Logic Analyser 1.3

by Vasantha Crabb <_merlin@operamail.com>
http://members.xoom.com/_merlin/

========================================================================
    INTERFACES
========================================================================
To protect both your calculator and the circuit under test from damage,
I recommend that you build some kind of interface circuit to connect
between the calculator's link port and the circuit under test.

The most basic interface circuit would simply be two power diodes, e.g.
1N4004, connected cathode to circuit, anode to link port to prevent the
input from being pulled above 5V.  However, this wouldn't keep the input
from being pulled below 0V.  To solve this problem, and to protect
against faulty diodes (don't laugh, I had a 1N4007 that broke over at
7V!), I recommend adding two zener diodes connected anode to ground and
cathodes to the link port inputs as shown in DIODES.GIF.  This circuit
will allow the calculator to be connected to 5V TTL circuits and 5-18V
CMOS.

For supply voltages lower than 5V, you'll need a slightly more complex
cicuit.  The one shown in MOSFET.GIF will work with TTL and CMOS with
supply voltages down to 3V.  Note that this circuit acts as an inverter.
Another approch would be to use a level shifter IC such as the 40109.
The input supply pin would be connected to the circuit under test's
supply rail and the output supply pin would be connected to a +5V rail.
This could be taken from your calculator's SPiNTERFACE port if you've
fitted one.
