Bump. I modified the cartridge so it works with a RAM chip. It's powered by a small battery when it's not plugged in the atari so it retains the data. I featured it on my website:
 http://8times8.host56.com/

Did they even try to repair these consoles? At least a few could be restored, right?

Does that work with online mode?

I just bought an atari2600 jr online for €10, so I can play atari while my original atari is waiting for its video upgrade to arrive in the mail (at least the components I need for it) I also just went ahead and bought castlevania for the NES, because I don't own a lot of NES games and I just want a few more classics. I already got mario and probotector (contra), but other than that I do not own any good games for the system.

It's just the engine by the way. Nothing fancy. I did not even have the world map engine done. Just two rooms. If someone would like to continue this project he would really only need a few snippets of code and the graphics. It's better to redo my bad code. I was not a very good axe programmer back then.

I made the linkprotocol bi-directional (r/w)
#define _N 255

// MACROs
#define PIN2L !(PIND & 0B00000100)
#define PIN3L !(PIND & 0B00001000)

#define CLR(x,y) (x&=(~(1<<y)))
#define SET(x,y) (x|=(1<<y))

byte data;
byte bitCount = 8;

// State _N (255) : Do nothing and wait for incoming interrupt. Carry out other tasks in the mean time.
// State  0      : Slave receives  (Receive bytes from master)
// State  1      : Slave sends     (Send bytes to master)

void bitReceive()
{
  if(bitCount){
    data >>= 1;
    if (PIN2L)
    {
      data |= 128;
    }
    bitCount--;
  }
}

void bitSend()
{
  if(bitCount)
  {
    //Set portD
    DDRD |= 4;
    if(data & 1)
      PORTD &= 251;
    else
      PORTD |= 4;
      
    data >>= 1;
    bitCount--;
  }
}

void setup()
{
  CLR(DDRD,2); //pinMode 2 is input
  CLR(DDRD,3); //pinMode 3 is input
  attachInterrupt(1,bitReceive,FALLING);
}

void loop()
{
  byte state = _N, address, cell[128];
  
  while(1)
  {
    if (!bitCount){
      // Pull down clock line to make sure the calculator does not send while the atmega
      // is processing the incoming data.
      DDRD |= 8;
      switch (state)
      {
        case 255:
        {
          bitCount = 8;
          address = data >> 1;
          state = data & 1;
          // When the mode is Slave Transmit the function bitSend() is attached to the interrupt.
          if (state){
            data = cell[address];
            attachInterrupt(1,bitSend,FALLING);
          }
          break;
        }
        case 0:
        {        
          cell[address] = data;
          bitCount = 8;
          // Return to mode _N.
          state = _N;
          break;
        }
        case 1:
        {
          bitCount = 8;
          // Re-attach the bitReceive function to the external interrupt on pin 3 and
          // return to mode _N.
          attachInterrupt(1,bitReceive,FALLING);
          state = _N;
          break;
        }
      }
      // Release the clockline and data line.
      DDRD &= 243;
      PORTD &= 251;
    }
  }
}This code compiles to less than 1k and it uses only 16 bytes of RAM + 128 for the data cells (used in this example so you can read and write from these cells from the calculator. These will get replaced by the SID registers).

Edit: When I put the content of the main loop in a function it compiles to over 2k I have to do this in order to be able to make this into a library and make the code more readable, but it's not optimized at all.

Yeah, but that would take away the original NES feel.