Summary: RailSync is a pair of differential signals present on LocoNet which communicate the command station's DCC packets to boosters.
The RailSync lines communicate a copy of the track signals to the Boosters for amplification and transmission to the rails. RailSync can power a limited number of LocoNet connected devices on small layouts, such as Utility Panels. As the number of power consuming LocoNet devices attached to LocoNet increases, the need to separately power those devices increases.
RailSync is a low power differential signal that mirrors the Digital Command Control packets on the rails. It is generated by the Command Station and sent out via LocoNet (pins 1 & 6) to devices such as Boosters, Occupancy Detectors and any other device which requires RailSync to operate. Boosters receive the RailSync data, amplify and output it to the track. RailSync can also be used as a source of power for low current devices such as throttles.
RailSync uses pins 1 and 6 of the LocoNet cable.
By using a differential signal, the receiving device can sum the two signals to cancel any common mode noise which may result in errors.
Common Mode Rejection Ratio (CMRR): By summing the differential signal, any noise present will be cancelled out.
- The signal is +5V on one line and –5V on the other.
- There is 3V of noise present.
The positive signal is connected to the non-inverting input of an operational amplifier (op-amp) while the negative signal connects to the inverting input of the op-amp.
- 5 – (–5) = 10 (Desired signal)
- 3 – 3 = 0 (Noise)
- Result is a 10V signal with no noise present.
An Op Amp performs a summing operation by subtracting the non-inverting input from the inverting input. 
The 3V noise signal is common to both lines (their amplitudes are identical), and their sum results in one line's noise cancelling the other.
For more information see:
- When used as an amplifier, the gain can be controlled using negative feedback, where the output is connected to the inverting input. Without negative feedback the gain would be infinite. By adding a resistor into the feedback loop, gain can be controlled, with no resistor the gain is 1 or unity.