FAQ:Can an unmodified locomotive operate together with a DCC locomotive at the same time?
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The standard allows for this, although some care needs to be exercised in its use. We refer to this type of operation as analog compatibility mode. The signal is symmetric around the 0 volt level which provides a 0 volt DC component. By expanding the length of the zero bits on the positive side of the signal a positive DC component can be added. Likewise, by lengthening the bits on the negative side of the signal a negative DC component can be added. Only the 0 bits can be lengthened in this manner.
The result is a non-zero average DC voltage which will run an unmodified locomotive. However, since the complete signal gets to the motor, the stretched zero side causes the motor to turn, and the following instant the unstretched side (the opposite polarity) tries to reverse the motor. The longer (stretched) side wins, but motors do run more noisily and generate more heat. Some high precision can motors (the ironless core type) may be permanently damaged. (See the following question.)