Digital Command Control Systems: A Model Railroaders Dream Comes True Part 1
By Keith Conn
The history of model railroading dates back to the early 1800’s when toy craftsmen produced wooden miniatures for children. The evolution of controlling these toys began with clockwork gearing and live steam. By the start of the twentieth century, model railroads were electrically operated. Turnouts, a section of track with movable rails to divert a train from one track to another, were mechanically controlled as were other signaling devices such as crossing gates. Since electric trains use external control and external power, both the speed and direction of the train are controlled by their operators.
Model railroads are controlled by a power pack. The power pack contains a transformer that reduces the input voltage of 110V AC to 12V, and a rectifier. The power pack has a rheostat to control the speed, a reversing switch to control direction, two output terminals that connect to the track, and two more terminals that supply AC voltage for accessories such as turnouts. As the rheostat, throttle, is varied, that variance is detected by the locomotive’s motor. The motor receives the delayed command to change speeds through the current passing from the power pack, to the rails which the locomotive’s wheels transfer the current to the motor. Direction is controlled by the reversing switch changing the polarity of the track.
This method of control is referred to as conventional or DC control. As a model railroad layout expands the DC control may create a hindrance because only one locomotive can operate on any single track. The current conducted by the track to the wheels on the locomotive can create a build up of dirt, dust, and grease which will reduce the performance of the locomotive and may cause intermittent problems. The creation of a realistic system with DC control results in complicated wiring configurations and many controlling devices.
DCC control was developed to create more realism for the modeler. DCC allows multiple locomotives to connect together and operate as a multiple unit (MU), or as separate trains with their own locomotive, and better performance since a decoder controls the motor of the locomotive and not the wheels so the dirt and dust build up is reduced. DCC was implemented by the National Model Railroading Association, NMRA, in the late 1980’s. NMRA created The DCC Working Group to study and adopt standards for using DCC controls. The group consisted of three men; Stan Ames, Rutger Friberg, and Ed Loizeau. They reviewed a few early manufacturers of DCC systems to determine the best protocols.
The standards were adopted by the NMRA at their annual meeting in Columbus, Ohio in 1992. The operation of DCC is essentially less complicated than conventional DC control. See Figure 1 as an example of the simplification.
Basic Operation of DCC Control
The standard employed by the NMRA for DCC is a similar packet technology that is used within the computer industry for local area networking (LANs). Within the LAN, each device has its own address, and communicates with other devices using a communication packet. This packet is composed of a minimum of 38 bits. The packet transmits information between devices within the network. The devices are a command station that transmits information to the decoders and only in rare cases, when specifically asked, does the decoder communicate to the command station, and the booster that takes in power from the power supply, signals from the command station, amplifies these signals and then supplies power and DCC signals to the track.
Each packet contains an address that indicates which decoder the packet is intended for. All decoders read all packets, but the decoder with the address that matches its own, will respond to the packet. The NMRA standard allows between 150 and 200 instruction packets to be transmitted every second.
Since DCC is a binary system, a “1” bit is encoded by a short positive pulse followed by a short negative pulse. The “0” bit is encoded by a longer positive pulse followed by an equally long negative pulse. Figure 2 represents these bits.
This bi-polar waveform has a few advantages. They include:
1) The average DC component of a bi-polar signal is zero volts.
2) With a bi-polar signal, the decoder only needs to look for a half-bit and it doesn’t matter the polarity the decoder has relative to the track. This allows the locomotive to face either direction, and still move forward when signaled.
3) A bi-polar signal does not polarize the tracks, thus resulting in the track not getting dirty as fast as conventional DC track because of polarization which tends to attract dirt and dust.
The one disadvantage is that a pure square wave signal has many high frequency components that cause interference with AM reception if the media is not shielded. Since the track cannot be shielded alternate methods are used. The natural paralleled rails tend to cancel many of the high frequencies. Another method the decoder uses to reject noise is implemented. The method is call edge detection. Every time the voltage in the bi-polar signal goes positive, the microprocessor is triggered in the decoder. In measuring the time between rise time triggers, the decoder can identify “1” bits, “0” bits, and noise.
The packet, as previously stated is composed of a minimum of 38 bits, starts with a preamble, address data byte, instruction data byte, error detection data byte, and finally an end bit. This is illustrated by Figure 3.
Packets can be between three and six bytes long, but most applications use a three to four byte packet. The preamble of each packet consists of a minimum of 10 “1” bits, then followed by blocks 1 byte in length separated by “0” bits, and concluding with an end bit of “1.” The byte blocks are used to transmit an address, speed and direction instructions, and an error byte. The DCC process is simply encoding information into a packet, transmit this packet over the track, receive the packet by the decoder, decode it back into the original command, and then deliver the correct voltage to the locomotive’s motor, light bulbs, or other items. The command station must continuously transmit packets because the DCC signal and the voltage to operate the locomotive’s motor are one in the same. The command station can either send idle packets or retransmit previously sent packets.
This concludes our discussion of part 1. Our next installment will discuss the DCC block diagram and coding of the locomotives.
Ames, Stan, Friberg, Rutger, and Loizeaux, Ed, 2003, Digital Command Control – The Comprehensive Guide to DCC, Allt om Hobby, Sewden.
Atlas Model Railroad Company, Inc., 2003, The Complete Atlas Wiring Book – Series Book #12, Atlas Model Railroad Company, Hillside, NJ.
Digitrax, Inc., 1999, The Digitrax Big Book of DCC, Digitrax, Norcross, GA.