Hi,

Here is a portion from the PBP manual
Since the serial reception is done in hardware, it is not possible to set
the levels to an inverted state to eliminate an RS-232 driver. Therefore
a suitable driver should be used with HSERIN.
Any idea what this means ? I don't know or may be I do

Using the hardware pins with a software serial (inverted) works but makes no sense as anyway you don't use the hardware peripheral. So time to spend on MAX or be happy with soft serial.