Signal tracer For long pulse trains, a cyclic redundancy check may be calculated and displayed, giving the tech insight into the content of circuits that are switching rapidly. Signal tracer A signal tracer is a piece of electronic test equipment used to troubleshoot radio and other electronic circuitry. Usually a very simple device, it normally provides an amplifier, and a loudspeaker, often battery-powered and packaged into a small, hand-held test probe. An optional diode detector is usually also provided, allowing the detection of amplitude-modulated signals. The technician injects a test signal into the device under test. Then, by using the signal tracer,
End-of-train device End-of-train device The end of train device (ETD), sometimes referred to as an EOT, flashing rear-end device (FRED) or sense and braking unit (SBU) is an electronic device mounted on the end of freight trains in lieu of a caboose. They are divided into three categories: "dumb" units, which only provide a visible indication of the rear of the train with a flashing red taillight; "average intelligence" units with a brake pipe pressure gauge; and "smart" units, which send back data to the crew in the locomotive via radio-based telemetry. They originated in North America, and are also used elsewhere