PIC UART Interrupt Not Triggering
TRISB1 needs to be set to 1 to configure RB1 (RX) as an input.  I'm not sure what the default is, so it may be ok.

You need to clear the receive interrupt flag (RCIF) by reading the receive register (RCREG).  In addition, since the receive register is doubled-buffered, you may need to read it more than once.

So your interrupt routine needs to look more like this:

extern interrupt isr(void)
{
    while (RCIF)
    {
        char ch;

        RB5 = 1;
        ch = RCREG;    // normally would go into an array and increment a counter
    }
}


I don't know if that is your only problem, since you indicate you are not getting into the interrupt routine at all.  But the above is the correct way to read the characters out of the receive buffer.

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EDIT:

Don't know if this will help or not, but in this post, before enabling interrupts, the code clears out the FIFO first.  (Their code also clears out the RCIF flag, but since it is readonly on your chip, that isn't needed.)

ch = RCREG;    // clear FIFO  
ch = RCREG;
ch = RCREG;

// then enable interruupts ...