I connect my SIM900A mini module with 4V. It’s LED is blinking fast about 1secons. When I send AT to the module it does not show anything it just show AT only. Why module does not sends OK responses.
I connected RXD 5v to TX and TXD 5v to Rx. It is not working for different boud rates.
I change coonnection RXD 5v to RX and TXD 5v to Tx. It is not working for different boud rates.
AT command send and not response only show AT in terminal.
Have a look at the hardware design guideline for the SIM800: https://cdn-shop.adafruit.com/datasheets/sim800h_hardware_design_v1.00.pdf. Although it is a different model, I think most of the info applies to the SIM900 as well.
In any case, on page 32 of that PDF it shows how to connect these “4v” devices to a 5v logic microcontroller. In my experience, the suggested circuit was the most stable way to connect the sim800 to a 5v Arduino, by far. It’s also relatively easy to do, just make shure to make all traces/cables as short as possible.
Besides that, you can try with 4800 baud or 9600 baud as starting points. Sometimes those are the default speeds for the serial port in the SIM900/800. Other times, it defaults to 115200, but depending on the device you are using, it might be to fast (an Arduino UNO for example does not have the speed to pull it off, I think).