Circumvention
Most unlicensed companies created circuits that used a voltage spike to knock the authentication unit in the NES offline.
A few unlicensed games released in Europe and Australia (such as HES games) came in the form of a dongle that would be connected to a licensed cartridge, in order to use that cartridge's 10NES lockout chip for authentication.
Tengen (an Atari Games subsidiary) took a different tactic: the corporation obtained a description of the code in the lockout chip from the United States Patent and Trademark Office by claiming that it was required to defend against present infringement claims in a legal case. Tengen then used these documents to design their Rabbit chip, which duplicated the function of the 10NES. Nintendo sued Tengen for these actions. The court found that Tengen did not violate the copyright for copying the portion of code necessary to defeat the protection with current NES consoles, but did violate the copyright for copying portions of the code not being used in the communication between the chip and console. Tengen had copied this code in its entirety because future console releases could have been engineered to pick up the discrepancy. On the initial claim, the court sided with Nintendo on the issue of patent infringement, but noted that Nintendo’s patent would likely be deemed obvious as it was basically U.S. Patent 4,736,419 with the addition of a reset pin, which was at the time already commonplace in the world of electronics. Therefore, while Nintendo was the winner of the initial trial, before they could actually enforce the ruling they would need to have the patent hold up under scrutiny, as well as address Tengen’s antitrust claims. Before this occurred, the sides settled.
A small company called RetroZone, the first company to publish games on the NES in over a decade, uses a multi-region lockout chip for NTSC, PAL A, and PAL B called the Ciclone which was created by reverse engineering Tengen's "Rabbit" chip. It is the only lockout chip in existence that will allow games to be played in more than one region. It is intended to make the games playable on the original NES-001 hardware that uses the 10NES lockout chip and the two other regions - the other region free alternative would be the top-loading NES, which does not feature the lockout chip. The Ciclone chip is the first lockout chip to be developed after the patent for the 10NES had expired.
Because the 10NES in the model NES-001 Control Deck occasionally fails to authenticate legal cartridges, a common modification is to disable the chip entirely by cutting pin 4 on the Control Deck's internal 10NES lockout chip.
Read more about this topic: 10NES