In coventional wiring, the voltage or current applied to the loco is controlled before the wiring reaches the track.  In the loco, the contact with the track is connected directly to the motor.  In the early minitrix locos that this site focuses on (ie the period up until 1982 when Trix stopped supplying British Outline models), the chassis of the model is connected directly to the negative power supply provided by the left hand track.

DCC does things differently.  Instead of a 12 volt supply, we have 16 volt.
Instead of Direct Current - DC - we have Alternating Current - AC
Instead of controlling the power / voltage from the controller, the controller transmits signals which are encoded in the AC power supply.
In DCC, the power supply to the track is always on.

In the loco, we place a small chip.  This has the job of picking up the power supply and the signals from the track, decoding the signals and doing something as a result of the information that it has decoded.  This includes:

• Reading the intended device.  Every loco receives all of the signals from the controller.  One of the first jobs of the chip is to find out if the signals are intended for this loco or for some other loco.  If the signals are intended for another loco, ignore them.
• Act on the decoded signals.  This may include (say) the instruction to move the loco forward at a speed of 104.  The chip takes the power from the track and supplies the motor with the appropriate current.

Of course, it has to do a lot of other stuff, and it does it all over and over again, many times per second.

 

From all of the above, two things are important for the DCC controlled layout.

• The electrical pickup from the track MUST be isolated from the electrical supply to the motor.
• Every loco (or other device) on the layout has to have a unique number programmed into the chip.

There may still be documents lurking around which say that you cannot put a DCC chip into a model that uses the chassis as an earth.  I read this a number of times when I first started with DCC in my Minitrix models, but it is rubbish.  You can, but you have to be aware that the chassis is being used for the negative power supply, and isolate the motor from it.

Basically, everything goes through the chip. The power from the track goes into it. The controlled power to the motor, lights and any other gizmos that you have, come from the chip.

This DCC chip or wiring socket has 6 wires

Orange		Motor +ve
Grey		Motor -ve
Red		Right Side Power Pickup (+ve)
Black		Left Side Power Pickup (-ve)
White		Forward Light
Yellow		Reverse Light

Blue - if a 7th wire is available, is the common return lead for the directional lighting on chips with 7 wires. 

In a chip with only 6 pins, there is no common blue wire for the lights.  Instead, the chassis connection is used as the common return. This results in the lights being lit for only one half of the AC cycle, so will appear slightly dimmer.  This is fine, since the bulb in the loco is connected to the chassis, and wiring it in any other way would be a pain.

Finally, a word on electrical continuity.  Before fitting a chip to any loco, make absolutely sure that it runs perfectly well on a DC track.  DCC seems to be much more prone to tiny breaks in the circuit - eg going over points, or slight bumps in the track which may lift the wheels momentarily.  In such circumstances the chip loses the signal.  From my own observations, if there is an inertia setting so that the loco accelerates gradually, then the effect of the momentary disconnection is that the loco suddenly stops, and then accelerates gradually back to its speed setting.  Other chips and controllers may behave differently.