Analog Conversion

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Short Definition

Allows a locomotive with DCC decoder to run on non-DCC powered tracks.

Analog Conversion is a mode of power conversion that is supported by most mobile decoders to allow a Digital Command Control decoder equipped locomotive to operate on a non-DCC equipped layout.

Warning - Using a Primary Address (CV 1) of "0" enables this feature and overrides the APS setting (Bit 2) of CV29. A combination of a Primary Address of 0 and CV12=1 (Power Source Conversion) will result in an immediate runaway when DCC power is connected to the rails.

Analog Conversion is defined as optional by the NMRA Digital Command Control Standard. It is not the same as Zero Stretching.

Also see the article on Power Conversion.

How It Works

Analog Conversion is an optional feature provided by a mobile decoder when it is operated on Direct Current supplied from a traditional analog controller. The DC voltage level must be sufficient to power up the decoder circuitry (usually only a few volts) so that it can operate. In the absence of a DCC signal, the decoder will default to driving the motor in the same direction as the polarity of the DC supply indicates. As the supply voltage increases the decoder will supply more power to the motor so that speed increases accordingly. When the direction is reversed on the analog controller, the decoder will correspondingly reverse the direction of the motor.

Different decoders may handle different analog controllers with varying degrees of success because many analog controllers do not simply vary the voltage to vary the speed. Some of them employ PWM or other techniques to modulate the motor speed to improve low speed performance.

Many mobile decoders will support automatic function controls in analog conversion mode. For example, only lighting the headlight in the direction of travel.

Fail Safe

The NMRA standards dictate fail safe modes which must be present.

When a command station is initialized, to states can exist:

  1. It has stored the previous state of the system
  2. It has no information regarding the previous state of the system.

If the system has no stored state, it shall send a minimum of 20 reset packets, followed by at least 10 idle packets, before sending any command packets. The 10 idle packets are required to ensure the decoders exit service mode.

When receiving power, the decoder will boot up and enter digital operations mode. If a minor power loss occurs, and the decoder has received valid instructions prior to the outage, it should resume operations. If not, the decoder shall return the locomotive to a complete stop.

Analog Conversion Notes

According to the NMRA Standard S-9.2.4, Fail Safe Operating Characteristics:

  1. If a multi-function (mobile) decoder has automatic conversion enabled, detects the absence of a DCC signal for more than 30 mSeconds, it can switch to an alternate power source. If the decoder converts to an alternate power source, it shall accelerate or decelerate in the direction specified until it reaches the speed that the analog power dictates, using the programmed acceleration /deceleration rates of the decoder.
  2. A decoder not in digital mode detects a DCC signal, it shall switch to DCC operation and
    1. Continue at the same speed as it was while operating under the alternate power source
    2. Operate at the last known digital speed
    3. Stop until a proper digital instruction is received
  3. When converting to digital, should the new instructions cause the locomotive to reverse, the locomotive shall come to a complete stop (if needed) until
    1. the direction information is the same,
    2. a DCC instruction packet is received that tells the decoder to stop,
    3. a reset packet is received, or
    4. track voltage drops to 0 volts for 500 mS.

Error Conditions

While operating in digital mode, the decoder shall have a Packet update timeout. If that value is exceeded, the decoder shall bring the locomotive or other devices controlled by it to a stop. The purpose is to ensure that all decoders receive updates from the command station which should help prevent runaway conditions. The user should be able to define the timeout value, or disable it.

Set Up

Analog conversion is not required by the NMRA standard but the method of enabling it is. CV29's bit 2 must be turned on. By default it is usually off. This bit means Enable Power Conversion. The NMRA standard allows for multiple types of power conversion and analog is only one of them. For decoders that support multiple types of conversion CV12 should be set to 1 for analog conversion.

See the article on Configuration Variables, as NMRA defines a number of optional alternative power modes, such as Hornby's Zero 1 or CTC-16.

Many users set the decoder to NMRA DCC Only, so if a decoder will not respond on a DC powered layout, check the decoder parameters are set to allow operation on analog power. This is done to prevent runaways should an unexpected condition occur on a DCC layout.
As always, read the manual for your decoder.

Further info: NMRA Standard S-9.2.4