FAQ:What is the Purpose of a Resistor used for Decoder Programming
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The purpose of this is to decrease the impedance the command station's program output sees, allowing more current to flow. This increases the likelihood that the command station will get enough current to indicate a positive acknowledgment from the multifunction decoder after a write operation.
Multifunction decoders acknowledge a successful write operation by pulsing the motor. If the impedance of the decoder or motor is too high the amount of current flowing may not be accurately detected by the command station.
By placing a resistor in parallel, the total resistance is reduced (see Ohm's Law for more details.) This effectively allows more current to flow when the motor pulse is initiated (total current = motor + resistor currents.)
The value of the resistor can vary, but many modellers use a 1kΩ (1000Ω) 1/4 or better for this purpose.