I wanna connect an existing device to PC using RS485/RS232. I have programmed the microcontroller correctly and it works when I haven't switch on the power of device. I mean after programming by PIC kit 2, I have tested it by connecting the related cable from TX pin to PC and have received the test messages of microcontroller by hyperterminal. but when I turn on the device I can not receive. I tested the output voltage of the TX pin, It shows 4.3v voltage. I used 1k pull-down resistor in this pin. the voltage removed but still I can not receive any message.
Suddenly one day, after programming the microcontroller and without disconnecting the in-circuit programming cable, I saw that the device is sending message. after checking all pins of PICKIt 2 programmer, I saw the pin GND of programmer is affecting the device and removes the voltage in pin Tx. I mean if I connect the GND pin of microcontroller to an external GND, It sends correctly the message but sometimes with a bit changes in some characters randomly like a noise.
briefly, there is a noise in Pin Tx of PIC microcontroller. How can I remove this noise when the device is connected to mains. I should mention that this device has some parallel boards: power supply board, MCU board and IO boards beside each other. the power supply board is very close to MCU board which including out microcontroller (around 2cm).
Should I use a bigger pull down resistor? Should I use capacitor in parallel with pull down resistor? which capacitor? Please guide me. I'm a EEE student working on a summer project. I appreciate a lot in advance.