how does a chip drive an output pin high or low? what internally causes this to occur?

Answer 1

Tom has it right. But I'll put it into my words. A chip has transistor's, it is a transistor that causes an output pin to be driven high or low. Actually, it's 2 transistors (one for high, one for low). They like to call this output configuration a Totem pole output because the transistors are electrically stacked on top of each other when you draw them on a schematic. The type of transistor depends on the chip, most chips today use FET (Field Effect Transistors), the chips of the past used BJT's (Bipolar Junction Transistors).

Mike is also correct but he's adding more data about how much current each transistor can pass. Sink current is the current when the output is low, Source current is when it's high. Sometimes they won't specify the output current but instead rate it as a "Fan-out" current. Meaning this output can drive say 15 inputs of other chips.

Answer 2

The others said it, but maybe I can clarify a little. This is a bit simpler than what you'd see at the actual output pin of an actual chip, but gives the general idea.

Transistors in computer chips act as on-off switches. In CMOS, output pins always come from a circuit that starts at the positive supply votlage, runs through two transistors in series, to the supply ground - the "zero" voltage in the circuit. The output pin connects to the point between the two transistors, half-way between supply and ground. The way the circuit is built, one transistor or the other is always on (passing current), but never both together.

A high output voltage happens when the transistor near the supply voltage is on and the transistor near the ground is off. There's a low-impedance path from the pin to the supply voltage, but a high impedance path to ground. Low output voltage happens when the transistor to supply voltage is off and the transistor to ground is on, so the easy current path goes from the pin to the lower voltage.

Answer 3

It depends on the chip and on the making method. Most chips will sink more than they source - that is you can drive more if you pull it low than if you set it high. Some require the output to be pulled high with a resistor and you pull it low, normally with a transistor. CMOS, as I understand it, switches current to the + or ground or to the + or - supply, but the amount it will source or sink is much lower than TTL.
http://www.fairchildsemi.com/an/AN/AN-77.pdf

Answer 4

There is a transistor in series with a resister across the high voltage rail and ground. The output pin is connected to the point between the transistor and the resister. Turning the transistor ON pulls the voltage down to near ground. Depending on the type of transistor and whether the resister is connected to ground or +5 volts, turning on the transistor pulls the output to whatever rail the transistor is connected to. When the transistor is off, the output is pulled to that side the resister is connected to.

Answer 5

The state of conduction, or non-conduction of the output gate. I believe that if you go back to your text books, the 'how" as well as the "why" of how a gate operates will be explained. If it isn't there, or you want more than that, then the manufacturer of s particular gate can probably get that information for you.