Hardware architecture of the GRAVITY fringe tracker. The realtime controller operates within the wgvfft Linux workstation. Another Linux workstation, wgvkalm, is dedicated to calculating the Kaiman filter parameters based on the observed OPDs. A Motorola CPU in the lgvttp workstation integrates the piston information with tip-tilt sensor data to accurately adjust the mirrors mounted on piezoelectric actuators in the tip-tilt piston (TTP). Additionally, an external workstation can provide the fringe tracker with piston setpoints and a mobility flag, which influences the controller behaviour.