 I can eliminate both the 240 reverser and
the 245 H bridge, correct? (I understand the current limits and the need to
stack them).

Thanks, Ken


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