Can i directly connect a gsm shield to a computer without using arduino?
Either buy (or build) an RS232 to TTL UART converter. You can use something like the MAX232 to do that if you don't want to buy one. There are other options for RS232 to TTL, such as this Simple RS232 Converter which uses a couple of transistors arranged as:



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Or if you are connecting it to a computer, you can either buy (or build) a USB to TTL UART converter (e.g. FT232RL) and use that to connect to the module. Or if you are using an Arduino, you can program the ATMega to act as a simple bridge between the GSM UART pins and the USB-UART converter on the Arduino board.