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foto del decoder per accessori DCCDecoder per accessori DCC
The decoder can check 8 accessories in a DCC system.
Each accessory can absorb up to 500ma.
The command can be impulsive (with duration between 10 msec and 2,55sec in steps of 10 msec) or constant.
The command packet DCC is the type D (See NMRA-DCC RP 9.2.1 D: Accessory Digital Decoder Packet Formats).
The characteristics of the decoder are programmable through the direct mode (see NMRA-DCC RP 9.2.3 Service Mode Instruction Packets for Direct Mode).
Three flashes of the LED indicate that the programming was successful.



 The programmable parameters are:
  • addresses; high bits (cv521 bits 0-2), low bits (cv513 bits 0-6) 
  • outputs mode (cv515-cv518): 0 = constant level, 1-255: 10msec-2,55sec.
 Limitations:
  • Do not activate more than four outputs simultaneously in constant mode.
  • The duration of the pulses has a tolerance of +/- 2%.
 
Construction

the schematic of the decoder is available at the link below:
constructive notes:
  • all resistors are 1/4w 5%. Theirs value is expressed in Ohms.
  • capacitors are ceramic 63volts, except where specifically noted
  • The driver may be replaced by ULN2801A ULN2803A, in this case the limiting resistors R4-R11 may be eliminated
  • PIC16F84 should have the frequency of 10 or 20MHz, but since these versions are difficult to find, you can use a 4Mhz. I have tried several 4Mhz without encountering difficulties.
  • The oscillating element can be a quartz or ceramic resonator. Some ceramic resonators do not require coupling capacitors (C7 and C8). Refer to the datasheet of the component for more information
PIC programming

 The firmware of the PIC is still evolving. The version currently available is 1.2.
It is derived by the work of D. Probst and HeikoSchroeter.

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