When I bought my first Lionel Command Control system, I saw the serial connector.  This usually starts my wheels turning and this time was no exception.  

Basically, Lionel Trainmaster Command Controlis an all digital method for controlling O Gauge model railroads.  This is sometimes referred to as TMCC.

Basically, there are command devices that will completely control almost every aspect of the layout.  The whole process is done with a combinaton of just a few units that are really quite inexpensive.

    1.  The Handheld Cab-1
    2.  The Command Base
    3.  A Power Supply
    4.  Various Switch and Accessory Controllers.
The Cab-1 is a handheld remote that allows you to move freely around transmitting signals to the Command Base.  The Command Base receives signals either from the Cab-1 or its serial port then sends the signals all around the layout.  These signals are carried by the actual wires connected between the Power Supply and the track or other accessories.  The process is similar to the carrier current system I used when I did my stint with radio back in 1972.  Then we used carrier current to distribute AM radio at our station WOCR in Ocean City, Maryland.

Basically a high frequence modulated carrier is imposed on the power line.  With TMCC, you apply a constant 18 VAC to the track.  Instead of varying the voltage to the track to change the engine's speed, the engine is sent digital commands over the carrier current, instructing the engine to run, stop, blow a whistle, ring a bell, generate other sounds or operate couplers.

Each device has its own programmable address. There are dozens of address possibilities making the system capable of operating more devices than one will ever need.

There are many reasons that one might desire using TMCC on a model railroad.  TMCC provides the ability to operate more than one engine on a single track.  It also allows more precision contol of the engines speed.  Forward and reverse are now completey controllable and nolonger are you required to run through the forward/reverse/stop/etc. everytime you want to change direction.

There are also audio playback systems built into the engines that enhance the trains performance.  Some sound systems come with simulated radio talk as well as station announcments.  This is nice, but what I really like, is the ability to operate other devices on the layout.  There are switch controls such as the SC-1 and SC-2.  The SC devices take commands from TMCC and give you relay closures that can be used for just about anything.