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Ask the Slot Expert: Reseeding the RNG in a slot machine

7 January 2015

My question pertains to the timing of the reseeding of the PRNG. Have done some extensive research on this subject, and would like to hear your thoughts on it. We know it is possible to do.

Can the PRNGs used in slot machines be set to reseed to a set of negative values by date/hour/minute/second/millisecond? How would it be possible for a casino to do this?

We have all seen days where you cannot win a thing on nearly every slot in the casino. This was happening long before SBG [server-based gaming] was being used. Why are there days like that when machines have not been played equally?

PRNG (Pseudo-Random Number Generator) is the correct name for the random number generator function in a slot machine. The number stream produced by the PRNG satisfies many of the tests for a random number stream but is not truly produced at random.

A PRNG function takes one or more inputs, performs some mathematical operations on them, and returns the result of the calculations. The input parameters are usually numbers previously generated by the function, but they don't have to be.

Slot machines frequently "reseed" the PRNG function by passing an independently derived number as input. That number could be the number of milliseconds in the current time, the coordinates of the spot where the player last touched the screen, etc. The PRNG is reseeded to make it harder for RNG cheats.

You may have realized that if the PRNG function consists of mathematical operations and prior results are used as the inputs for the next number, the stream of numbers from the PRNG is completely predictable if you know the PRNG function. RNG cheats use inside knowledge of the PRNG function to predict future outcomes. By reseeding the PRNG, the software running the machine causes the PRNG to jump to a different point in its sequence, hopefully before RNG cheats have been able to figure out where it originally was in the sequence.

So, reseeding the PRNG is done all the time as an anti-cheating tactic. It is not done to switch machines from paying money to taking money.

Remember that the stream of numbers from a PRNG is just a stream of numbers. The stream has no long-term payback. One stream is not more generous than another. If you want to change the long-term payback on a machine, you have to change the reel layouts. The PRNG function stays the same.

We've all seen times when a machine, a bank of machines, or all the machines in a section of the casino have been hot. And maybe we've seen the exact opposite the next day. I've had times when I just couldn't lose on the machines in a section of Circus Circus and then, on my next visit, I just couldn't win.

These experiences are just a normal and expected result of randomness. Reseeding is not causing the machines to switch from "pay" to "take" cycles.


Can the casino lower one or all of its machines to pay out at a lower percent? It sure seems that they do that. The machines seem to win more in the mornings than they do on Friday and Saturday night.

Yes, the casino can change the long-term paybacks on machines in most jurisdictions. Depending upon the jurisdiction, the casino can make the change itself. In others, reps from the gaming commission either have to witness the change or make the change themselves. In some jurisdictions the logic drawers in the machines are sealed and the no one can make any changes. And depending on jurisdiction, the casino either has to report the change or get permission to make the change.

If a machine supports downloadable games, changing the machine doesn't require changing chips in the machine. Instead, the change can be sent to the machine over the casino's network from a central server.

Now, that being said, casinos do not change machines at whim. They do not make machines pay back more (or less) in the morning than in the evening or change them for weekends or holidays or special events. The effort and cost of changing machines is too prohibitive.

Even with downloadable games, casinos cannot make this sort of change so frequently. John Grochowski interviewed a gaming control board member in one of his recent columns, and that regulator said the board does not allow casinos to change machines so frequently.

I suspect you might have a case of select memory. You can do an experiment to see if your impressions are really happening. Pick a machine you like to play and play 1,000 or more spins on it in the morning and 1,000 or more spins on it on Friday or Saturday night. Keep track of the number of hits you get and the number of spins you play in each time period. I predict that the hit frequency will be very close in both time periods.


John Robison

John Robison is an expert on slot machines and how to play them. John is a slot and video poker columnist and has written for many of gaming’s leading publications. He holds a master's degree in computer science from the prestigious Stevens Institute of Technology.

You may hear John give his slot and video poker tips live on The Good Times Show, hosted by Rudi Schiffer and Mike Schiffer, which is broadcast from Memphis on KXIQ 1180AM Friday afternoon from from 2PM to 5PM Central Time. John is on the show from 4:30 to 5. You can listen to archives of the show on the web anytime.

Books by John Robison:

The Slot Expert's Guide to Playing Slots
John Robison
John Robison is an expert on slot machines and how to play them. John is a slot and video poker columnist and has written for many of gaming’s leading publications. He holds a master's degree in computer science from the prestigious Stevens Institute of Technology.

You may hear John give his slot and video poker tips live on The Good Times Show, hosted by Rudi Schiffer and Mike Schiffer, which is broadcast from Memphis on KXIQ 1180AM Friday afternoon from from 2PM to 5PM Central Time. John is on the show from 4:30 to 5. You can listen to archives of the show on the web anytime.

Books by John Robison:

The Slot Expert's Guide to Playing Slots