Bus Termination

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Bus Termination is a topic that comes up a lot.

The theory is reflections that occur when the digital signal hits the end of the 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.

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 on the bus. 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.

Now for the math:

Calculating Values

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 0.01uF capacitor

1000 X 0.000 000 01

or 1 X 0.000 01

τ equals 0.000 01 seconds (10uS), or 1/0.000 01 is 100,000

You can substitute values for the resistor and capacitor, as long as the result is 1/100,000. You are not tied to any specific 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.

What It Does

The terminator reduces distortion of the waveform by minimizing reflections caused by the open end of the bus. Capacitors are a dead short to AC signals, such as the digital waveform used for DCC. The resistor prevents it from being a dead short between the two bus lines.

When a high frequency signal hits the end of a wire, it bounces back. The reflection is reduced in amplitude, has the same polarity, but out of phase, (displaced in time), so it distorts the waveform. Since a square wave is made of multiple frequencies, some will be reduced and others increased by the addition of the reflection, causing the distortion. The amount of displacement in time depends on the length of the wire.

The RC network appears as a load, and absorbs some of the energy, reducing any reflections. Some of the energy will also be returned to the command station via the neutral bus line.

Who Needs to Terminate Their Bus Lines

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.