Digital Command Control
Digital Command Control ( DCC ) is a standard for digital train , signal and switch control of model railways . The standard is based on the development of the German company Lenz Elektronik (for Märklin). The oldest digital decoders according to DCC are essentially the locomotive decoders delivered in the 1980s by Arnold (N gauge) and Märklin ( digital direct current for H0-Hamo and 1 gauge).
Märklin later parted with Lenz and the system called Digital DC at the time ; Lenz further developed the system and tried to get it recognized as a norm by the NMRA .
technical basics
Since around the end of the 1990s, the DCC system has been the most widely used digital system in the 2-rail, 2-conductor area; the Märklin-Motorola digital system is only predominant in the 3-rail, 2-conductor area (Märklin H0) , although a new system was brought onto the market in 2004: Märklin Systems (mfx). Another digital system that has also been in existence for a long time is SelecTRIX (mainly for N gauge).
With DCC, both the voltage required to operate the trains and control information are transmitted from a digital center to the decoder via the track . Turnouts and signals (solenoid accessories) can also be controlled via DCC signals. The subject of standardization is solely the protocol (the data traffic on the tracks), but not the communication between the control center and other input devices, boosters (= current amplifiers) etc. In Europe, XpressNet (originally X-Bus from Lenz) plus and are important for the input side LocoNet (developed by Digitrax). LocoNet integrates input, (stationary) feedback and booster control in one "bus", whereas with X-Bus / XpressNet two additional signal buses are added.
The DCC standard has evolved over time. Initially there were only 14 speed steps, 99 locomotive and 256 solenoid addresses, the current standard provides for 14, 27, 28 and 128 speed steps and 10,239 locomotive addresses; Most locomotives and some decoders are limited to 9999 locomotive addresses. Despite the further development of the system, the current control units and decoders are downward compatible so that the weakest component always determines the overall possibilities.
The DCC standard has meanwhile been expanded to include a mechanism for reporting information from the locomotive decoders to the control center via the track (Railcom).
The interface plugs have been standardized for easy installation of locomotive decoders, even by non-specialists. The six-pole plug according to NEM 651 is intended for confined spaces; the eight-pin connector according to NEM 652 standards offers an optional common return conductor for lighting and other functions as well as a free pin for another additional function.
Details
Packet format
The data rate on the track is approx. 8000 bits per second. Each DCC packet begins with a 10-14 bit long preamble, which is used to synchronize the decoder. An address byte, a command byte and a check byte then follow in a standard package, each of which is introduced by a start bit. The entire package is terminated by a stop bit. The test byte is calculated as an XOR link using the address and command byte.
However, this standard package is no longer the norm today, as it is only suitable for controlling locomotives with the aforementioned restrictions. For turnout and other function decoders as well as modern locomotive decoders, an extended packet format was defined that can contain between three and six data bytes.
Motor control
A locomotive decoder converts the non-sinusoidal alternating current in the track into direct current with the help of a bridge rectifier . The motor is supplied with this direct current via clocked transistors . In contrast to analog operation, it is not the voltage but the pulse duty factor that determines the motor speed ( pulse width modulation ).
Reading out of decoders
Decoders save configuration-relevant data in eight-bit-wide registers (configuration variable, CV). There are up to 1024 CVs. The DCC standard prescribes CV1 (base address), CV7 (decoder version), CV8 (manufacturer ID) and CV29 (decoder base configuration) as mandatory CVs. The meaning of some other CVs is also standardized, others can be assigned manufacturer-specifically.
DCC was originally designed as a unidirectional protocol for the sake of simplicity. The only way to transfer information back to the control center was the so-called acknowledgment pulse (ACK). The decoder increases its power consumption by 60 mA for 6 ms by switching on its motor or another consumer. This increased power consumption is recognized by the control center and interpreted as confirmation for the last programming command. This also makes it possible to read out the value of a specific CV. With older decoders, this query is only possible byte by byte, so that in extreme cases up to 256 possible values of a byte of a CV have to be queried until an ACK comes from the decoder at some point. Newer decoders can be queried bit by bit, so that the maximum number of required queries per CV is reduced to eight. Only one query has to be carried out for each of the 8 bits.
Railcom
The Lenz company and a “Railcom working group” with other manufacturers of DCC components developed an extension of the DCC protocol that enables the decoders to send data over the track during operation. Railcom was standardized by the NMRA in 2007 as RP 9.3.1. In order to create space for the Railcom transmissions, pauses are cut into the DCC signal from the control center. Up to 8 bytes can be transferred in a break. The evaluation of the Railcom data streams can be done either locally (i.e. mostly at the level of separate track circuits) or globally (mostly at the level of the DCC control center). A classic application of a local detector is vehicle detection in the context of track occupancy reports. A global detector can no longer locally assign a transmitting decoder, but applications such as registering a locomotive with the control center are conceivable.
Mixed operation
Since it is basically possible to send Märklin-Motorola digital system and DCC signals at the same time (to be precise, alternately in the millisecond range) on one track, many 3-rail-2-conductor (Märklin H0) systems in particular use both systems at the same time. Almost all digital control units (control centers) on the market are able to generate DCC and Märklin-Motorola signals. Märklin's own current center, the Central Station 3, can also generate DCC signals in addition to Märklin-Motorola and mfx and thus control locomotives and turnouts with DCC decoders.
Web links
- DCC specifications of the NMRA (English)
- DCC specifications (German)
- DCCWiki (English)
- DCC Eigenbau (German)
- OpenDCC self-made control center, decoder, speed controller, booster (German)