Märklin Digital was among the earlier digital model railway control systems. It was a comprehensive system including locomotive decoders (based on a Motorola chip), central control (Märklin 6020/6021), a computer interface (Märklin 6050), turnout decoders (Märklin 6083), digital relays (Märklin 6084) and feedback modules (Märklin s88/6088). The initial system was presented at the 1979 Nürnberg International Toy Fair, released in Europe in 1985 and the USA in 1986 under the name Digital H0. Operation Conventional analog control of model railways works by varying the track power and any locomotive on the track will respond by running at a speed roughly proportional to the power. For multiple trains sidings must have a switch to isolate trains standing there and leave the track dead. For multiple controllers the layout must be divided into sections isolated from each other and each with its own controller and current supply. All accessories such as signals and turnouts require individual switches and cables, making wiring very complex. With analog systems fine control of locomotives requires knowledge of the individual characteristics; gradients and curves require constant adjustment and low speed running is both difficult and liable to stalling. Any train lighting will vary in intensity with the power and be off when the locomotive is stopped. Digital control supplies constant power to the track with the power being switched many times a second to provide the "bits" of data (0 and 1) necessary for control (such digital power is neither DC nor AC). Every locomotive must be fitted with a decoder circuit which will interpret instructions and individually control the motor. Each decoder has its own address, instructions sent from the controller have a corresponding address so that while every active decoder will receive the instructions only the addressed decoder will respond. Once a locomotive is running it will continue and so even with one controller several trains can be running. Many locomotives may be on the track and individually controlled. Train lighting will always be at full intensity, even when the locomotive is stopped. Signals and turnouts may also be provided with decoders and controlled digitally. The track consists of 2 electrically-grounded rails, and a row of center studs over which the engine's pickup "shoe" glides acquiring the digital-pulsed 16V signal and power. This symmetrical track design solved the problem of loop-back topology (a problem encountered in polarized 2-rail). Conceptually, the entire layout may be controlled