Slot machine technology
Computerization
With microprocessors now ubiquitous, the computers inside modern slot machines allow manufacturers to assign a different probability to every symbol on every reel. To the player it might appear that a winning symbol was 'so close', whereas in fact the probability is much lower.
In modern slot machines, the reels and lever are for historical and entertainment reasons only.
Random number generators
All modern machines are designed using pseudorandom number generators ("PRNGs"), which are constantly generating random numbers, at a rate of hundreds or perhaps thousands per second. As soon as the "Play" button is pressed, the most recent random number is used to determine the result. This means that the result varies depending on exactly when the game is played. A fraction of a second earlier or later, and the result would be different.
It is important that the machine contains a high-quality RNG implementation, because all PRNGs must eventually repeat their number sequence (see Art of Computer Programming Volume 2 Chapter 3, Random numbers), and if the period is short, or the PRNG is otherwise flawed, an advantage player may be able to 'predict' the next result. Having access to the PRNG code and seed values Ronald Dale Harris, a former slot machine programmer, discovered equations for specific gambling games like Keno that allowed him to predict what the next set of selected numbers would be based on the previous games played.
Most machines are designed to defeat this by generating numbers even when the machine is not being played, so the player cannot tell where in the sequence they are, even if he knows how the machine was programmed.
Linked machines
Often machines are linked together in a way that allows a group of machines to offer a particularly large prize, or "jackpot". Each slot machine in the group contributes a small amount to this progressive jackpot, awarded to a player who gets, for example, a royal flush on a video poker machine or a specific combination of symbols on a regular or nine-line slot machine. The amount paid for the progressive jackpot is usually far higher than any single slot machine could pay on its own.
In some cases multiple machines are linked across multiple casinos. In these cases, the machines may be owned by the manufacturer, who is responsible for paying the jackpot. The casinos lease the machines rather than owning them outright. Megabucks may be the best known example of this type of machine. Megabucks Nevada starts at $10,000,000 after a jackpot. (Prior to September 2005, Megabucks Nevada reset to $7,000,000.) The new penny Megabucks video game also has a jackpot that starts at $10,000,000.
Fraud
Mechanical slot machines and their coin acceptors were sometimes susceptible to cheating devices and other scams. One historical example involved spinning a coin with a short length of plastic wire. The weight and size of the coin would be accepted by the machine and credits would be granted. However, the spin created by the plastic wire would cause the coin to exit through the reject chute into the payout tray. This particular scam has become obsolete due to improvements in newer slot machines.
Modern slot machines are controlled by EPROM computer chips and, in large casinos, coin acceptors have become obsolete in favor of bill acceptors. These machines and their bill acceptors are designed with advanced anti-cheating and anti-counterfeiting measures and are difficult to defraud. Early computerized slot machines were sometimes defrauded through the use of cheating devices, such as the "slider" or "monkey paw" used by notorious slot machine cheat Tommy Glenn Carmichael. However, more recent attempts at defrauding slot machines involve manipulating the EPROM, such as by directing microwaves toward it to disrupt its proper functioning. Casino insiders such as Ronald Dale Harris have also been discovered manipulating the software in slot machines in order to defraud casino operators.
