Bus Termination

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Summary: Bus termination is used with some DCC systems to reduce ringing and reflections which may cause problems on the Power Bus

RC Terminator or "Snubber"

Bus Termination is a topic that comes up a lot.

The theory is reflections occur when the digital signal hits the end of the power bus, so additional precautions must be taken to minimize those reflections . It is similar in concept to terminations used on high speed computer data and address buses to prevent corruption of data and erratic operation. Some computer interfaces, such as SCSI, must be terminated by design. Ringing occurs during phase switches (inversions) by the booster's output driving an inductive load.

Many have pointed to this as a solution to decoders becoming corrupted or erratic operations, when the truth may lie within poor wiring practices and dirty track. For a termination to be effective at eliminating an issue, it must be at the source of the problem, which is never the end of the bus. In many cases the application of terminations is not necessary and does not solve the problem.

Bus Termination

Most modern, well designed decoders will reject almost any combination of transmission-line ringing, RFI, and any other mismatch effects that may be caused by the layout's power bus. This decoder oriented design solution is far superior to filters for unpredictable wiring arrangements. It is impossible to design a "one size fits all" solution to such an issue, as every layout has different wiring arrangements which will alter the parameters. Any additional bus termination would require measuring every power bus on the layout and calculating a custom termination for each one.

Any Radio Frequency Interference created by the electronics on the layout will be handled by the decoder and command station.

At the frequencies Digital Command Control employs, many of the arguments for termination just don't fit the parameters. Transmission Line Propagation at the frequencies used by Digital Command Control are manageable without any extra precautions such as terminators. Many layouts do not have the complex wiring configurations or very long bus runs which would be an issue. Most issues can be traced back to bad wiring practices. Claims of excessive voltage spikes may in fact be a result of poor wiring or other issues.

Before embarking on adding terminators, the track signals should be examined with an oscilloscope to determine if there is distortion, and wiring examined and corrected first. Adding terminators may introduce new issues into the equation, such are excessive waveform distortion. 

How to Construct a Terminator

A bus terminator is really an RC circuit. Which means Resistor Capacitor.

In this instance, a resistor is wired in series with a capacitor, and the combination is then connected across the bus wires. The time constant is selected to be small enough that it will allow the capacitor to charge and discharge rapidly, important when the phase of the rail changes.

Now for the math:

Calculating Values

Charge and discharge cycle of an RC Circuit. 5τ is required for a full charge/discharge of the capacitor.

To work with signals in the Digital Command Control frequency range, the time constant Tau (τ) required is 1/100,000.

To calculate this value, 1/τ equals RC.

For example: a 1000 ohm resistor and a 10,000pF capacitor

1000 × 0.000 000 01
or 1 × 0.000 01

τ equals 0.000 01 seconds (10µS). To fully charge the capacitor 50µs is required (5 time constants). In this example the Cutoff Frequency (Fc) is 16kHz, so the RC circuit will not have a large effect on the DCC signal.

  • Fc = 159155/(τ is in μs) or 1/(2πRC)

Substitution of values for the resistor and capacitor is possible, providing the resulting time constant is 5 to 10µs. You are not tied to any specific value. The recommended values from one manufacturer is a 100,000pF capacitor, 50V, with a series resistor value of 47 to 100Ω. This results in a time constant of 4.7 to 10µS. The 100Ω resistor is the preferred value. For scales such as HO and N, a half watt resistor is the minimum. For larger scales, such as O and S, at least 1W is the minimum. Large scales, such as those used for garden railways, for example, should use 2W resistors. The capacitor should be at least 50WVDC, non-polarised.

It is also permissible to use two capacitors in parallel to increase the total capacitance of the RC circuit, if a non-standard value is needed.

What It Does

The terminator helps counteract the impedance of the power bus. It also filters frequencies above that of the DCC signal on the bus, reducing distortion. Should spikes occur, it offers a low impedance path back to the booster, instead of allowing them be be reflected and further distort the signal. The terminator also reduces distortion of the waveform by minimizing reflections caused by the open end of the bus.

Impedance Issues

Most signal quality issues are related to the bus impedance, specifically its inductance. Poor wiring, either through the selection of the incorrect wire gauge, or poor workmanship, have direct inputs on the impedance of the bus.

For more information on wiring, see the Wiring for Digital Command Control pages. Also read Wire Sizes and Spacing, and the section regarding twisitng bus wires.

Who Needs to Terminate Their Bus Lines

The bus forms a transmission line. At DCC frequencies there are no real issues to be dealt with. The need for a terminator is directly related to the design and implementation of the output (driving) stage of your booster. Some designs already incorporate an RC snubber within the booster to deal with impedance issues related to the inductance of the load being driven. The output impedance of the booster is also a consideration.

This is an issue related to long bus runs. Most layouts will not have any problems with the bus wiring which would require installation of bus terminators. Should you experience erratic operation, this would be one possible option. Some manufacturers recommend terminating the bus. Others do not recommend the addition of terminators.

Further Reading

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