user13592: I'm playing with a 7 segment display.
The following works on my breadboard:
5V -> 330 Ohm Resistor (same row, row 9)
330 Ohm Resistor -> Com Port (row 9 to row 15)
h led (the dot/row 17) -> ground rail

I get the dot to light up.
However, the following does not light up:
5V -> h led (the dot/row 17)
Com Port -> 330 Ohm Resistor (Row 15 -> row 38). 
330 Ohm Resistor to negative rail (row 38 -> negative rail)

I realize that the row numbers are meaningless -- things just need to be on the same row. I've included them to make it easier to visualize.
So basically, if the circuit goes:
5V -> Resistor -> Com, then I can use the ground wire to light up any part I want.
If the circuit goes Com -> Resistor to Ground, I can't use the 5V to light up any part. 
I'm not sure why given that all the stuff is in serial?

user368: Because you have a common-anode display and you're trying to use it as a common-cathode display. LEDs, like all diodes, only conduct freely in one direction.

13592: <p>I'm playing with a 7 segment display.</p>

<p>The following works on my breadboard:</p>

<pre><code>5V -&gt; 330 Ohm Resistor (same row, row 9)
330 Ohm Resistor -&gt; Com Port (row 9 to row 15)
h led (the dot/row 17) -&gt; ground rail
</code></pre>

<p>I get the dot to light up.</p>

<p>However, the following does not light up:</p>

<pre><code>5V -&gt; h led (the dot/row 17)
Com Port -&gt; 330 Ohm Resistor (Row 15 -&gt; row 38). 
330 Ohm Resistor to negative rail (row 38 -&gt; negative rail)
</code></pre>

<p>I realize that the row numbers are meaningless -- things just need to be on the same row. I've included them to make it easier to visualize.</p>

<p>So basically, if the circuit goes:</p>

<p>5V -> Resistor -> Com, then I can use the ground wire to light up any part I want.</p>

<p>If the circuit goes Com -> Resistor to Ground, I can't use the 5V to light up any part. </p>

<p>I'm not sure why given that all the stuff is in serial?</p>
 368: <p>Because you have a common-anode display and you're trying to use it as a common-cathode display. LEDs, like all diodes, only conduct freely in one direction.</p>
