DCC Sniffer, Schematics


  • Processor:
    An Atmel Atmega162 or Atmega8515 can be mounted, the processor is operated at 5V. If the board is run as packet analyzer or speed meter, an external quartz (HC49) with 16MHz should be used.
  • Power supply:
    The power supply can be selected alternatively from the USB or from the DCC side. On the DCC side, a SOT23 linear regulator (LM3480) by National is used, this is cooled with copper areas. A series resistor helps to reduce the power dissipation. The reconfiguring of the power supply is done with solder jumpers. Do not use both power supplies simultaneously.
  • USB:
    A chip from FTDI is used. The accompanying PC driver simulates a serial interface (e.g. COM3). On the USB bus each participant has a VID and PID (VID: Vendor ID, PID: Product ID). The DCC Sniffer has a PID within the VID of FTDI:
      VID: 0x0403
      PID: 0xBFD9
      Product Description: 'USB-IF DCC Sniffer V1.0'
  • DCC input:
    The DCC input is optocoupled which avoids disturbing ground loops with the measurement PC. A SMD version of the common 6N136 is used.
  • Speed Measurement:
    An infrared reflective light barrier (CNY70 by Vishay) enables contactless measuring of the rotational speed of a motor. The sensor is connected to the capture input of the AVR. The capture can be done either directly at the input of timer 1 or with the help of the analogue comparator. The reference for the comparator can be generated either via a resistor divider or with RC lowpass. This RC lowpass is fed by a PWM, produced by timer 2. This allows to set the switching threshold of the analogue comparators via software.
  • Trigger input:
    An additional trigger input (also equipped with an optocoupler) enables the measurement of a complete system reaction. Starting from the sensor event until the appropriate DCC command is on the track.

Fuses for AVR

    The settings for the fuses (values DF 98 F9, for using bootloader) can easily created with palmavr. (If there is no bootloader entfallen, the fuses are DF 99 F9.)