Sounds simple,

I presume I only need one comparator so I would make CMCON = %100101
I would then connect RA1 (VIN-) to ground and feed RA2(VIN+) from RB3 (PWM signal)
I would connect my Tx to RA2 and pick another unsused pin (RB5)

I figure I would get the inverted signal from RA2 by reading CMCON.7

But how to I pass the value of CMCON.7 to RB5?

Or am I going the wrong way?

This is what works feeding just one pin of the Tx
Code:
Main:

;reset timer
hpwm 1, 127,40000       ;   Send burst, Ch1, D.Cycle50%, Freq 40Khz
pauseus 150             ;   length of burst.
CCP1CON = 0             ;   Turn off PWM
pause 100               ;   Wait to prevent picking up 
                        ;       noise generated by Tx
; detect timer overflow
; do the maths
; display results
GOTO Main

Mike