Digital Signal Processing Reference
In-Depth Information
can also reverse the motor. Integrated H-bridge modules are available that can
minimize the number of discrete components used. One such example is the
National Semiconductor LMD18200 integrated H-bridge module. The
LMD18200 supports TTL and CMOS compatible inputs allowing the FPGA
board's output pins to be connected directly to the H-bridge inputs. A H-bridge
typically requires two digital input control pins (i.e., forward, reverse, and
stop). The H-bridge switches the train's power supply and in addition to the
FPGA output pins that drive the H-bridge inputs, a ground connection is
required between the train's power supply and the FPGA power supply.
Figure 8.12
A portion of a DCC train signal is seen above.
The zero crossing rate of a DCC signal is
used to send data bits for train speed commands. The DCC signal is also rectified in each train's
engine to provide 7-22V DC power for the train's electric motor and decoder circuits.
For the train sensors, Sharp GP2L26 infrared (IR) photointerrupter sensors can
be used to detect when a train passes each sensor point. These sensors emit IR
light from an LED and detect when the light is reflected back with an IR
detector circuit. These sensors are very small (3mm x 4mm) and can fit
between the rails on the track. Wires can be run down through the roadbed to a
central protoboard where the discrete components needed for interfacing to this
sensor are connected. Many model trains have dark underbodies, and the IR
photo sensors can not always detect the trains passing over them. To increase
the visibility of the trains to the photo sensors, pieces of reflective tape can be
taped to the bottom of the trains.
Preamble
Address Data Byte
Instruction Data Byte
Error Detection Data Byte
11111111111
1000
11
0
111
00
111
0
1
0000
1
0000
111
Data Byte Start Bit
Data Byte Start Bit
Packet Start Bit
Packet End Bit
Figure 8.13
An example DCC model train speed and direction command packet.
