How does the serial(RS232) transmission work in Simulink?

I want to send numerical values to my PIC microcontroller via Simulink. With the most common configurations(baud=9600, databits=8, parity =none, stop bits=1). I only receive the same value in the 65..127 range. Anything other than those values, and i get strange values for ex. (tx=0 rx=-192). I reckon this has to do with the ASCII encoding,but i don't understand why and how it affects my transmission?

Could somebody clarify the method of transmission in Simulink? I'm using Serial Configuration, Serial Send and Serial Receive blocks from the Instrument Control Toolbox