I am using atmega 2560 and SIM808. When I send an AT command to SIM808 it returns correctly. But sometimes I have seen that SIM808 returns the same AT command. Sometimes it occurs due to lack of power in SIM808 and due to Serial problem. But another time I have seen that, power and Serial are ok but it returns the same AT command. Why this type of problem occurs ?
My default modems usually echo back whatever you send. So if you send
AT it will first respond with
AT followed by the
>> AT << AT << OK
Echoing can be turned off with the command
ATE0 and turned back on with
>> ATE0 << ATE0 << OK >> AT << OK >> ATE1 << OK >> AT << AT << OK