Random Number Generator
There are many modern computerized slot machines. They programme the odds. The reels and lever are present for historical and entertainment reasons only. The positions of the reels depend on a Random Number Generator. The machine's software includes a Random Number Generator.
Random numbers are constantly generated by the RNG at an extremely high rate. As soon as the Play button is pressed, the most recent random number is used to determine the result. 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. Most RNGs will repeat their number sequence. Poor programming determines such behavior. It is relatively easy to build pseudo RNGs with very long periods. The computer that is able to complete a single period in the expected lifetime of the universe hasn't been invented yet. Ronald Dale Harris, a former slot machine programmer, was the only one who had access to the pseudo RNG code and seed values. He discovered equations for specific gambling games like Keno. The fact that the next set of selected numbers would be based on the previous games played was revealed. However even if nobody uses the machine the RNG continues to pick numbers. So the player cannot see the secquence.

Percentage Of The Payout
Slot machines are typically programmed to pay out as winnings 82–98 per cent of the money that is wagered by players. The term theoretical payout percentage is used to denote it. The minimum theoretical payout percentage varies among jurisdictions. Law or regulation typically establish it. A certain winning pattern is determined by every casino individually. This is the amounts they pay and the frequencies of these pay-outs. The winning patterns on slot machines are carefully selected to yield a certain fraction of the money played to the house. As for the rest of the money it goes to the players.
When a slot machine's software is written its theoretical payout percentage is also set. Changing the payout percentage requires a physical swap of the software. Being a time-consuming process it is done not very often. Certain jurisdictions presuppose the EPROM to have a tamper-evident seal. It can only be changed in the presence of Gaming Control Board officials. The audit of slot machines is made by some jurisdictions make sure that they contain only approved software.
Thanks to the newly developed technology the game, the odds, and the payouts would be able to be changed by the casino's slot manager remotely. The change cannot be done instantaneously, but only after the selected machine has been idle for at least four minutes. 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 Linking
Linking the machines in a special way is made 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 a specific combination of symbols. A single slot machine's jackpot is considerably less than the progressive jackpot.
Sometimes multiple machines form multiple casinos. In these cases, the machines may be owned by the manufacturer, who is responsible for paying the jackpot.

Near-miss
The reel display of modern slot machines is controlled by computer software. Combinations that are close to winning combinations can be displayed on the slot machine intentionally.
The term near-miss programming is used to denote this practice.
There is a related phenomenon that is also sometimes called near-miss. It is winning percentages that control the chance of a winning combination appearing on a pay line. 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. This phenomenon only occurs where abbreviated physical reels are used to display a win pattern based upon the RNG. Video slot machines have virtual reels and the symbols that appear around the winning line are usually an accurate depiction of how the reels were mathematically modeled.
There was an investigation held by the Nevada Gaming Commission concerning the issue of a near-miss above or below the pay line. They ruled that this was legal, so long as the near-miss above or below the pay line was not specially programmed. That means that the possibility of near-miss occurrence should be equal to the possibility of any other combination. It is impossible to program the machine to show winning combinations more frequently than other combinations above or below the pay line.
Near-miss programming, where a near miss is inaccurately displayed is not allowed in Australia either. To audit manufacturer's practices regulators use stop motion cameras.

Fraud
Mechanical slot machines and their coin acceptors were sometimes considered to be cheating devices and other scams.
Coin acceptors have become obsolete in favor of bill acceptors and modern slot machines are controlled by EPROM computer chips. These machines and their bill acceptors are designed with advanced anti-cheating and anti-counterfeiting measures and are difficult to defraud. One of the recent attempts at defrauding slot machines is directing microwaves toward it to disrupt its proper functioning.

There are a lot of myths and superstitions about slots.
You can spot a winning slot machine.
The only way you'll spot a winning slot machine is when the bells are ringing and the money is dropping. Slot machines use random numbergenerator and that's exactly that. Random.
Casinos place loose slots near the cashier, aisle ways, etc.
If this were true, it would be the only machines anyone would play. A casino mixes slots all over the casino floor. No one with any experience at playing slots would make a statement like this.
Count the number of symbols on the reels to determine your odds.
This is mathematically impossible, remember the RNG (random number generator) determines the stops. Thereare literally millions of combinations on a 3 reel slot machine, so there is no way of determining odds by the symbols on the reels alone.
Casinos can tighten or loosen slot machines from the back office.
In order to change how a slot willpay out is by changing the computer chip. No casino can just flip a switch.
A slot machine that has not been paying is due to hit.
Absolutely false. The RNG assures this. You have an equal chance of winning on a losing machine as you do on a machine that just paid out the jackpot.
Pulling the slot machine handle produces more wins than pressing the spin button and visa versa.
Again false. It makes no difference, as the computer only uses these as mechanisms to start the game. Only the computer determines a win or a loss and it has already determined if you are a winner or loser even before the first reel stops spinning.
Playing with hot coins improves your chances.
It is false. Nothing can affect the outcome, the machine doesn’t care and doesn’t know whether your coins are cold or hot.
Casinos set slot machines to payback more on the weekends.
One more rude mistake. The chances and odds are always the same. It doesn’t matter when you play.

Slot machines are honest because state gaming commissions make sure they are. What you can really doubt is the honesty of some manufactures.
When the first slots only appeared cheating was impossible because they worked mechanically, with independent wheels spinning and then stopping randomly. Modern slot machines differ form the old ones greatly. Now the integral part of any slot machine is a chip, which is programmed to select numbers randomly. The selected number determines the symbol which then comes out. The computer code that generates the random numbers is responsible for whether the machine is “loose” or “tight,” so gaming inspectors look at the machine’s code to make sure that a slot machine doesn’t cheat. The machine has to pay out either to the legal rate or better. Of course the latter happens very seldom.
Because slot machines don’t pay off with every spin, it’s difficult for customers to tell whether a slot machine’s chip has been programmed to pay off less often than is legally required. Inspectors look at brands of machines and their chips before they are installed in casinos, but they can’t inspect each machine. If inspectors approve the brand, they can produce machines, and change them. But these changes must cover legal requirements. Manufacturers can change chips to make a machine pay less as long as the inspectors have approved that particular chip.
For example, the Nevada Gaming Commission caught American Coin using chips that did not allow poker royal flushes. Or a chip that shows near misses of a jackpot. The effect was two-fold: the machine didn’t pay off, and customers continued to play the machine, thinking that they were very close to a big jackpot. If you think that you have come across such a machine you have a right to complain. In the case if the complaint the inspectors will visir the casino and check whether the machines have a legal chip. Manufacturers that cheat are fined or they can lose their license.