P89LPC936 UART working on Keil MBC900 evaluation but not when using only VDD and VSS without the ev

Hi dear friends,
I am using P89LPC936 of philips for my project and i use keil MCB900 evaluationboard. When i test my projects on the MCB900 everything works perfect when sending data trough Rx and Tx pin on the P89LPC936(not after rs-232 tranciever). My GSM phone recives AT-commands perfectly but when i want to use the single chip on a sperate wrapboard using Vdd and Vss only UART does not seem to work properly. The bytes I read from the Rx and Tx seem to be correct but my GSM does not respond to it. I am totally stuck. Tried to change the output mode of the pins but no results. Any clue or help is really appreciated
Thank you,
Sarsam

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