Coax Continuity Tester: The object of this project was to play around with
logic circuits (TTL) and computer software for logic circuit design and
analysis. It is trivial to test the continuity of an ordinary length of
coax using an ohmmeter. Touching the probes to the center conductor at
both ends should show near zero resistance, and the same for the
shield. Probing between the center conductor and shield
indicate an open circuit (infinite resistance).
The tester pictured above has three light emitting diodes (some
additional wiring is not visible under the PCB). When a cable
connected between the two SO-239 jacks (or the RCA phono jacks),
pressing the left pushbutton (left end of circuit board) tests the
conductor. Pressing the other button tests the shield. The
green LED indicates that the tested conductor is good. The red ‘Open’
LED indicates a break in continuity, while the other red ‘Short’ LED
indicates a short circuit. Since the unit is battery powered I thought
it would be prudent to include a power indicator as well (lower left
I first drew the circuit using the
program Logisim. After making a couple of
simplifying revisions I constructed the project on a breadboard and it
appeared to work as intended. At this point I became curious
whether I could write out a logical expression for the ‘Good’
indicator, based on the four inputs (center left, center right, shield
left, and shield right). Using pencil and paper to construct an
equation I made mistakes, inevitably ending up with mismatched
parentheses or a missing or superfluous ‘not’ symbol. So
instead I wrote a simple computer procedure that accepts a list of
equations as input (each equation corresponding to a single logic gate)
and additionally a target expression (something to solve for).
By iteratively substituting symbols the procedure converts
the target to a form that references only input variables (i.e.,
when no further simplification is possible).
Using this ‘helper’ utility I was able
to obtain the desired expression
for the ‘Good’ indicator.
Next, a Google search for ‘logic simplifier software’ yielded several
relevant resources. The one that I ended up using is called Logic
Friday (another wonderful free program). My goal
was to create three expressions, one for each indicator LED, then to
simplify each of them, and in the end produce a simplified combined
circuit. However, when I entered the ‘Good’
expression into Logic Friday the program’s
truth table included two unexpected rows. After wasting a lot of time
proofreading the circuit and double-checking its breadboard
implementation a question popped to mind: Had I really tested the exact
combination of inputs that unexpectedly evaluate as good? The
truth table was saying that if one of the coax conductors was open and
the other not, AND if both buttons were pressed simultaneously the ‘Good’
indicator would illuminate. Upon testing these exact conditions that
what happened. This observation led to a
minor change in the logic, after which the truth table exhibited by
Logic Friday consisted of only two rows, corresponding to a good center
conductor and good shield.
I am omitting details from this
account but supplementary information may
be found here.
logic diagram shown below is the circuit that was implemented (photo at
the top of this description). It is revision 4 of the original
design. (Note that the two fault LEDs are themselves inverted).
Logic Friday includes a ‘Minimize’
option that accepts a messy equation as input, and produces a minimized
version of the same logic. It is then possible to
translate the minimized expression back into a circuit. Combining separate minimized circuits without
further simplification leads to the following diagram.
Since the diagram above represents a
combination of three independently derived circuits, it includes some redundant
elements (duplicate inverters). In general, minimizing logic in a small-scale circuit
such as this one is not necessarily the same as minimizing the circuit,
due in part to the way that gates are packaged in ICs. Other
considerations include the ready availablity of ICs or their cost, if
they must be purchased.
Coax continuity tester: TTL/coax_tester_demo.mp4
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