Checking the RCIF/TXIF or RC1IF/TX1IF (for the 18Fseries) flags I think is enough to see if the UART is ready to send or has received a byte.
Of course the checks and byte read/write to the UART are not free and need your precious time each.
Maybe you have to move to a higher grade PIC like the 24F or even on the 32bit ones but then you also have to say good bye to PBP.
Ioannis
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