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Continuous null character received in sim900

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Newbie level 5
Sep 13, 2012
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This is first time, I am working with sim900. When I power on the module, I receive continuous null character in hyper terminal.I set baud rate 9600 in PC. I don't know what is the problem in that.

give any advice regarding this ..

thanks in advance.

hi sathshya

I m not working on sim900, is sim900 uses usart communication?

Hi, first upalod the schematic. What are the states of Netlite and status LED.

I power ON the module and wait some seconds.then,When I give 1 sec pulse to PWRKEY pin through SW, status led is glow.But netlite led is not glow. I already inserted Sim card.

hi Dhakeparag81,

I connected sim900 with PC hyper terminal . And check the at commands and I faced the above problem(previous post)

give suggestion for this issue.
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did you check your connection rx pin goes to tx of pc and vice versa
if so,
then you might be didn't initialize correctly
set the baud rate properly
if parity is consider the check whether it is odd or even

same type of problem i faced in programming pic18f
in which i didnt initialize the serial port and i get continuous null character
then by setting TXEN transmit enable bit to 0 at starting i m done.

Hi sathshya,

go to the link **broken link removed** and read Table19. It states all about netlite. Also see figure 22 in the document. Are the connections made to the module are exactly the same. If you are not using hardware flow control Just connect TXD, RXD and Ground pins. Also if check that the VRTC pin is not open. It should not be open as per document. If you have not connected RTC battery connect it to ground (0V).

Hi dhakeparag81,
According to sim900 datasheet , module already set in auto baud rate configuration.
If we need to change baud rate means send AT+IPR command and set the baud rate.But my problem is sim900 send continuous null character so that I didn't send any command and it didn't response also..

Hi babar_ali,
As per the reference circuit,I connected the Netlite pin. My doubt is, when power ON the module, Netlite automatically glow or Do I send any AT command?. or Will module register automatically in the network?

I am not using hardware flow control so i connected TXD ,RXD, GND pin only

In datasheet,VRTC pin can be keep open (page 16).

advice me
thanks in advance...

Hi sathshya,

When ever you power on the module Netlite automatically starts blinking weather its gets the network or not.

One thing you should tell me that is this a demo board of SIM900 or do you have made the PCB by your self? If you made the PCB by yourself did you kept in ming he keep out area? Are there enough decoupling capcitors at the power supply?

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