GSM Modem/Module not responding to AT commands after firmware Upgrade??

   by Dost Muhammad Shah    137 Comments  →
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I get a lot of queries about this issue, so I thought I should write a small post about it. For example the latest one was

Hello Muhammad,
I recently update SIM900 to firmware 1137B12SIM900M64_ST.cla using “Simcom – sim900 Customer flash loader V1.01″ at the baud rate 1498000. Everything went perfectly like but after I restart the module I get “þIIIIþþþ” response from SIM900 at 115200 baud rate, I could not get any AT commands working. Kindly do help me.

Some modems support AutoBaud by default. Auto Baud feature allows the modem to be used with any baud rate, and for this to be used the Modem waits for a input string “ATr” it uses this string to detect the baud rate being used. As the manual describes the modem when in Auto Baud will be looking for this string after it is powered and this should be in CAPS otherwise modem wont be able to detect the correct baud rate. Sim900 has the auto baud by default. Some modems don not support Auto Baud so for these modems after the firmware has been updated, you should use the default baud rate, 9600 and 115200 are the most common.

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if you want to fix or change the baud rate you should use the AT+IPR command. For example if you need to fix it to 4800.


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  1. Hi, i have a problem with SIM900 S2-1040S-Z1K18… i had simulate Serial port by arduino and with 19200 of baudrate when i switch on the SIM900 i get right answer: RDY +CFUN: 1 +CPIN: READY Call Ready *PSUTTZ: 2016, 10, 2, 8, 39, 3, “+8”, 1 DST: 1 But if i try to send an AT command i don’t get an aswer, or a feedback that the command was sent. i tried with different power, by arduino or external source, change pin… i don’t know the reason, maybe to high bauderate? but ii can change it if the modem doesn’t receive my commands? it could be possibile that serial pin rx is burned, by a change rx tx? Thanks Reply
  2. Hi Sir, I am using Sim900A.. handling http at commands using user guide..Even AT+HTTPACTION command replied +HTTPACTION: 0, 200 ,10when server is ok.Some time 601 or 604 error came.but server and data package all are ok.when this error came, reset the at commands(ATZ) and also http process going on.but that error repeatedly..after reset(power off and on) the module only working. any idea about without reset the module. Reply
  3. I am facing an issues my sim808 was working fine, but after few days it stop responding AT command, it do send a text message on my cell but not responding my AT command i need to send data our GPRS will you please guide me how i can troubleshoot it. Reply
    • if it sends SMS that means the AT commands are accepted. May be there is issue in your serial port setup or reading from serial port… Reply
  4. hi sir thank u very much for your respond. my device info, we are connected gsm900a rx to aurduino rx and gsm900a tx to arduino uno tx, and connected gsm ground to aurduino uno ground. i supplyed 12v adapter power to gsm module, and arduino uno connected to my laptop with usb. iam calling to gsm module its ringing ,but does not responds my at commands on serial monitor of aurduino studio. my commands ofter typing AT command, doesnot showing any data on my serial monitor, also iam trying different commands
    also iam trying below on ofter changing myconnections, rx to tx and tx to rx of aurduino uno and gsm and ground to ground arduino uno code
    void setup()
        void loop()
        bool bOK = false;
        while (Serial.available() > 0)
        char inChar = (char);
        bOK = true;
        Serial.println("AT+CMGF=1"); // sets the SMS mode to text
        bool bOK = false;
        while (Serial.available() > 0) {
        char inChar = (char);
        bOK = true;
        Serial.print("AT+CMGS=\""); // send the SMS number
        Serial.print("hi  this is test from aurduino ."); // SMS body
    pls give me the proper sollution for this. thank u Reply
  5. hi have using sim900a for aurduino uno r3 the bhoard is working but not responding AT COMMANDS FOR message send and reciving purpus pls give me the sollution for that and reasion Reply
    • does it reply to other AT commands? Please share the log of the commands you send and the replies and the initialization commands you use to setup the modem. Most probably it might be because the modem is set to PDU mode for SMS but I can’t be sure of that until you provide more details. Did you use AT+CMGF=1 command? Reply
  6. Hello sir, I am using SIM808 module from I have inserted SIM properly and given required power from the adapter and also connected with USB, It shows me “Silicon Labs CP210x USBx to UART Bridge (COM2)” in device manager in computer. But when I give AT command it does not reply me anything. I can make a call to SIM and can hear tone, but It does not respond to AT commands. Please help me out Reply
    • you need to setup the serial port correctly. Rx->Tx and Tx->Rx. Furthermore you have to send AT\r so that the modem knows what baud rate your are trying to communicate. Once you receive back OK you can fix the baud rate by using AT+IPR=x command Reply
  7. my gsm module is not working with putty terminal as i have used 12 v and 1 ampere supply . I am getting the phone call over it but its not responding to AT commands . plzz help me out with this. Reply
  8. Hey there i have a SIM808 (with GPS GPRS and Bluetooth) which is not talking at all. How can I reflash the firmware? Reply
      1. Were you able to communicate before?
      Most probably there wont be problems with firmware. There can be issues with baud rate settings or communication setup! Reply
      • no i wasn’t able to communicate. i tested different baudrates and the module should be still on self adjusting mode. the strange thing is, i only got one a ÿ character back and now never again. the module seems also to be running but i can’t get a connection Reply
        • When you realize that ÿ character, you should send back AT followed by carriage return as described in the post. Make sure AT IS IN CAPS Reply
  9. I am using SIM900A talky version. It has no reset pin on board. It was working fine until now but suddenly after powering it on, its getting hot and a red light is constantly glowing with continuous beep sound. no matter how many times i switch it on or off but still it doesnot accepts any AT commands. what to do now? where is the reset pin? can you atleast tell me the reset pin number in sim900A chip? Reply
    • when the modem is getting hot that is a sign of a damaged modem… I have several of them …. sadly “its dead”. Reply
  10. Good day, I got simcom sim800L and make all connections, run a simple test example of AT commands its Okay, now try to call or sent sms is not going while the AT command shows ready for call and sms. my gsm carriers are MTN nigeria and Airtel Nigeria. I need a helping hand i think of updating the firmware? Reply
    • You must be having some other problem and that you have to fix instead of doing a firmware upgrade. I can’t help unless you provide your code or commands and their responses . Reply
  11. Hi , Iam facing problems with sim900 module. Iam not getting any response from it. I tried calling to the sim card which is inserted into it.But not receiving any ringing tone. Reply
      • Hi, i am also facing the same problem where i do not get a response from the module and yes the module is power up correctly. I have also tried sending AT command and i am getting symbols as replies. Reply
        • symbols as response !! Can you share a screenshot? seems to be problem elated to baud rate mismatch… Please provide the following dettails
        • baudrate
        • exact symbols you get
        • Reply
          • ÿ ÿ ÿ shows that the modem is in auto baud rate mode. You should send AT\r a number of times until you get OK from the modem. (DO NOT FORGET TO SEND THE COMMAND AT IN CAPS).
          • Hi, I have tried you suggestion of sending AT\r commands to the modem but i am getting the same response and sometimes no response at all. If it helps i think the problem with my modem is the baud rate because it was working fine until i change the baud rate through arduino coding and i do not know how to change the baud rate back. Is there a way to change the baud rate back or reset the whole modem?
  12. Hi, I am interfacing sim900a to kinetis controller vi rs232. The controller sends at command to gsm modem ,but doesn not receive anything from gsm modem..I tried connecting gsm modem to pc and it works fine.also ,the controller is also able to send and receive data. Kindly let me know what could be the issue Reply
  13. Hi. Thank you for replying to my post, my query is will sim900A_revisoion 1137B14SIM900A32_ST work for HTTP commands… bcoz when i issue the HTTP commands it is returning is happening in all of the SIM900A modems with the same firmwar Reply
  14. Hi. Thank you for replying to my post, my query is will sim900A_revisoion 1137B14SIM900A32_ST work for HTTP commands… bcoz when i issue the HTTP commands it is returning is happening in all of the SIM900A modems with the same firmware Reply
  15. Hi, Very glad of you providing information about SIM900 modems.I was just struck with upgrading a SIM900A series gsm modem to SIM 900 modem Firstly, the revision read by SIM900A modem reads as follows AT+GSV SIMCOM_Ltd SIMCOM_SIM900A Revision:1137B14SIM900A32_ST As i was unaware of this revision, I loaded the firmware of SIM 900 which is named as follows 1137B09SIM900M64 normal_ST Since then the modem stops responding,at all the baud rates I’ve tried.. I suspect the revison above is 32 and below i flashed is of can i get it back to normal state.. And In this SIM900A_32 revison the HTTP commands are giving errors,pls help me out .. Thank you Reply
  16. SIM900A stop responding after hours of standby is there any way to reset it by commands when it hangs i am sending hourly data to FTP server but after 3 to 4 hours it stops reponding i have to restart my controller and gsm both i have cheked controller is working it have two uart one is responding other is tied to gsm sim900a please sir let me know is there any way to reset by commands Reply
  17. HI i Have updated SIM900A with i am getting continues blinking of network status led with garbage data on TX after updateting firmware (64bit older was 32) i have tried all available baud rate to get what exectly transmitting from modem but i am getting only YYYYYYY Reply
    • You need to match the 64M and 32M softwares as they represent the amount of memory and often wont work interchangeably. From your case it seems it worked. Your modem is now in autobaud mode. So switch off modem-> switch on –> send “AT\r” command a few times. It will recognize your baud rate Reply
  18. Hi i have sim900a module. it has tx rx g 5v pins. i connected them with arduino using software serial. pins 10,11. i am able to send AT commands. call sms everything works. but i dont get any response for any of the commands. is it possible that the tx of gsm board is not working? Reply
  19. Hi, I have to communicate a HMI with a GSM modem. I tried communicating using hyper terminal from PC and I get perfect replies from the modem. However, when I send commands from HMI to the modem,I receive the same given input command in the output as well. Request to please help. Reply
  20. I ‘am also trying to flash SIM900A, mine one got folowing release AT+GSV SIMCOM_Ltd SIMCOM_SIM900A Revision:1137B07SIM900A32_ST And doesn’t work here in Italy. Module does not connect to the GSM net. All the flashing tool report now this error: 00’00 “000 – 24/12/2015 19:49:02 00’00 “000 – Opened COMM port 00’00 “000 – Power On / Reset Target 00’04 “672 – Sending RAM program 00’06 “375 – RAM program send completed 00’06 “391 – RAM program loaded. 00’06 “391 – Waiting for ACK RAM program. **00’06 “391 – ERROR RAM program ACK Not received! (0)*** what does it depends on? i’m stuck Reply
      • Thanks. Actually I kept down “NRESET” pin while clicking on “start download”, then i release pin 16, and download starts. Ok it works! But I couldn’t find a firmware working. All those I tried don’t work. System reports “Error The type of FLASH Does not match the firmware…”. It will likely depends on the fact that flash size is only 32Mb… any suggestion? Reply
  21. i have sim900a modem and i made a shield of it . during using it suddenly my vbat and ground giving connectivity and modem went hot . now its continuously giving VBAT and Ground Connectivity is that means my modem damaged ? Reply
    err 00 update option does not support this Flash
    Err 00 Error during get mindful device
    I tried using the program download Tools SIM900 Series Customer 1.8.exe It GENERATED
    00'00 "000 - 24/12/2015 19:49:02
    00'00 "000 - Opened COMM port
    00'00 "000 - Power On / Reset Target
    00'04 "672 - Sending RAM program
    00'06 "375 - RAM program send completed
    00'06 "391 - RAM program loaded.
    00'06 "391 - Waiting for ACK RAM program.
    00'06 "391 - RAM program ACK received.
    00'06 "500 - Flash Man ID: 0x01
    00'06 "500 - Dev Flash ID: 0x007E
    00'06 "500 - Flash Ext ID1: 0x0063
    00'06 "500 - Flash Ext ID2: 0x0001
    00'06 "500 - Flash Name:
    00'06 "500 - Manufacturer: Spansion
    00'06 "500 - Flash Size: 128 Mb
    00'06 "516 - ERROR The cla file does not match the Target List! (0) ------
    00'23 "922 - 24/12/2015 19:49:26
    • Hi, you have a Sim900A with Spansion memory chip. you should use firmware that is for this variant! As you already erased the Flash you need now to atleast flash with the previous version you had. Check this post for firmware with Spansion tag in the name . Reply
  23. I having one problem with 900.Max 232 becomes hot and also the LM317 (onboard) when i connected to supply.Power and network status are i changed the Max 232 smd ic.Now the lm317 and max 232 becomes normal,but i can’t communicate with pc.But Network registration is done. Reply
  24. Hello Muhammad. I need your help please, I bought a sim908 without board only the module, I designeda little board to connect this to the breadboard, at this moment the nethligh is flassing at 800 ms off, but the probel is AT commands, I mean, when turn on the module I send “AT” command, but the mdule response with the same command, I tried with “AAAAT” “A” or “ATr” but nothing happen, Ihave read the manual and say: “”DTR shall be pulled up to invalid state at least 2 seconds by DTE”, do you know what should Ido? Thanks in advance. Reply
      • Hello i am using SIM800 mounted on board module and i am having this issue, when i send AT from serial terminal i receive AT, any AT command which i am transmitting is echoing back to me, no command is having any effect. Also when i call on GSM module it sends “.RING.” to serial terminal also when i am sending msg to module i am also getting notified in some format which is correct according to datasheet. I can not find a way to make my at commands effecting please suggest something. Thanks in advance. Reply
  25. Hi I’ve tried flashing the SIM900A with a variety of different 64bit firmware versions (for both sim900 and sim900a). the flash completes successfully (as reported by the tool). I’m using an arduino nano as a USB-Serial controller (disabling the 328p chip by shorting RST to GND). In every case, without fail, after flashing the module (3.8.2 mini) remains in bootloader mode after rebooting – with both power and netlight flashing rapidly and continuously. Do you have any experience of breaking out of this loop? or is there perhaps a step in the flashing process that the loader does not intermediate? thanks Justin Reply
    • module (3.8.2 mini)
      What is that?
      with both power and netlight flashing rapidly and continuously.
      I havent experienced or came through any such situation . I’ll recommend to contact simcom support. Reply
      • thanks Dost. the board is just a break out module. cheap from china. with a max232 level shifter so that i can take 5v and 3.3v TTL i/o. i contacted sitcom support a few days before trying this board! no answer at all ;-( Reply
          • Hi, what are the labels with those leds? Can you check if your sim900A gets HOT??
          • the chip doesn’t get hot, no. I’ve tried feeding it with different voltages between 4 and 6v but not change in temperature. the silk screen for both LEDs just references a diode (Dn). however the LED at the edge of the board connects to pin 52 (netlight) and to a via that is labelled “status”. which is annoying as there is no connection to pin 66. the LED in the middle of the board is connected to pin 4 (RI). both have resistors in series. difficult to get my probe on them but I’d guess they are 1kΩ.
          • Applying power more than 4 volts is ging to damage the pin. I have no idea why the RI LED should blink like that……. 🙁

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