serial communication problems related to supply

Hei I’m trying reposting this question in another tread. I have a 12 bit paralleloutput i have to get into an computer. the output is from a RF dekoder where eight of the outputs also are connected to some astable multivibrators and four of them to a D/A converter. the problem is that when I connect the arduino to the paralelsignals, Vin to the +5V supply channel on my breadbord and the arduinos GND pin to the breadboards GND and plug in the USB cable the astable multivibrators are activated as if the arduino is making its own output even though all pins are set to be input.

the code for the arduino program is below. It is based on the “Virtual Color Mixer” example:

/*Variabeldeklaration, jeg definerer 
her på hvilke ben hver af tonesignalerne
går ind på Atmel microcontrolleren. 
*/
int c = 0;
int d = 1;
int e = 2;
int f = 3;
int g = 4;
int a = 5;
int b = 6;
int C = 7;

//samt hvilke ben volumen går ind på
int vol0 = 8;
int vol1 = 9;
int vol2 = 10;
int vol3 = 11;

//en variabel der indeholder volumen
int vol = 0;

void setup() 
//Denne funktion kører en gang når microcontrolleren starter
{ 
  pinMode(c,INPUT);
  pinMode(d,INPUT);
  pinMode(e,INPUT);
  pinMode(f,INPUT);
  pinMode(g,INPUT);
  pinMode(a,INPUT);
  pinMode(b,INPUT);
  pinMode(C,INPUT);
  pinMode(vol0,INPUT);
  pinMode(vol1,INPUT);
  pinMode(vol2,INPUT);
  pinMode(vol3,INPUT);
  
  Serial.begin(9600); //jeg starter seriel kommunikation med computeren
}

void loop() //Denne funktion kører kontinuert efter at setup() har kørt
{
  //først gemmes den binære værdi af volumen i vol  
  vol = digitalRead(vol3)*8+digitalRead(vol2)*4+digitalRead(vol1)*2+digitalRead(vol0);
  
  /*
  jeg sender en række af tekststrenge til computeren der ser således ud:
  c<værdi på ben 0>
  d<værdi på ben 1>
  e<værdi på ben 2>
  f<værdi på ben 3>
  g<værdi på ben 4>
  a<værdi på ben 5>
  b<værdi på ben 6>
  C<værdi på ben 7>
  v<værdi af variablen vol>
    
  "newline" karakteren som afslutter hver tekststreng samt hvad
  hver tekststreng starter med fortæller om hvilken variabel der bliver overført
  */
  Serial.print("v");
  Serial.println(vol);
  delay(200);
  Serial.print("c");
  Serial.println(digitalRead(c));
  delay(200);
  Serial.print("d");
  Serial.println(digitalRead(d));
  delay(200);
  Serial.print("e");
  Serial.println(digitalRead(e));
  delay(200);
  Serial.print("f");
  Serial.println(digitalRead(f));
  delay(200);
  Serial.print("g");
  Serial.println(digitalRead(g));
  delay(200);
  Serial.print("a");
  Serial.println(digitalRead(a));
  delay(200);
  Serial.print("b");
  Serial.println(digitalRead(b));
  delay(200);
  Serial.print("C");
  Serial.println(digitalRead(C));
  delay(200);
}

The delays are for debugging.

the scematic over the circuit can be seen here:

the jumpers are symbolising the connections to the arduinoboard

I really hope someone has a clue what’s going on.

okay I found the problem myself. When I connected the arduino to an external powersource it began to misbehave. when it was powered by the usb itself but still connected to the rest of the circuits ground it worked as intended. Well at least with a little test I have just done on a prototyping board. I have still to try it when it’s connected to the encoder. I’ll write how that went tomorrow.

okay that didn’t went well when the arduino was connected to the circuits ground but when I removed that connection it worked like a charm.