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Q: What is a command station?

Main article: Command Station

Command Stations (sometimes called a Central Station) are the very heart of a Digital Command Control system. They receive commands from a throttle network (such as Digitrax's Loconet), process them, and decide if it needs to make a standardized digital packet to send to all the decoders on a DCC system. They don't do the actual work; they tell other things to do the work. Please see command stations for full details.

Q: Why allow zero stretching at all?

Zero Stretching in the DCC Waveform

Zero Stretching or Analog Mode is not part of the NMRA DCC Standard.

Some manufacturers added the analog mode to create a compelling reason to adopt Digital Command Control in the early years, as modellers (particularly those with large fleets of motive power) were concerned about the costs of conversion to DCC.

This allowed DC locomotives to be controlled without adding a multifunction decoder. It is not a crucial feature today unlike 30 years earlier when DCC was introduced. It is now a legacy feature that some DCC systems still incorporate.

Analog compatibility mode is only one use for zero stretching. Other possible uses include the following:

Providing a stretched zero after each packet for the purpose of superimposing locomotive feedback to the command station.
Allowing generation of the command control signal with a computer using a standard serial port, which may not be possible without slightly stretching some zeros.
There are probably other uses that will become apparent as time goes on. Note that the above-mentioned uses are still very much in the experimental stages. The point is that preserving the ability to stretch zeros allows for the possibility of some interesting things.

Edit FAQ Related Articles: Power Conversion, Zero Stretching Categories: Beginner

Q: What is a digital circuit breaker?

Should you want to combine analog and digital operations on the same layout, a device needs to be placed on the analog side to prevent the voltages from combining when you go over a gap. This can be accomplished by using a large choke or by a digital circuit breaker. A digital circuit breaker must be used if the “0” bits are elongated (the DC component of the signal is non-zero).

Edit FAQ Categories: Beginner, Power, Wiring

Q: What is an Indexed CV?

The NMRA DCC Standard S-9.2.2 allows for more CVs than the 1-1024 limit specified. While many of those CVs are reserved, the NMRA anticipated the need for additional CVs in the future.

NMRA Standard S9.2.2 defined the ranges of 1 – 256 and 512 – 1024 as normal CVs. CVs from 257 to 511 are treated as Paged Index CVs, with the page accessed through CVs 31 and 32, the index registers, or Master CV.

This accomodates up to 65,536 additional pages in that range. The result is a memory space which can accommodate more than a million additional configuration variables.

Indexed CVs give the designers of multifunction decoders more options, especially true of those with sound.

See the article on Configuration Variables for more details.