Introduction to DCC/Moving a Locomotive
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For the operational examples below, consider again the Abstract Functional Block Diagram. In the diagram, note the locomotive headlights, indicating their forward directions.
In the diagram above, imagine an Operator who wants to move Locomotive 9901 ahead...
| DCC - How It Works Operationally: An Operator Wants to Move #9901 Ahead | ||
|---|---|---|
| From the Operator's Perspective | What's Going On Under the Covers | Result on Layout |
| The Operator arbitrarily selects the 3rd Throttle and enters the address of the locomotive to be controlled - 9901. | 3rd Throttle saves address 9901. All subsequent commands from 3rd Throttle will be sent to Decoder 9901. | None. |
| The Operator selects Forward direction. | 3rd Throttle saves direction as Forward. All subsequent commands from 3rd Throttle will specify Locomotive 9901 is to move in the forward direction. | None. |
| The Operator increases throttle speed. | 3rd Throttle interprets the Operator's throttle action as, "Set speed to Speed Step 4" [presumed to be previously set to "Speed Step 0"], and sends a Speed Step 4 / Forward command, addressed to 9901. The Command Station constructs, and then broadcasts on the Booster Bus, to all Boosters, a DCC packet addressed to 9901, specifying Speed Step 4 / Forward. All Boosters simultaneously place the packet on their respective tracks. All Decoders receive the packet. All Decoders except Decoder 9901 ignore the packet, as it is not addressed to them. Decoder 9901 executes the command and applies a low-level [speed step 4], forward voltage [forward direction] to the locomotive's motor. | Locomotive 9901 moves forward, approaching Locomotive 9902 . |
| The Operator decreases throttle speed. | 3rd Throttle interprets the Operator's throttle action as, "Set speed to Speed Step 0 / Forward", and sends a Speed Step 0 command addressed to 9901. The Command Station, Boosters and Decoders all respond as above, but now for the Speed Step 0 / Forward command addressed to Decoder 9901. | Locomotive 9901 comes to a stop, adjacent to Locomotive 9902 . |