Im currently doing a project and was told to serial monitor my GSM Sim900a module to make sure the connection works. The USB to TTL converter I used is a PL2303 module. The GSM module is not connected to any board. The only connection used are the cross connection of the Tx and Rx pins and also the power and ground pins.
So the problem I faced is that, After typing AT command, it is supposed to reply OK but replies AT instead. When I used a different baud rate, it replies with an inverted '?' or sometimes just an empty space.
Could be a short-circuit of TX/RX somewhere. Is everything you send echoed back?
I've checked the connections, there are no connection problems. I've checked the jumper wire I used to connect the TX to RX and vice versa. Yes everything is echoed back, some are blank spaces that can't be seen but when it echoes 'AT' or '?', I can see that there were blank spaces.
Having thought about this, a short-circuit would echo back regardless of baud rate.
Post a link to your module and maybe someone who has used one of these might be able to help.
I suspect that your GSM module is not in 'command mode' and is just echoing everything it receives. For some devices, sending "+++" will switch it to command mode. Try that and then see if it responds to "AT" with "OK". On other devices there is a pin you have to pull low or high. Look at the user's manual for the device.
