Serial to I2C bridge for Arduino Uno

i2cbridge.ino

#include <Wire.h>

char inputString[200];
uint8_t ipos = 0;
boolean stringComplete = false;

void setup() {
  Serial.begin(9600);
  Wire.begin();
  ipos = 0;
}

int parseHex(char *g){
  uint8_t a,b;
  a = g[0];
  b = g[1];
  if (a>'f') return -1;
  if (a<'0') return -1;
  if (a>'9' && a<'A') return -1;
  if (a>'F') a-=32; //abcdef -> ABCDEF
  a = a-'0';
  if (a>9) a-=7;
  if (a>15) return -1;
  
  if (a<0 || a>15) return -1; 

  if (b>'f') return -1;
  if (b<'0') return -1;
  if (b>'9' && b<'A') return -1;
  if (b>'F') b-=32; //abcdef -> ABCDEF
  b = b-'0';
  if (b>9) b-=7;
  if (b>15) return -1;
  
  if (b<0 || b>15) return -1; 


  return a*16+b;
  
}

char output[3];

char *toHex(int i){
  output[0] = i/16;
  output[1] = i%16;
  output[0] += '0';
  output[1] += '0';
  if (output[0]>'9') output[0] += 7;
  if (output[1]>'9') output[1] += 7;
  output[2] = 0;
  return output;
}

int defaddr = 0;

void loop() {
  int addr;
  int val;
  int xpos;
  int len;
  if (stringComplete) {
    //Serial.print("Go: ");
    xpos = 2;
    addr = parseHex(inputString);
    //Serial.println(toHex(addr));
    if (addr == -1) {
      xpos = 0;
      addr = defaddr;
    }
    switch (inputString[xpos++]){
      //commands
      case '!':
        //set addr
        defaddr = addr;
        Serial.print("Default address set to ");
        Serial.println(toHex(addr));
        break;
      case '.': 
        Serial.print("Write to ");
        Serial.print(toHex(addr));
        Serial.print(":");
        Wire.beginTransmission(addr);
        while(1) {
          val = parseHex(inputString + xpos);
          xpos += 2;
          if (val==-1) {
            if (inputString[xpos-2]=='?') {
              Serial.print(" ? ");
              xpos--;
              len = parseHex(inputString + xpos);
              if (len==-1) {
                Wire.endTransmission();
                newString();return;
              }
              Wire.requestFrom(addr,len);
              while(len--) {
                val = Wire.read();
                Serial.print(toHex(val));
                Serial.print(" ");
              }
            }
            Serial.println("*");
            Wire.endTransmission();
            newString();return;
          }
          Wire.write(val);
          Serial.print(toHex(val));
        }
        break;

      case '?': 
        len = parseHex(inputString + xpos);
        if (len==-1) {
          newString();return;
        }
        Serial.print("Read ");
        Serial.print(len);
        Serial.print(" bytes from ");
        Serial.print(toHex(addr));
        Serial.print(": ");
        Wire.requestFrom(addr,len);
        while(Wire.available()) {
          val = Wire.read();
          Serial.print(toHex(val));
          Serial.print(" ");
        }
        newString();return;
    
    }
    // clear the string:
    newString();
  }
}

void newString(void) {
    stringComplete = false;
    ipos = 0;

}

//---------------

void serialEvent() {
  while (Serial.available()) {
    char inChar = (char)Serial.read();
    if (ipos==199) return;
    inputString[ipos++] = inChar;
    inputString[ipos] = 0;
    if (inChar == '\n') {
      stringComplete = true;
    }
  }
}