Summary: Analog Conversion is a Power Conversion Mode supported by most Multifunction Decoder to allow a locomotive with a DCC decoder installed to operate on an analog (Direct Current) layout.
Analog Conversion is an optional feature provided by a multifunction decoder when operated on Direct Current supplied from a traditional analog controller. The DC voltage must be sufficient to power the multifunction decoder circuitry (usually only a few volts) so that it can operate. In the absence of a DCC signal, the multifunction 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 multifunction decoder will supply more power to the motor so that speed increases accordingly. When the direction is reversed on the analog controller, the multifunction decoder will correspondingly reverse the direction of the motor. 
Different multifunction decoders handle different analog controllers with varying degrees of success. Some analog controllers employ PWM or other techniques to modulate the motor speed to improve low speed performance.
DCC System Initialization
The NMRA standards dictate fail safe modes which must be present.
When a command station is initialized, two states can exist:
- It has stored the previous state of the system
- 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 ten idle packets ensure any multifunction decoders exit service mode.
When receiving power, the multifunction decoder will boot up and enter digital operations mode . If a minor power loss occurs, and the multifunction decoder has received valid instructions prior to the outage, it should resume operations. If not, the multifunction decoder shall return the locomotive to a complete stop.
Analog Conversion Notes
According to the NMRA Standard S–9.2.4, Converting between Power Modes:
- If a multi-function decoder has automatic conversion enabled detects the absence of a DCC signal for more than 30 milliSeconds, it is permissible to 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 until the available analog power level is achieved, using the programmed acceleration /deceleration rates of the decoder.
- A decoder not in digital mode detects a DCC signal, it shall switch to DCC operation and
- Continue at the same speed as it was while operating under the alternate power source
- Operate at the last known digital speed
- Stop until a proper digital instruction is received
- When converting to between Alternate Power Sources, should the new instructions cause the locomotive to reverse, the locomotive shall come to a complete stop (if needed) until:
- the direction information is the same,
- a DCC instruction packet is received that tells the decoder to stop,
- a reset packet is received, or
- track voltage drops to 0 Volts for 500 mS.
Furthermore, S–9.2 states: Some DCC decoders manufactured prior to the NMRA standards require a valid baseline packet be received every 30 milliseconds to prevent analog power conversion. 
While operating in digital mode, the decoder shall have a Packet 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.
Analog conversion is not required by the NMRA standard but the method of enabling it is. CV29's bit 2 (APS) must be turned on. By default, it is off. This bit means Enable Power Conversion. The NMRA standard allows for multiple modes 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.
Many users set the multifunction decoder to NMRA DCC Only, so if a multifunction 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 multifunction decoder.
Warning Regarding Operation of a Multifunction Decoder on an Analog Layout
Multifunction Decoders can be very demanding with respect to signal quality on the track. Operation of a multifunction decoder on analog power comes with risks, as dirty track power can destroy a multifunction decoder.
Low-cost analog power packs, and train set quality power packs can create voltage spikes, irregular waveforms and other issues as part of their design, especially those equipped with Pulse Power for better low speed operations. The amplitude of these spikes can be enough to permanently damage the multifunction decoder, rendering it useless.
See the warning from Rapido Trains regarding analog operations and multifunction decoder damage for more information.
Homebrew DCC Systems and Power Supplies
It is possible for homemade analog supplies or DCC Boosters to damage multifunction decoders. Analog DC supplies may introduce voltage spikes which can damage a decoder. Improperly designed or constructed DIY DCC Boosters can introduce a DC voltage or spikes to the track that can damage a DCC multifunction decoder.
Use DCC systems which meet the NMRA DCC Standards to operate locomotives with DCC multifunction decoders installed, to avoid damage.
Further info: NMRA Standard S-9.2.4
- When a Multifunction Digital Decoder that has automatic conversion enabled detects the absence of the NMRA digital signal for more than 30 milliseconds, it is permissible for the decoder to convert to an alternate power source.
- Digital Decoders must be in digital operations mode in order to interpret NMRA digital packets
- S–9.2, Frequency of Packet Transmision: If automatic conversion is enabled, Digital Decoders must remain in digital mode and not convert to using any alternate power signal so long as the time between Packet Start Bits is less than or equal to 30 milliseconds in duration. If automatic conversion is disabled, Digital Decoders must remain in digital mode regardless of the timing of Packet Start Bits. It shall be possible to configure Digital Command Stations to transmit at least one complete packet every 30 milliseconds as measured from the time between packet start bits.
- A value range of 1 through TIMEOUT_MAX sets the time-out to the chosen value. The minimum value of TIMEOUT_MAX will be 20 seconds. It may be longer at the manufacturer's discretion. A value of 0 disables this function
- NMRA Standard S-9.2.4