Before I dive into this article I must warn you that it is quite technical. More often than not most gamblers still don’t understand the concept of the Random Number Generator (RNG) and how it’s calculated.

Let’s start with the inventor of the EPROM (Erasable Programmable Read Only Memory) first. The EPROM was invented by acclaimed Israeli engineer, Dov Frohman. It’s significant because not only does it retain data (regardless of power cuts) but it is easily programmable and ideally suited to the modern slot machine. On May 15, 1984, Inge Telnaes, a Norwegian theoretical mathematician applied patent 4448419. This patent enabled slot machines to get beyond the limited number of physical stops on a mechanical reel. By using a virtual reel, slot machines now offered larger payouts which paved the way for the mega jackpot.

All modern slot machines contain EPROMs. The EPROM is programmed by the manufacturer to deliver a particular payout percentage, for example 95.5%. Casinos normally order a number of slot machines each with varying payout percentages. The machines with the highest payouts are normally placed strategically in the casino. Slot machines with lower payouts will occupy the remaining spots.

Normally it doesn’t take long before regular slot players figure out where the higher payout machines are located on the casino floor. As a result the casinos keep on moving them around making it harder for players to play just the higher payout machines. The manufacturers of the slot machines also program the EPROM. The idea behind this is to map a random number to a virtual stop on a reel on the slot machine that can then be mapped to physical symbols in a specific location on that reel. This is where the Random Number Generator (RNG) comes in. 

What is a Random Number Generator (RNG)?

The RNG generates a random number that is divided by a fixed number (the number of virtual stops) to produces a remainder (between 1 and N) that corresponds to the N possible combination of symbols pertaining to each stop on a virtual reel of a specific slot machine. The specific symbols each contains a stop on a virtual reel that appears in the slot machine’s display window. Not all slot machines have the same probability of occurring. For example, a jackpot symbol only appears once on a reel while there may be many occurrences of the lesser payout symbol on the same reel. 

How Is The RNG Calculated?

Once the random number is generated and the remainder is obtained, it is then mapped to a virtual stop on a reel to determine the corresponding symbols that will be displayed in the slot machine window. Here’s another example:

  • Let’s use a slot machine with 3 reels and 3 visible symbols per reel (one above the middle line, one on the middle line and one below the middle line.
  • Let’s assume the EPROM chip’s program, using the RNG, returns a random number 10653942.
  • Suppose there are 6 unique possible symbols (S1-S6) on a given reel, then there are 6 choose 3 possible combinations of these symbols or 20 {(6x5x4x3x2x1)/[(3x2x1)x(3x2x1)]} possible outcomes. Simply put, there are 20 virtual stops on each reel. This is a very simplistic example an no current slot machine would ever offer such a small number of possible outcomes per reel. 
  • To control the payout percentage for the slot machine let’s add 12 more stops to bring the total number of stops to 32. To be honest it’s still too small for a modern slot machine but good enough for our example. 
  • The program then divides the random number by 32 to determine the remainder 22. In other words 32×332935+22=10653942. The program then looks at the 22nd virtual stop on the reel (from a table) to identify the 3 symbols (say Sa, Sb, Sc) that will be displayed on the first reel of the slot machine (one symbol Sa above the middle line, one symbol Sb on the middle line and one symbol Sc below the middle line).
  • This process is repeated two more times until the symbols for all the reels are determined.
  • The program then runs an algorithm to decide the payout by checking the alignment of all 6 symbols based on the lined played. 
  • If one considers the 32 possible virtual stops on each reel, there are 32,768 (32x32x32) possible outcomes on any given spin. All of this takes place in a fraction of a second and is triggered when you press the spin button or pull the handle. 

By adding or deleting stops on the reels (as we have done in our example), the manufacturer can control the probability of getting specific payouts. In this manner the overall payout percentage is controlled in the long run. 

The following table (using the example above for a $1 slot machine) describes it best.

    Symbol (CS)     Pays       Reel 1      Reel 2     Reel 3       Occurrences       Payout 
        3S1    $500        2         3        2              12      $6,000
        3S2    $150        3         3        4             36      $5,400
        3S3    $100        3         3        3             27       $2,700
        3S4      $50        5         5        5            125       $6,250
        3S5      $10        8         8        8            512       $5,120
        3S6      $5       11        10       10           1,100       $5,500
Total Win                   1,812      $30,970
Total Bet                $30,768
       Return                  94.5%

*Note this is a simple example and does not represent any particular machine.

Although the explanation of how the EPROM and the RNG work on a slot machine is complex, the following tips can stand you in good stead when you play slots. Most people play slots, sad but true. More than 70% of casino revenues are derived from slot machines. The majority of slot players only know two things; bet the max amount and hit the spin button quickly. This is a great strategy for the casino but a lousy one for the slot player.  

1. The EPROM and RNG is never reset and runs constantly, together they generate thousands of random numbers every second. Don’t feel bad when you leave a machine and the next player hits the jackpot on it. Even if you stayed at the machine you would not have pressed the spin button at the exact same instant the next person did to win the jackpot. 

2. The result of a particular spin is already determined before the reels stop spinning. Don’t be fooled by this, every spin is random. Many of the virtual stops on the reel are designed to trigger a near miss. Usually a win on a line you cannot play. It goes without saying that the manufactures of slot machines capitalize on the near miss effect. 

Researchers found that manufacturers program their games to tease players with near misses about 30% of the time. This is a number psychologists have found optimal for getting gamblers to keep on playing. 

3. Even if the screen on a slot machine shows a jackpot, you will not be paid unless the EPROM confirms that the jackpot has been hit. If you win a large progressive jackpot don’t be surprised if the casino moves everybody away from the machine while a technician checks the EPROM to confirm that there really is a jackpot. If the EPROM indicates any signs of tampering be it deliberate or accidental, the casino will not pay out the jackpot regardless of what the screen shows. 

Always read the small print on every slot machine carefully. There are many outstanding lawsuits against casinos where players believe they have won a jackpot but the casino refuses to pay them. In most cases these cases are ruled in favor of the casino.