Voltage on UART pins

i was checking the voltage of pins TX1,TX2,and TX3 while opening the port using Serial1.begin etc.What i found out is that the voltage of TX pins stays at 1.6V(approx),and if i use delay() function the voltage becomes 3.3V.I am not sure why this is happening.I checked the schematics of Arduino Due and cant find anything there.Any information is helpful.Thanks.
Bikash

What kind of data are you sending to the TX pin. What are you using to check it. Most multimeters will not work on serial data. You need a digital scope, instead.

@promacjoe i am sending Hex data to Tx pin (0xAA, 0xBB, 0x06, 0x00, 0x00, 0x00, 0x01, 0x01, 0x03, 0x03).Yeah i figured it out that digital multi-meter wont work,i need oscilloscope.But i am not sure if the Hex value i am sending is actually in Hex or just ASCII value.Thanks for your reply.

Serial.write for HEX
Serial.print for ASCII