[FONT="]DCC Simplified [/FONT][FONT="] Digital Command Control (DCC) is a system used to power and control a model railroad system. The operative words are Power and Control. The command/booster is the power company and communication center for the layout. It supplies power to the rails via the feeders. Power is always supplied to the rail unless we shut down the system or isolate a block or section of track from the booster. Type of Power DCC uses Direct Current (DC) at various voltages depending on the gauge of the models. Typically 12 volts through 22 volts. This voltage is intended to be constant, meaning that a N-Scale system will run at 13.9v +_ it will not change to 2 or 8v like you see on a DC track. (N-Scale power example) A DCC control/booster sends out power to the track in a form of alternating DC power. What is alternating is the Positive and Negative polarity. This type of power is call a Bipolar DC signal. The booster sends out +13.9 volts and -13.9 volts. This voltage alternates between the two polarities. This is the important aspect of DCC. Notice that bipolar DC is called a SIGNAL and not power. This point will be key to understanding DCC. [/FONT][FONT="][/FONT][FONT="]In my opinion DCC is not Alternating Current (AC). The DCC booster has a chip set that changes the polarity on its leads that formulates a digital signal. Some call this process as AC but I will not. Whenever the term AC is used the first thing that comes to mind is the AC from the wall. This is not the type of power that DCC utilizes. NMRA refers to the DCC output as a DC waveform. So, don't get caught in the banter of AC verses DC. Just understand that a digital signal is incorporated in the power supplied by the booster. Understanding the Signal Aspect Referring to the DC signal that the booster sends out across the track the + or - 13.9 volts is both a power source and a means to transmit data. The power is actually carrying a digital signal across it. Recalling that the + or - 13.9 volts alternates between one another one can see that there will be a correlation in time. Meaning if the booster is going to put +13.9 volts on the track how long is that voltage going to stay on the track and when does it alternate down to -13.9. This is the point where the signaling aspect comes into play. We all understand that a digital signal is 1's and 0's formulated in a format known as binary (00010011). In DCC the duration (how long the voltage stays on a particular polarity) is 58 micro seconds for a 1 and 100 micro seconds for a 0. The 13.9 volts (+or-) is used as a means to send a signal out. This is another form of morse code. Dit Dit Dit dash dash dash Dit Dit Dit (S.O.S). So if the booster wanted to send out an S.O.S it would send voltage out for 58 micro seconds 3 times for the dit and voltage out for 100 micro seconds 3 times for the dashes and another 3 58 micro second burst for the last dit dit dit. End results dit dit dit dash dash dash dit dit dit Inserting data onto a power line and creating a signal system is basically what DCC is. Summing it up. The duration of the negative and positive voltages creates a signaling method for DCC at the same time providing a power source for the track. Decoder The decoder gets its powered from the + and - 13.9 volts. It has a bridge rectifier that converts the track power to DC which powers the decoder. It reads the digital signal (1's and 0's or +-13.9v transitions) in the form of data packets and converts them to commands that operate the loco or misc equipment. The bridge rectifier converts the DCC power to DC for the motor to operate. This is why you have to isolate the DC motor from the wheels and pickup. In all actuality the decoder is your DC transformer. Putting it together The booster send DC power out to the track. That bipolar power alternates + to -. Your throttle send commands to the command portion of the booster which creates data packets. The data packets are encoded which dictates how long the + or - voltage durations are send out by the booster to the track. These data packets are broadcast and picked up by all decoders. The decoders are powered from the booster power/signal. They read the data packet coming across the track. Each packet is labeled with the address of the decoder. When a decoder reads a packet with its address on it will follow those command instruction. Light, horn, forward or what ever. The principles behind DCC is to power and communicate with the decoders over the same median/track. Hope this help out. Inobu [/FONT]
Your first post is a thank you. That must be a good experience. Lot of good people here that are willing to help. Sometime sticky but that is a small percentage. Glad it gives a little insight. Inobu
The DCC signal IS AC. It is true that the AC most people are familiar with is a sine wave, but that certainly doesn't mean that AC has to be a sine wave. The DCC signal is a square wave and has a varying frequency, but neither of these features precludes it from being AC.
DCC uses Pulse width modulation to comminicate the signal. The voltage levels are constant for all of the pulses.
Here we go again: What precludes it from being the traditional AC is the set frequency that AC is created in. The AC known as Alternating Current the type from the wall socket is different. It has a sine wave that is driven by a frequency shift. In America that is 60hz. The DCC voltage is not driven by a set frequency and does not change in direction. AC changes direction of its electrons at what ever frequency it is created in. Just because you see water falling from the sky does not mean it is rain. Just because the polarity changes does not me it is AC. Inobu
That is correct. The width of the pulses is 58 micro seconds for 1 and 100 micro seconds for 0. The change in polarity indicates the next bit. It too has a width that determines the 1 or 0 state. ................................+13.9.............................. ................................- 13.9............................ The decoder has a bridge rectifier that uses the + and -13.9 for power and the decoders read those voltage durations as 1 and 0 (command packet) and carries out the instructions. The packet looks like these 1 and 0 but are formulated as data. Like address 003 and then instruction by way of CV's. This is why you set and program the CV's on the decoder. The packets are sent based on the directions you send from the throttle/cab. The decoder reads the CV's looks at the CV you programed and then carries out the instructions. It is the manipulation of the DC voltage by alternating between + and - and the duration that creates a data signal or packet. Inobu
"DC" that is continuously alternating it's polarity IS square wave AC. The LMD18200 can not output sine wave AC, but it can output square wave AC. DCC is square wave AC.
What you are saying is correct in that it is current that is alternating. Not the type of Alternating Current that comes from your wall socket. If you think they are one in the same then you are mistaken. This is why I keep saying that the terminology that people are using is incorrect. AC verses current that is alternating. Inobu
From my Electical Engineering text: Alternating current is defined as any waveform that has a consistent and periodic change in slope. This means that PCM can be considered alternating current. A traingular sawtooth waveform is also alternating. It is not reserved only for sinusoidal waveforms.
Look up Inverter An inverter is an electrical device that converts direct current (DC) to alternating current (AC); A DCC booster does not perform this function. It is designed to modulate data over the DC signal placed on a track. Inobu
You did not state that DCC is not "traditional" AC, you said it is not AC. If by "traditional" AC you mean a set frequency sine wave, then DCC is quite different. The AC most people are familiar with has a set frequency and is a square wave, but that is not the only type of AC. AC can be a square wave and it can have a varying frequency and it will still be AC. What do you mean it doesn't change directions?!?! Everytime it's polarity changes the direction of it's electron flow will change just like any other AC power source.
That change in slope is defined by the frequency or Hertz in which it was created. DCC does not generate the frequency which makes up and completed you definition of AC. Inobu
Robert, Lets simplify this further. So you are telling me that if I take a Kato DC transformer and set the speed control to max 12v quickly rotate the forward and reverse switch .I have created AC. The same type of AC that is from the wall? Or Am I merely changing the + and - DC power flow from the transformer to the track. That is merely alternating the current polarity which is what the chip does. It is not an inverter. Inobu
I certainly do not think they are one in the same and never said so, in fact I have been clear to point out the differences; however, if you think that the type of AC that comes form your wall socket is the only type of AC there is then you are mistaken. DCC and household current are different, but they are still both AC. The incorrect terminology that people are using is assuming that AC refers to a set frequency sine wave.
And this is simple? Hahahahahaha! Sorry, about that. It just struck me as funny. The OP's, original post is a great tutorial for a newbie. A good start. Nice work Inobu! Now, wait until we get into a discussion on setting CV's. This is going to get real interesting. Does a flow chart....sound familiar?
Unfortunately I allow to get my self to get involved in this banter that brings no value. The thread speaks for its self. Inobu
Tomato, tomato. A horse! A mule! Relax, guys. You're both right. Strictly speaking, the DCC signal at the rails is an AC signal because it varies in polarity from positive to negative voltage (and therefore current) and back in a periodic fashion. It is most certainly not - and has nothing to do with - the AC sine wave we are all used to from our household wiring. but it *is* AC. However, when you consider the signal functionally - what the decoder does with the signal, it's not so bad to think of it as a "bipolar DC" signal. Immediately upon picking up the signal, the decoder splits the signal and does two things... it runs it through a bridge rectifier and converts it into a constant +12V DC to drive the electronics and power the motor. Having a symmetrical waveform ensures that there are no dead spots in the power delivered (consider what the output of the rectifier would be if the negative half of the waveform wasn't there). At the same time, the decoder half-wave rectifies the signal (discarding the negative half) so that it can measure the pulse widths and extract the commands. Either way, the signal is only utilized in a DC manner. Thus, it can help to think of it as a DC signal instead of AC. So you're both right. It's AC - but not the same AC as we get from the power mains. And it's DC - but only in the way it's used.
