LocoNet and RailSync Voltages

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Summary: Troubleshooting Loconet

Troubleshooting LocoNet Voltages

If either LocoNet or RailSync voltage is low, the layout may experience several different symptoms:

  1. Difficulty turning on track power when using additional Boosters.
  2. Throttles may not operate correctly or may have no display at all if they do not have a battery installed.
  3. Detection and/or Transponding may not operate correctly.

Use a good quality Digital Voltmeter (DVM) to diagnose:

  1. If the LocoNet data lines (pins 3 & 4, green/red) have voltages between 8 and 14V with no message traffic present. This confirms that the booster is outputting enough voltage.
  2. If the RailSync lines are OK.
    1. The two RailSync lines are a differential signal, driven with a 22-ohm output impedance so as they are loaded the voltages will drop, indicating the current load. These two lines should not be connected together, this will cause excess current draw and low voltages.
    2. Differential means that if signal A is HI, the corresponding B signal will be low. This allows the pair to be summed together (A – (−B)) while cancelling any common mode noise.

How to Check for LocoNet Voltages

Prepare a short LocoNet cable with one end stripped to expose the wires inside the jacket.

  1. Strip the Red and Green wires and twist them together.
  2. Strip the Black and Yellow wires and twist them together.
  3. Plug this short LocoNet cable directly into one of your command station’s LocoNet Ports.
  4. Disconnect LocoNet from the layout at your Command Station so that you are measuring LocoNet voltage at the Command Station without the layout attached.
  5. Use a good quality multimeter (on the 20-volt DC range) to measure the DC voltage between the red/green and black/yellow pairs.
  6. Connect the red/green wires to the + (positive lead) of the DVM and the – (negative) DVM lead black/yellow wires and measure the voltage.
  7. The voltage on the LocoNet data wires should be at least +8V DC. The highest voltage depends on the command station but is usually +12V to +14V.
  8. Very large layouts will benefit from using a LocoNet Repeater (LNRP)
If more than 40 devices are on a single leg of LocoNet, or you get frequent interruptions as you plug in devices, this may indicate the need for an LNRP.
  • LocoNet Repeaters can be used to isolate areas of the layout from each other
    • Allows for troubleshooting with LocoNet fault indications
    • Proper operation of LNRP REQUIRES at least a 14VDC power supply.

How to Check RailSync

Use the short LocoNet cable prepared for checking LocoNet voltage above.

  1. Plug this short LocoNet cable directly into one of your command station’s LocoNet Ports.
  2. Disconnect LocoNet from the layout at your Command Station so that you are measuring RailSync voltage at the Command Station without any of the layout LocoNet attached.
  3. Measure the no load RailSync voltage at the Command Station without the layout plugged in. Measure both RailSync voltages separately and add them to remove and analog zero stretching changes and to be sure both wires are carrying RailSync.
  4. Use a DVM to measure from the white wire pin1 to black or the yellow ground (pin 2 or 5).
  5. Use a DVM to measure from the blue wire pin 6 to the black or yellow ground (pin 2 or 5).
  6. Add these two DC voltages, the total should be +12V to +14V DC, with the command station unplugged from the layout. This is the unloaded RailSync measurement.
  7. Measure the Loaded RailSync Voltage at the Command Station with the layout LocoNet plugged into the Command Station.
  8. Plug the LocoNet Cable from the layout in to the Command Station.
  9. Use a DVM to measure from the white wire pin1 to the black or yellow ground (pin 2 or 5) on the short LocoNet Cable plugged in to the Command Station.
  10. Use a DVM to measure from the blue wire pin 6 to the black or yellow ground (pin 2 or 5).
  11. Add these two DC voltages, the total should be +7 volts DC, with the command station plugged into the layout. This is the loaded RailSync measurement.
  12. Compare the Unloaded and Loaded measurements for RailSync; this voltage difference is a measure of how much power the RailSync loads are drawing.
  13. If voltage is below 8 volts, make sure all UR92s, UR91s, PM42s, etc. are powered using PS14 or similar power supplies as required.
  14. If this does not bring the voltage above 8 volts, power all throttles with batteries and/or power UP5 and similar panels using PS14. Multiple UP panels can share a single power supply by daisy chaining them together as shown in their instruction sheets.