Interfacing GSM Modem

I made it by luck !! i wanna understand why ??

I can't recall if it's mark or space but 5V is just high enough for RS232.

ahmedragia21 said:

Im using 89S52, 8051 architecture, but guys why RTS,DTS should be tied to 5V ?

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I think RTS and DTR, not RTS and DTS ?
I am facing same problem with you, could you please post the pin connection here ?

RTS is for Request To Send, it means if buffer at receiver empty, so transmitter can transmit data. It can be set '1' or reset '0' depends on buffer status.
DTR is for status connection, and must be set '1'.

If your modem can disable this handshake, you may use 3 wires only: TX, RX and GND.