The Modern Slot Machine

Most modern slot machines are designed to look and feel like the old mechanical models, but they work on a complete different principle. The outcome of each pull is actually controlled by a central computer inside the machine, not by the motion of the reels.
The computer uses step motors to turn each reel and stop it at the predetermined point. Step motors are driven by short digital pulses of electricity controlled by the computer, rather than the fluctuating electrical current that drives an ordinary electric motor. These pulses move the motor a set increment, or step, with great precision (see this page to find out more about step motors).

But even though the computer tells the reels where to stop, the games are not pre-programmed to pay out at a certain time. A random number generator at the heart of the computer ensures that each pull has an equal shot at hitting the jackpot.

Whenever the slot machine is turned on, the random number generator is spitting out whole numbers (typically between 1 and several billion) hundreds of times a second. The instant you pull the arm back (or press the button), the computer records the next few numbers from the random number generator. Then it feeds these numbers through a simple program to determine where the reels should stop.

Here's how the complete process plays out in a typical three-reel machine.

You pull the handle, and the computer records the next three numbers from the random number generator. The first number is used to determine the position of the first reel, the second number is used for the second reel and the third number is used for the third reel. For this example, let's say the first number is 123,456,789.

To determine the position of the first reel, the computer divides the first random number by a set value. Typically, slot machines divide by 32, 64,128, 256 or 512. In this example, we'll say the computer divides by 64.

When the computer divides the random number by the set value, it records the remainder of the quotient. In our example, it finds that 64 goes into 123,456,789 a total of 1,929,012 times with a remainder of 21.

Obviously, the remainder can't be more than 64 or less than 0, so there are only 64 possible end results of this calculation. The 64 possible values act as stops on a large virtual reel.

Each of the 64 stops on the virtual reel corresponds to one of the 22 stops on the actual reel. The computer consults a table that tells it how far to move the actual reel for a particular value on the virtual reel. Since there are far more virtual stops than actual stops, some of the actual stops will be linked to more than one virtual stop.

Computer systems have made slot machines a lot more adaptable. For example, players can now bet money straight from a credit account, rather than dropping coins in for every pull. Players can also keep track of their wins and losses more easily, as can the casinos. The operation is also simpler in modern machines -- if they want to, players can simply press a button to play a game, rather than pull the handle.

For the manufacturers and slot proprietors, one of the main advantages of the computer system is that they can easily configure how often the machine pays out (how loose or tight it is).