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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  • "works perfect when sending data trough Rx and Tx pin on the P89LPC936(not after rs-232 tranciever)."

    How do you know it is "perfect?"

    More likely, it is marginal and it just barely works - ie, it works by luck, not by design.
    Therefore it takes only one minor difference to tip it over the edge from "just barely working" to not working at all.

    Have you verified the voltage specs in the Manual for your GSM phone?

    Beware: some GSM modules use "unusual" voltages like 4.2V - you really must read the manual carefully, and design accordingly.

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  • "works perfect when sending data trough Rx and Tx pin on the P89LPC936(not after rs-232 tranciever)."

    How do you know it is "perfect?"

    More likely, it is marginal and it just barely works - ie, it works by luck, not by design.
    Therefore it takes only one minor difference to tip it over the edge from "just barely working" to not working at all.

    Have you verified the voltage specs in the Manual for your GSM phone?

    Beware: some GSM modules use "unusual" voltages like 4.2V - you really must read the manual carefully, and design accordingly.

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