It's useful to think of the wire like a pipe which you can send soda. Let's pretend a device can send two kinds of soda. If a device pumps red soda, then the wire is transmitting a 0. If a device pumps green soda, the wire is transmitting a 1.

A device can also pump no soda at all. In this case, the wire is at high impedance, which means it has neither value 0 or 1 (or perhaps more precisely, it has a random value of 0 or 1, which changes depending on when it is read).

When a device is pumping soda into the pipe, it can only pump red or green soda. No other device is allowed to pump soda onto the same wire. If some other device attempts to pump soda, then the wire will contain a garbage value. We assume there is a garbage value even if two devices are pumping soda of the same color.

The device that is pumping the soda is said to write a a value to the wire. We want to guarantee that there is, at most, single writer (there may be none).

Devices may "read" the wire as well. The device can "sample" the soda, and determine if it's red or green. If the device attempts to read the value of a pipe when it is empty, or if the device attempts to read the value when two or more devices attempt to pump soda, then we assume the value read is random. That is, it can either be a 0 or a 1, but we don't know which, and this value can change.

In order for us to make a stable system, we want devices reading when a pipe contains soda pumped by a single device.

More than one device can read from the wire, but at most one device can write to the wire.

When you learn to program, you often think of values in discrete units. For example, suppose you want to run the statement: z = foo( x + y ). You think of x + y being computed, then this value sent to foo, then foo computing a return value, and this return value being stored in z.

Each event occurs in a discrete step.

However, it's better to think of a wire like water being sent to your home, or like electricity flowing down the wires. It's constantly flowing. This creates a more accurate image of what's happening in a circuit.

In reality, electrons are floating at some potential of either 0 or 5 volts (though these days, it's sometimes 3.3 volts) where 0 volts represents the bit 0, and 5 volts represents the value 1. If no voltage is asserted on the wire, the the voltage is ambiguous and essentially "floats".

These electrons are flowing through the wire, and devices can measure the potential of the wire to determine if there is a 0 or 1 on the wire.

We want to avoid two devices trying to assert (i.e. "write") voltages on the wires.

The following chart describes the behavior.

The output is one of four values: 0, 1, Z, and ?. 0 and 1 should be obvious.

? occurs when two devices attempt to write to the wire at the same time. When a device reads from the bus it reads a value that's either 0 or 1, so it's unknown. We want to avoid having two devices write at the same time.

Z means that no device is writing to the wire. Reading a value from the wire also results in a value that's 0 or 1, but it's not known which. We want to avoid having a device read the wire when no device is writing to a wire.