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In page Texas Instruments:

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One of the common unifying forces that has united the community over the years has been the rather contentious relationship with TI regarding control over its graphing calculators.[citation needed] TI graphing calculators generally fall into two distinct groups—the older ones powered by the Zilog Z80 and the newer ones running on the Motorola 68000 series.[4] Both lines of calculators are locked by TI with checks in the hardware and through the signing of software to disable use of custom operating systems. However, users discovered the keys and published them in 2009. TI responded by sending invalid DMCA takedown notices, causing the Texas Instruments signing key controversy.[5]