Anti-Lock Brake System (ABS)
On air brake trailers ABS is defined by 49CFR 571.121 (FMVSS 121)
Anti-lock brake system or ABS means a portion of a service brake system that automatically controls the degree of rotational wheel slip during braking by:  
(1) Sensing the rate of angular rotation of the wheels;
(2) Transmitting signals regarding the rate of wheel angular rotation to one or more controlling devices which interpret those signals and generate responsive controlling output signals; and
(3) Transmitting those controlling signals to one or more modulators which adjust brake actuating forces in response to those signals.
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