A data link is simply a connection between two communicating devices, in this case the command station and the decoder. Now, it might be tempting to think of the data link as a physical attachment of some kind, and in this case it is, but what is more useful for our purpose of DCC (and all other types of communication as well), is that there is a data link relationship in the information that travels across that connection. When the decoder recognizes data sent to it by the command station, a logical data link exists between them.
In order to work correctly, the data that the decoder thinks it is getting from the command station must be the same as the data that the command station is actually sending. Obviously, it’s very important that whatever is between the command station and the decoder not cause any corruption of the data. That means that the quality of the link between the command station and decoder is highly dependent on the condition of the track and the electrical pickups on the wheels. In this respect, both DCC and DC controls have the same requirements. Clean tracks and good electrical connections make for good operation.
A data link doesn’t just come into being by itself. The simplest form of data link in communications theory is a point-to-point system in which there is no ambiguity about the source or destination of the information exchange. In DC (also called analog) control, this very situation exists. When some information is sent from the power pack – the information in this case happens to be the voltage – the unmistakable destination is the single locomotive on that segment of track. This particular example is the most simplistic in content as well, as there is really only one bit of information, the voltage, being sent across the data link. Since we are trying to do something a lot more sophisticated with our trains, we need to have something with a little more capability.
In order to do this, we need some mechanism by which to send two pieces of information to whatever it is we want to control. First, we need to send a message that identifies just what we want to have happen, and then we need to send a message saying how much should happen. For example, if we want to turn on a locomotive headlight we need to say, “turn on the headlight”. Then we need to say, “Set the headlight to half brilliance.” That’s impossible to do when all we have is a knob that controls the track voltage.
But suppose we think a bit and come up with an idea that if we send the full track voltage for a few seconds, then turn the knob back and send half the track voltage for a few more seconds, we can somehow represent that half-headlight condition. We have just discovered the concept of a remote control system. Naturally, DCC is much more complex than that. However, the same principle applies to virtually any control system in existence today.
Operating Model Trains With Electronics and Circuitry
In the early days of electronics, there were many control systems that operated by the transmission and sensing of different levels of voltage or different frequencies of tone sent on the data link (which is commonly called a control link). These systems operated well when there was a clearly discernible difference between the control states. However, the nature of all transmission is that there is some loss of fidelity over long distances because the signal tends to become weaker and get lost in noise. The remote end of the link will start to make mistakes because what it sees as a command has in some way become corrupted by the noise of the intermediate transmission medium. Compensation was done with amplifier units between the transmitter and receiver, which increased the signal back up to a usable level. However, the amplifier not only increases the signal, it also increases the noise, so at some point it becomes useless to add more amplifiers. Moreover, the hardware to recognize the different levels of signal becomes very complex very quickly. An analog circuit is typically capable of only identifying one value of interest, so for complex actions the circuitry gets huge and difficult to make portable.
