Stand-Alone Responder to a Marklin Keyboard i2c bus message.

gistfile1.txt

// Wire Slave Receiver
// by Nicholas Zambetti <http://www.zambetti.com>

// Demonstrates use of the Wire library
// Receives data as an I2C/TWI slave device
// Refer to the "Wire Master Writer" example for use with this

// Created 29 March 2006

// This example code is in the public domain.


#include <Wire.h>

constexpr const uint8_t kCentralAddr = 0x7F;
constexpr const uint8_t myAddr = kCentralAddr;

typedef struct DataMessage {
  uint8_t destination = myAddr;
  uint8_t source = 0;
  uint8_t data = 0;
} DataMessage;

DataMessage lastMsg;
bool messageReceived;

void setup() {
  messageReceived = false;
  Wire.begin(myAddr);                // join i2c bus with address #8
  Wire.onReceive(receiveEvent); // register event
  Serial.begin(115200);           // start serial for output
  Serial.println(F("slave_receiver ready."));
}

void loop() {
  // delay(100);
  if (messageReceived) {
    // Craft a response
    DataMessage response;
    response.destination = lastMsg.source >> 1; // guesswork: shift by 1. Observation: Keyboard that sent with 20 gets its response on 10.
    response.source = lastMsg.destination << 1; // guesswork: shift by 1. Observation: Central that received on 0x7F sends from 0xFE
    response.data = lastMsg.data;
    SendMessage(response);
    messageReceived = false;
  }
}

void SendMessage(DataMessage& msg) {
  Wire.beginTransmission(msg.destination);
  Wire.write(msg.source);
  Wire.write(msg.data);
  Wire.endTransmission();
    Serial.println(F("Response sent."));
}

// function that executes whenever data is received from master
// this function is registered as an event, see setup()
void receiveEvent(int howMany) {
  if (!messageReceived) {
    lastMsg.destination = myAddr;
    lastMsg.source = Wire.read();
    lastMsg.data = Wire.read();
    Serial.println(F("Message received."));
    messageReceived = true;
  } else {
    Serial.println(F("Buffer full, lost message."));
  }
/*
  while (1 < Wire.available()) { // loop through all but the last
    uint8_t c = Wire.read(); // receive byte as a character
    Serial.print(c, HEX);         // print the character
    Serial.print(' ');
  }
  int x = Wire.read();    // receive byte as an integer
  Serial.println(x, HEX);         // print the integer
  */
}