[Astronouth7303]

I was lieing in bed this morning when it hit me: Since the prog port is RS-232, and so is a computer, there is a chance you could connect LEGO's old COMM tower with the prog port on the RC or the EDU. You would probably need to make a custom cable, but it might work.

Basically, to control the RCX, you send it IR messages via the IR tower. From there it's a simple matter of programming the two.
The LUGNET thread for sending messages in VB can be found here.

Here's some FRC code to do it:

#define SendByte(Byte) {TXREG = Byte; Wait4TXEmpty();}
void SendRCXMessage (char Message)
{
	//Header
	SendByte(0x55);
	SendByte(0xFF);
	SendByte(!0xFF);
	//Opcode for Message
	SendByte(0xF7);
	SendByte(!0xF7);
	//Message Byte
	SendByte(Message);
	SendByte(!Message);
	//Checksum
	SendByte((0xF7 + Message) & 0xFF);
	SendByte(!((0xF7 + Message) & 0xFF));
}

The RCX, unfortunately, only has a 1 byte buffer for IR, so I thought of a 1 byte setup to set each motor to a certain speed.
The RCX has outputs A, B, and C. They can be Forward, Backward, or stopped, and have a speed of 1-8. This can be expressed as -8 to 8.

If you just want 2 motors, you can set both in the same byte in the range of -7 to 8. Use the first 4 bits for A and the second 4 for B. In NQC:

// A Basic RC Skid-Steer Program (Recieve)

#define mLeft OUT_A
#define mRight OUT_C
#define nOffset 7
#define nBit 16

int Left = 0;
int Right = 0;

task main()
{
 start GetMessage;
 start RunLeft;
 start RunRight;
}

task GetMessage()
{
 Left = Message() / nBit - nOffset;
 Right = Message() % nBit - nOffset;
}


task RunLeft()
{
 while (true)
 {
  SetPower (mLeft, abs(Left));
  if (Left > 0) OnFwd(mLeft);
  if (Left < 0) OnRev(mLeft);
  if (Left == 0) Off(mLeft);
 }
}

task RunRight ()
{
 while (true)
 {
  SetPower(mRight, abs(Right));
  if (Right > 0) OnFwd(mRight);
  if (Right < 0) OnRev(mRight);
  if (Right == 0) Off(mRight);
 }
}

(And yes, the RCX does support primitive multi-threading)

In the FRC:

void Set2RCXMotors(signed char Left, signed char Right)
{
 char Temp = 0;
 //Check Left value
 if (Left > 8)
  Left = 8;
 if (Left < -7)
  Left = -7;
 //Check Right value
 if (Right > 8)
  Left = 8;
 if (Right < -7)
  Left = -7;
 
 Temp = (Left+7) << 4;
 Temp |= Right+7;
 SendRCXMessage(Temp);
}

If you want to control all 3 outs directly, then you'll have to specify which one. The first 2 bits are what out, 3 is 0, 4 is direction, 5-7 are power-1, and 8 is "Is on?". So, in a nutshell: MM0D PPPOn.
On the RCX:

// A Basic RC 3 Motor control

int Left = 0;
int Center = 0;
int Right = 0;

task main()
{
 start GetMessage;
 start RunLeft;
 start RunRight;
 start RunCenter;
}

task GetMessage()
{
 int Mess;
 Mess = Message;
 //Left
 if ((Mess/64) == 1)
 {
  if (Mess & 0x10)
  {
   Left = ((Mess/2) & 8) & ((Mess & 1) != 0) ;
  }
  else
  {
   Left = -(((Mess/2) & 8) & ((Mess & 1) != 0));
  }
 }
 
 //Center
 if ((Mess/64) == 2)
 {
  if (Mess & 0x10)
  {
   Center = ((Mess/2) & 8) & ((Mess & 1) != 0);
  }
  else
  {
   Center = -(((Mess/2) & 8) & ((Mess & 1) != 0));
  }
 }
 
 //Right
 if ((Mess/64) == 3)
 {
  if (Mess & 0x10)
  {
   Right = ((Mess/2) & 8) & ((Mess & 1) != 0);
  }
  else
  {
   Right = -(((Mess/2) & 8) & ((Mess & 1) != 0)));
  }
 }
}


task RunLeft()
{
 while (true)
 {
  SetPower (OUT_A, abs(Left));
  if (Left > 0) OnFwd(OUT_A);
  if (Left < 0) OnRev(OUT_A);
  if (Left == 0) Off(OUT_A);
 }
}

task RunRight()
{
 while (true)
 {
  SetPower(OUT_C, abs(Right));
  if (Right > 0) OnFwd(OUT_C);
  if (Right < 0) OnRev(OUT_C);
  if (Right == 0) Off(OUT_C);
 }
}

task RunCenter()
{
 while (true)
 {
  SetPower (OUT_B, abs(Center));
  if (Left > 0) OnFwd(OUT_B);
  if (Left < 0) OnRev(OUT_B);
  if (Left == 0) Off(OUT_B);
 }
}

And on the FRC side:

void Set3RCXMotors(signed char Left, signed char Center, signed char Right)
{
 char Temp = 0;
 //Check Left value
 if (Left > 8)
  Left = 8;
 if (Left < -8)
  Left = -8;
 //Check Center value
 if (Center > 8)
  Left = 8;
 if (Center < -8)
  Left = -8;
 //Check Right value
 if (Right > 8)
  Left = 8;
 if (Right < -8)
  Left = -8;
 
 //Motor
 Temp = 0x40;
 //Direction
 Temp |= ((Left<0)? 0 : 0x10); //Ooooh! A Ternary!
 //Power
 Temp |= absdif(Left, 1) << 1;
 //Is On?
 Temp |= (Left != 0) & 1;
 SendRCXMessage(Temp);

 //Again for the center
 //Motor
 Temp = 0x80;
 //Direction
 Temp |= ((Center<0)? 0 : 0x10); //Ooooh! Another Ternary!
 //Power
 Temp |= absdif(Center, 1) << 1;
 //Is On?
 Temp |= (Center != 0) & 1;
 SendRCXMessage(Temp);

 //Again for the right
 //Motor
 Temp = 0xC0;
 //Direction
 Temp |= ((Right<0)? 0 : 0x10); //Ooooh! Another Ternary!
 //Power
 Temp |= absdif(Right, 1) << 1;
 //Is On?
 Temp |= (Right != 0) & 1;
 SendRCXMessage(Temp);
}

There are obviously a few bugs to work out (I wrote this on the fly!), but I think you can get the idea. (My favorite NQC IDE, BrixCC, can be found Here.) I'll give every one a few hours to chug through that.