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<title>Transmission Gate</title>
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<h1><img  align="center" src="../../../../en/icons/transmis.gif" width="32" height="32">
<em>Transmission Gate</em></h1>

<p><table>
<tr><td><strong>Library:</strong></td>
	<td><a href="index.html">Wiring</a></td></tr>
<tr><td><strong>Introduced:</strong></td>
	<td>2.7.0</td></tr>
<tr><td valign="top"><strong>Appearance:</strong></td>
	<td valign="top"><img src="../../../../en/img-libs/transmis.png" width="44" height="43"></td></tr>
</table></p>

<h2>Behavior</h2>

<p>A transmission gate has three inputs,
called <em>source</em>, <em>n-gate</em>, and <em>p-gate</em>;
and it has one output, called <em>drain</em>. When diagrammed, the <em>source</em>
input and <em>drain</em> output are drawn connected by two plates;
Logisim draws an arrowhead to indicate the direction of flow from input to output.
The two <em>gate</em> inputs are drawn as lines connected to plates parallel to
each of the plates connecting <em>source</em> to <em>drain</em>.
The <em>p-gate</em> input's line has a circle, while the <em>n-gate</em> input's line does not.</p>

<center><table>
<tr><td></td><td align="center"><em>p-gate</em></td><td></td></tr>
<tr><td valign="middle"><em>source</em></td>
  <td align="center" valign="middle"><img src="../../../../en/img-libs/transmis.png" width="44" height="43"></td>
  <td valign="middle"><em>drain</em></td>
<tr><td></td><td align="center"><em>n-gate</em></td><td></td></tr>
</table></center>

<p>The transmission gate is simply the combination of two complementary
transistors. Indeed, the same behavior can be achieved in Logisim by using just
one transistor. However, designers sometimes prefer to use matched pairs of
transistors due to electrical issues with draining voltage
that is more complex than Logisim attempts to simulate.</p>

<p>The values at <em>n-gate</em> and <em>p-gate</em> are expected to be
opposite to each other. If <em>p-gate</em> is 0 while <em>n-gate</em> is 1,
then the value found at <em>source</em> is transmitted to <em>drain</em>.
If <em>p-gate</em> is 1 while <em>p-gate</em> is 0, then the connection is broken,
so the value at <em>drain</em> is left floating. In all other cases, <em>drain</em>
receives an error output &mdash; unless <em>source</em> is floating,
in which case <em>drain</em> is floating as well. This behavior is summarized by the following table.</p>

<center><table>
<tr><th><em>p-gate</em></th><th><em>n-gate</em></th><th><em>drain</em></th></tr>
<tr><td align="center">0</td><td align="center">0</td><td align="center">X*</td></tr>
<tr><td align="center">0</td><td align="center">1</td><td align="center"><em>source</em></td></tr>
<tr><td align="center">1</td><td align="center">0</td><td align="center">Z</td></tr>
<tr><td align="center">1</td><td align="center">1</td><td align="center">X*</td></tr>
<tr><td align="center">X/Z</td><td align="center"><em>any</em></td><td align="center">X*</td></tr>
<tr><td align="center"><em>any</em></td><td align="center">X/Z</td><td align="center">X*</td></tr>
</tbody></table>
<p>* <em>If <em>source</em> is Z, <em>drain</em> is Z; otherwise <em>drain</em> is X.</p>
</center>

<p>If the Data Bits attribute is more than 1, each <em>gate</em> input is still
a single bit, but the <em>gate</em> values are applied simultaneously to each of the
<em>source</em> input's bits.</p>

<h2>Pins (assuming component faces east, gate line top/left)</h2>

<dl>

<dt>West edge (input, bit width matches Data Bits attribute)</dt>
<dd>The component's <em>source</em> input that will transmit to the output
if triggered by the <em>p-gate</em> and <em>n-gate</em> inputs.</dd>

<dt>North edge (input, bit width 1)</dt>
<dd>The component's <em>p-gate</em> input.</dd>

<dt>South edge (input, bit width 1)</dt>
<dd>The component's <em>n-gate</em> input.</dd>

<dt>East edge (output, bit width matches Data Bits attribute)</dd>
<dd>The component's output, which will match the <em>source</em> input
if <em>p-gate</em> is 0 and <em>n-gate</em> is 1, or it will be floating if
<em>p-gate</em> is 1 and <em>n-gate</em> is 0. For all other values on <em>p-gate</em> and <em>n-gate</em>,
the output is an error value.</dd>

</dl>

<h2>Attributes</h2>

<p>When the component is selected or being added,
Alt-0 through Alt-9 alter its <q>Data Bits</q> attribute
and the arrow keys alter its <q>Facing</q> attribute.</p>

<dl>

<dt>Facing</dt>
<dd>The direction of the component (its output relative to its input).</dd>

<dt>Gate Location</dt>
<dd>The location of the gate input.</dd>

<dt>Data Bits</dt>
<dd>The bit width of the component's inputs and outputs.</dd>

</dl>

<h2>Poke Tool Behavior</h2>

<p>None.</p>

<h2>Text Tool Behavior</h2>

<p>None.</p>

<p><a href="../index.html">Back to <em>Library Reference</em></a></p>

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