Using the Iowa Scaled Engineering – Arduino DCC Decoder Shield

Iowa Scaled Engineering have developed a very flexible and capable DCC Decoder Shield that interfaces the Arduino to a DCC System and supports CV Read/Write via the CV ACK facility.

Here is a photo of the DCC Decoder Shield connected to an Arduino UNO showing the jumper settings:


Once you have configured the jumpers you can load up the code below to exercise the DCC Interface.

The example code will allow you:

  • See all the DCC Packets printed as HEX values to the Arduino Serial Monitor
  • Read and Write CVs
  • Reset to Factory Defaults by writing a value of 8 to CV8
#include <NmraDcc.h>

// This Example shows how to use the library with the Iowa Scaled Engineering ARD-DCCSHIELD
// You can find out more about this DCC Interface here:
// For more information refer to the file: here:
// This demo assumes the following Jumper settings on the ARD-DCCSHIELD
// JP1 - I2C Pull-Up Resistors - Don't Care
// JP2 - (Pins 1-2) I2C /IORST JP2 - Don't-Care
// JP2 - (Pins 3-4) - DCC Signal to Arduino Pin - OFF
// JP3 - I2C /INT and /OE - Don't-Care
// JP4 - DCC Signal to Arduino Pin - D2 ON
// JP5 - Arduino Powered from DCC - User Choice
// JP6 - Boards without VIO - User Choice
// JP7 - Enable Programming ACK - 1-2 ON 3-4 ON 
// It is a very basic DCC Accessory Decoder that does nothing except allow CV Read/Write and 
// you can also print every DCC packet by uncommenting the "#define NOTIFY_DCC_MSG" line below

// You can also print other Debug Messages uncommenting the line below
#define DEBUG_MSG

// Un-Comment the line below to Enable DCC ACK for Service Mode Programming Read CV Capablilty 
#define ENABLE_DCC_ACK 15 // This is A1 on the Iowa Scaled Engineering ARD-DCCSHIELD DCC Shield

NmraDcc Dcc ;

struct CVPair
 uint16_t CV;
 uint8_t Value;

CVPair FactoryDefaultCVs [] =

uint8_t FactoryDefaultCVIndex = 0;

void notifyCVResetFactoryDefault()
#ifdef DEBUG_MSG
 Serial.println("notifyCVResetFactoryDefault: Settings CVs to Factory Defaults") ;
#endif // Make FactoryDefaultCVIndex non-zero and equal to num CV's to be reset 
 // to flag to the loop() function that a reset to Factory Defaults needs to be done
 FactoryDefaultCVIndex = sizeof(FactoryDefaultCVs)/sizeof(CVPair);

// This function is called by the NmraDcc library when a DCC ACK needs to be sent
// Calling this function should cause an increased 60ma current drain on the power supply for 6ms to ACK a CV Read 
void notifyCVAck(void)
#ifdef DEBUG_MSG
 Serial.println("notifyCVAck") ;
 // Configure the DCC CV Programing ACK pin for an output

 // Generate the DCC ACK 60mA pulse
 digitalWrite( ENABLE_DCC_ACK, HIGH );
 delay( 10 ); // The DCC Spec says 6ms but 10 makes sure... 😉
 digitalWrite( ENABLE_DCC_ACK, LOW );

void notifyDccMsg( DCC_MSG * Msg)
 Serial.print("notifyDccMsg: ") ;
 for(uint8_t i = 0; i < Msg->Size; i++)
 Serial.print(Msg->Data[i], HEX);
 Serial.write(' ');

void notifyCVChange(uint16_t CV, uint8_t Value)
#ifdef DEBUG_MSG
 Serial.print("notifyCVChange: CV: ") ;
 Serial.print(CV,DEC) ;
 Serial.print(" Value: ") ;
 Serial.println(Value, DEC) ;
 Value = Value; // Silence Compiler Warnings...
 CV = CV;

void setup()
 Serial.println("NMRA DCC Iowa Scaled Engineering ARD-DCCSHIELD Example");
 // Setup which External Interrupt, the Pin it's associated with that we're using and enable the Pull-Up, 2, 1);
 // Call the main DCC Init function to enable the DCC Receiver
 Dcc.init( MAN_ID_DIY, 10, CV29_ACCESSORY_DECODER, 0 );

 Serial.println("Init Done");

void loop()
 // You MUST call the NmraDcc.process() method frequently from the Arduino loop() function for correct library operation
 if( FactoryDefaultCVIndex && Dcc.isSetCVReady())
 FactoryDefaultCVIndex--; // Decrement first as initially it is the size of the array 
 Dcc.setCV( FactoryDefaultCVs[FactoryDefaultCVIndex].CV, FactoryDefaultCVs[FactoryDefaultCVIndex].Value);