Tandem's products for this market began with the Integrity line in 1989, using MIPS processors and a "NonStop UX" variant of Unix. It was developed in Austin TX. In 1991, the Integrity S2 used TMR, Triple Modular Redundancy, where each logical CPU used three MIPS R2000 microprocessors to execute the same data thread, with voting to find and lock out a failed part. Their fast clocks could not be synchronized as in strict lock stepping, so voting instead happened at each interrupt. Some other version of Integrity used 4x "pair and spares" redundancy. Pairs of processors ran in lock-step to check each other. When they disagreed, both processors were marked untrusted and their workload was taken over by a hot-spare pair of processors whose state was already current. In 1995, the Integrity S4000 was the first to use ServerNet and moved toward sharing peripherals with the NonStop line.