C51 Some problems

I mean a code running on my target.

The target port P5 initialized as an input port:
...
P5MDOUT = 0x00;
P5=0xFF;
...

Then there is such fragment in my program:
...
TXbuffer[2] = str[P5];
...,
where str[...] is an array of unsigned chars.
On P5 comes some determined signals.
As a result I have in TXbuffer[2] an transformed by str[] image of this input signal.
But when programm processing in the target without Debug Session I have constant in TXbuffer[2] while input signal on P5 are changes.
I detect it by reading TXbuffer[2] when realize CAN-interchange of data with my target.