Random Number Generator
There are many modern computerized slot machines. The odds are programmed. In modern slot machines, the reels and lever exist only for historical and entertainment reasons. The positions of the reels depend on a Random Number Generator. It is included into the machine's software.
The RNG generates random numbers at an extremely high rate. When you pres the Play button the result is determined by the most recent random number. The result is different depending on exactly when the game is played. The result is different every fraction of a second.
The RNG was noticed to generate not exactly random numbers. Indeed, most RNGs will eventually repeat their number sequence. The cause of it is in poor programming. It is relatively easy to build pseudo RNGs with very long periods. There is no computer that is able to complete a single period in the expected lifetime of the universe. The access to the pseudo RNG code and seed values was known to Ronald Dale Harris, a former slot machine programmer. Specific gambling games equations were discovered by him. They allowed to predict that the next set of selected numbers would be based on the previous games played. But it is impossible for most machines, because numbers are picked by the RNG even when no one plays. So the player cannot see the secquence.

Percentage Of The Payout
The percentage of winnings paid out of the money that is wagered by players is 82–98 per cent. The term theoretical payout percentage is used to denote it. Jurisdictions influence the minimum theoretical payout percentage. It is typically established by law or regulation. A certain winning pattern exists nearly in every casino. The amounts they pay and the frequencies of these pay-outs are reflected in it. The main criterion for selecting the winning patterns on slot machines is that they should yield a certain fraction of the money played to the house. The rest of the money is given back to the players.
A slot machine's theoretical payout percentage is set at the factory when the software is written. Changing the payout percentage after a slot machine has been placed on the gaming floor requires a physical swap of the software. It is done infrequently because this is a time-consuming process. One can find a tamper-evident seal on the EPROM in certain jurisdictions. Gaming Control Board officials should be called for changing it. Other jurisdictions constantly ensure that slot machines contain only approved software.
The Nevada Gaming Commission is working now with Las Vegas casinos on technology that would allow the casino's slot manager to change the game, the odds, and the payouts remotely. Only after the selected machine has been idle for at least four minutes can the change be done. After the change is made, the machine must be locked to new players for four minutes and display an on-screen message informing potential players that a change is being made.

Machines That Are Linked
Offering of a particularly large prize, or jackpot becomes possible when a group of machines is linked in a special way. If a player gets a specific combination of symbols the progressive jackpot from this group of machines is given to him. The amount of the progressive jackpot is far higher than any single slot machine's.
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.

Near-miss Programming
The reel display of modern slot machines is controlled by computer software. That is why the slot machine can be programmed to display combinations that are close to winning combinations.
Such showing combinations that are similar to winning combinations more frequently than would occur randomly is called near-miss programming.
This term is also used for a related phenomenon. The chance of a winning combination appearing on a pay line is controlled by the winning percentages programmed into the slot machine. The combinations above and below the payline are all roughly equally randomly distributed. There are roughly equally randomly distributed combinations above and below the pay line. Only if abbreviated physical reels are used to display a win pattern based upon the RNG this can occur. In video slot machines the symbols that appear around the winning line are usually an accurate depiction of how the reels were mathematically modeled.
The issue of a near-miss above or below the pay line was also the subject of the Nevada Gaming Commission investigation. It was considered to be legal as long as the near-miss above or below the pay line was not specially programmed. In other words, the near-miss must be just as likely to occur as any other combination. The machine can't show winning combinations more frequently than other combinations above or below the pay line.
Australia also forbids to use near-miss programming, where a near miss is inaccurately displayed. Stop motion cameras are used by regulators for manufacturer's practices audit.

Slot Machines Fraud
Sometimes mechanical slot machines and their coin acceptors seem to belong to cheating devices and other scams.
EPROM computer chips controls modern slot machines and coin acceptors have become obsolete in favor of bill acceptors. They are designed with advanced anti-cheating and anti-counterfeiting measures and are difficult to defraud. Nowadays microwaves are used to defraud slot machines.