In this lecture, we present a circuit component called a tri-state buffer. A tri-state buffer is an integral circuit component of random access memory, as well as for communication between components such as registers. The tri-state buffer is drawn with one data input bit, one control input bit, one output bit, and is described by this truth table. The capital Z here symbolizes high impedance or resistance, so that very little current will pass through the buffer. The control bit of the tri-state buffer x as a valve that shuts current on and off from a circuit. When the control bit is 1, the input voltage of the buffer is sent to the output, and current flows through the buffer. But when the control bit is 0, the buffer will become a high impedance circuit, and negligible current will flow through the circuit. At this point, I want to address a common misconception that 0 volts of output from a gate means that nothing is output from the gate. Current will always flow through a circuit regardless of the voltage, but if the impedance is great enough, then current will stop. So remember, the current is flowing through a circuit when the output is 0 or 1, but no current is flowing through a circuit when the tri-state buffer is a high impedance circuit. We can use tri-state buffers, much like how we use multiplexers. In a multiplexer, we use a select bit to choose which data inputs value is sent to the output. And now, obviously, we can use the tri-state buffer's control bit to select which data inputs value is allowed to pass to the output. However, if you make a mistake in the design of a tri-state buffer selection circuit, the results are much more catastrophic than a mistake in the design of a multiplexer. If two tri-state buffers are enabled at the same time, we could damage the circuit. The illustrate why this happens, we need to remove the digital abstraction and look at the voltages and currents. If two components that I'll put to the same wire transmit different voltages, the current in the system will be governed by the resistance of the wire and the difference of voltage. Because the resistance of a wire is essentially 0, the components will try to draw infinite current. This type of current draw can damage our circuits. To summarize, a tri-state buffer is essentially a current valve that can shut off the output from specific components. The tri-state buffer allows us to select one output for many to send to a wire or bus. It must make absolutely certain that two components do not transmit to the same wire or bus.