Here are schematic drawings and theoretical explanation by Cam Bickel
(My thanks to Cam for allowing me to share them with you.)
The CAB-1 handheld unit is the primary user interface for TMCC. The unit has a microprocessor which reads the keys and knob and has outputs to a beeper and a transmitter. The microprocessor generates commands based on the sequence of keys pressed and the rotation of the knob. The beeper is used for feedback. When a key is held the command is resent about 10 times a second.
The transmitter sends the commands to receiver equipped units - the command base and Powermaster. The carrier frequency is determined by a plug in crystal approximately 27MHz. The carrier is only on when a command is being sent. This saves power and allows multiple Cab-1s to time multiplex at the same carrier frequency.
The command base has a receiver for the Cab-1 signal. The Local Oscillator for this receiver is also crystal controlled. The receiver crystal is 455KHz lower in frequency than the Cab-1 crystal. This creates a 455kHz IF which is FM demodulated and passed through a edge detector circuit. This signal goes into a microprocessor which validates receipt of a command and passes it on.
The microprocessor also has a serial input and output. The serial input allows insertion of commands from another source - a computer, a DCS TIU, a command recorder. Commands are sent out on the serial output to control hardwired command accessories like TPCs, and so on.
The microprocessor also outputs command data to frequency modulate a 455kHz oscillator. The output of this oscillator is boosted and capacitively coupled to the common of the track system. Although the frequency of this oscillator is controlled by independent LC components it is not a coincidence that it matches the IF frequency above. This allows the same or similar components to be used in the on-board receivers in the engines. The low side of the command base circuits is tied to earth ground through the three prong wall transformer. This is an important part of the track transmission system.
The command signal (455khz) to the track is constantly on. This allows detection of this carrier so engines know they are in a command environment. The commands are sent by FSK (Frequency Shift Keying) using an asynchronous serial format.
The command set and the binary coding for the commands is in the command base manual.
The Powermaster is essentially an electronic throttle controlled by the Cab-1. The receiver portion is very similar to that in the command base. The controller switch and a zero crossing detector allow the microprocessor to control the amount of time the ac output is on during each half cycle. By playing with the timing of the controls conventional engine speed is controlled and the dc offsets for conventional bell and whistle are generated.
These are not verified 100% but are useful for understanding how TMCC works and what circuits are like at the connectors.
Click on the following drawings to show them full size
|Command Base 1 of 4||Command Base 2 of 4|
|Command Base 3 of 4||Command Base 4 of 4|
|Powermaster 1 of 4||Powermaster 2 of 4|
|Powermaster 3 of 4||Powermaster 4 of 4|
|LCRU 1 of 4||LCRU 2 of 4|
|LCRU 3 of 4||LCRU 4 of 4|