Can you check the PIC is running at 4MHz? And that the trim byte is correctly set (my code may be using the wrong value).

Try pulling GP4 high with a 10k resister (it should use the internal weak pull-up but...)

>1.9V power to the callipers is ok?
As long as the scale works at that voltage and the output signal is about 1.5v peak, it should be OK.

If there is a scale connected, Zx should respond withRx: ( the colon : is used as the line terminator) and Ix should respond with Ix,[mode number e.g. 0],[vernier type string Decimal or Binary], [measuement type Inch or MM]:

Without a scale it should send Nack (15h) each time the watchdog times out.

Have you tried it with a scale connected?

Bill