Zero Stretching

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Summary: Zero Stretching is an optional feature which allows operation of a locomotive without a Multifunction Decoder installed.

Short Definition
Method of running a DC powered locomotive on DCC powered rails.

Also known as Zero Bit Stretching in the Digital Command Control specifications. May be informally referred to as Address 00.


Zero Stretching

It may be possible to power a single analog (non-decoder equipped) model locomotive by itself or in addition to DCC equipped locomotives through a method known as Zero Stretching. Refer to the documentation for the Digital Command Control system in use, as it is an optional feature.

Cautions

This feature was created to make DCC appealing during the early days. It enabled migration while minimizing fears relating to the cost and installation work required to upgrade to DCC. Zero Stretching is not part of the NMRA DCC specification and as such, not all Digital Command Control systems support Zero Stretching.

Note
Some locomotive manufacturers do not recommend operating a non-DCC decoder equipped locomotive on a DCC system. Refer to the instructions before trying your new locomotive on a DCC powered track.

Decoderless Locomotives on a DCC Layout

DCCSIG.PNG

To operate a locomotive which is not equipped with a decoder, the DCC waveform on the rails is modified. This modification affects the "zero pulse". The DCC waveform has a DC value of zero (15V - 15V = 0), so when zero stretching or analog mode is not in use, a non-decoder equipped locomotive will not move. The DCC waveform normally consists of symmetrical pulses, two of which equal one cycle. Under these conditions no torque is produced in the motor.

As shown, the Zero Pulse consists of a pulse, with a nominal duration of 100 microSeconds (µS). The complete cycle is 200µS. Under normal conditions the voltage of the pulse is canceled by the second one in the cycle.

How Does it Work?

To implement Zero Stretching, the zero bit cycle is modified. One half of the period can be made a lot longer in duration than the other, which will cause current to flow for a longer period of time. By controlling which half of the cycle is longer, the direction of the motor can be determined. The command station will make the necessary changes to the DCC signal to create the desired effect, based on the throttle's input. As locomotive speed is increased the command station creates more Address 00 packets

Since the data packet consists of eight bits, a total of 16 pulses will appear on the rails (two per bit). Which is 1.6mS in total. It is possible for the stretched bit to equal 1.5mS. The speed is determined by the amount of bit stretching and the rate at which Address 00 packets appear. Zero Stretching will not produce as much torque as using pure DC will.

DCC Waveform, illustrating zero bit stretching. Any pulse between 90 and 110µSecs is a zero pulse, two of which equal one cycle.

Important Warnings

Caution: Because of the frequency of current flow changes and the harmonics present, Direct Current motors will heat up much more quickly than they would on an analog power source, and some motor types can be seriously damaged with only a brief encounter with DCC track. Many motors will buzz and hum when presented with this type of power.

In the early days of DCC there was an unfounded fear that zero stretching would cause motors to overheat, destroying them and melting the body. Those fears were never realized.

  1. Do not leave locomotives that are not equipped with a decoder on the track
  2. All direct current locomotives will respond to the signals created by Zero Stretching. Which may have unintended results.

Coreless Motors

Coreless and other precision motor typess should not be used on a DCC powered track (unless a multifunction decoder is installed). Normally, current flow is limited by the voltage applied and the Back EMF a motor generates in operation, but the DCC waveform is full voltage all the time. Coreless motors have very low inductance in their windings. The DCC waveform's frequency results in a low reactance which is not enough to limit the current. Coreless motors lack the iron core to sink the heat generated by excessive current flow, which will damage them very quickly.

Bandwidth

As locomotive speed increases, more bandwidth will be demanded of Address 00, which can have an impact on response times when more than 5 DCC equipped locomotives are also in operation. This technique is a power bus bandwidth hog due to the need to constantly send packets addressed to 00.

  • Signal Corruption: It is possible that some motors may induce spikes and other noise into the power bus which could cause problems.

PSX Circuit Breakers

  • PSX Circuit Breakers are not compatible. The device relies on DCC power for operation, and may not work at some speed settings.

See Also

DCC Power Digital Packet