Yes they run off different voltages with common ground. I have tested and my uC can send and receive.



Both EA and ES are enabled. I am not splitting the interrupts (SSTAT = 0x00)



In my ISR (which works with TI) I check which one is enabled (RI or TI)



No other interrupts are active.



REN is set. My SCON is 0x50 (Mode 1 UART, REN enabled)




I actually did get my code from Code Architect.


After doing some more testing I have discovered that if I continuously check for RI being enabled in my idle loop, it does get enabled when key is pressed. But if I take that out and try to let my ISR take care of it, it does not get detected.

Here is what my ISR looks like:

void uart_isr
  (void) interrupt 4 using 1
{
if (RI)
  {
    RI = 0;
    uartGet = SBUF;
    LCD_Command(0x0C,uartGet);
  } 

  if (TI)
  {
    TI = 0;
    mtxbusy = 0;
  }
}

If you are interested this is my UART transmit function that works fine.

void uart_transmit
  (unsigned char value)
{
  while(mtxbusy);

  mtxbusy = 1;
  SBUF = value;
}

Thank you for the tips though.