Library: | Wiring |
Introduced: | 2.7.0 |
Appearance: | ![]() |
A transmission gate has three inputs, called source, n-gate, and p-gate; and it has one output, called drain. When diagrammed, the source input and drain output are drawn connected by two plates; Logisim draws an arrowhead to indicate the direction of flow from input to output. The two gate inputs are drawn as lines connected to plates parallel to each of the plates connecting source to drain. The p-gate input's line has a circle, while the n-gate input's line does not.
p-gate | ||
source | ![]() |
drain |
n-gate |
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.
The values at n-gate and p-gate are expected to be opposite to each other. If p-gate is 0 while n-gate is 1, then the value found at source is transmitted to drain. If p-gate is 1 while p-gate is 0, then the connection is broken, so the value at drain is left floating. In all other cases, drain receives an error output — unless source is floating, in which case drain is floating as well. This behavior is summarized by the following table.
p-gate | n-gate | drain |
---|---|---|
0 | 0 | X* |
0 | 1 | source |
1 | 0 | Z |
1 | 1 | X* |
X/Z | any | X* |
any | X/Z | X* |
* If source is Z, drain is Z; otherwise drain is X.
If the Data Bits attribute is more than 1, each gate input is still a single bit, but the gate values are applied simultaneously to each of the source input's bits.
When the component is selected or being added,
Alt-0 through Alt-9 alter its Data Bits
attribute
and the arrow keys alter its Facing
attribute.
None.
None.