ttl circuitry uses 5V signals. 5V is a high state. 0V is a low state. Of course no Voltage is exact. Consider 4.9999999999V that's not really 5V. So every particular ttl device has defined voltage ranges which it will react to as a high or low state. A connection to ground provides 0V and is the typical way to signal a low state. No connection theoretically provides 0V and typically registers as a low state. A non connection is called a float. It is not good design practice to use a float to signal a low state. If we want to default a signal to low we use a pull down circuit. A connection to ground through a high resistance. This provides the default low signal. Then to signal the switch to a high state a connection can be made to the +5V rail. Because the pull down circuit has a high resistance the +5V connection easily overrides it to provide the necessary V to signal the high state. Similiarly when we want the default signal state to be high we use a pull up circuit and to signal a low state we override it with a connection to ground.
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Originally Posted by dynomatt
Can you explain that a bit more for the nuffies like me?
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