SIM808 return the same AT command 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 OK response:

&gt;&gt; AT
&lt;&lt; AT
&lt;&lt; OK


Echoing can be turned off with the command ATE0 and turned back on with ATE1:

&gt;&gt; ATE0
&lt;&lt; ATE0
&lt;&lt; OK
&gt;&gt; AT
&lt;&lt; OK
&gt;&gt; ATE1
&lt;&lt; OK
&gt;&gt; AT
&lt;&lt; AT
&lt;&lt; OK