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My SIM800L V2 RoHS (the blue one) although it connects to the network and registers just fine, it's not responding to AT commands sent over the PL2303 (USB-STC-ISP) USB-To-TTL Adapter through which it's hooked up to the PC, I used multiple Serial communication software/terminal (Putty, RealTerm, Termite, eltima serial port monitor...etc) and i made sure that every terminal would send using both NL and CR (CR/LF as it is labeled in some of them, basically "\r\n" at the end of each command) and i also tried communicating over most common baudrates (9600, 115200, 57600, 38400, 19200...etc) but none of them got a response so the terminal/software being used is not the problem, so i thought maybe the problem is with the PL2303 so i tested it using the loopback method (connecting Both the TX and the RX of the adapter to each other) and i got a response through Putty, i also connected the ground (GND) of the adapter to the second ground (GND) pin of the SIM800L module (as showen in the diagram below) but still nothing, so i'm not sure what the problem is exactly. I hope somebody can help me here, that would be very much appreciated, thanks in advance. enter image description here

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  • \$\begingroup\$ Did you try swapping TX & RX? \$\endgroup\$
    – brhans
    Sep 17, 2021 at 15:02
  • \$\begingroup\$ @brhans Although it didn't make since but i already tried that, and still no response. \$\endgroup\$
    – sMooDi
    Sep 17, 2021 at 15:34

2 Answers 2

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First thing to do is open the datasheet of the sim800l and ckeck that your board meets the requirements for the serial port. There are different recommended designs based on the voltage of the supply. Next, is your ac/dc adapter outputing the same voltage as the logic level of your serial to usb adapter?

From the datasheet again, the module might be in auto baudrate detection, in which case it wants to recieve "AT" before anything else.

I see two different ground sources connected together here, which is dangerous for ground loops. you could have damaged your device doing that depending on the design they did.

If everything checks out and your supply to the module is good, I would plug a logic analyser on tx and rx to see if anything happens there.

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  • \$\begingroup\$ The Gnd points are connected together on the board, this is not a problem. This is very typical for USB connected peripherals with power pack supplies. \$\endgroup\$ Sep 17, 2021 at 15:18
  • \$\begingroup\$ @AntoineLev thanks for the response, first, i couldn't find the datasheet for this model but based on bits of info i gathered from the internet, yes this sim800l meets the serial port requirements and already has a built-in regulator circuit and TTL level converter on board so it would work fine on 5v (btw the ac/dc adapter is 5v 5a), second for the auto baudrate, again no datasheet however the command i'm using to test it is the "AT" command, third for the ground loops thanks for warning me i didn't know that, but i tried connecting them both to the adapter ground but that didn't help..... \$\endgroup\$
    – sMooDi
    Sep 17, 2021 at 15:26
  • \$\begingroup\$ ....finally for the logic analyzer, i don't own one and don't know of any way i can get my hands on one atm, is there any other way i can test this? \$\endgroup\$
    – sMooDi
    Sep 17, 2021 at 15:27
  • \$\begingroup\$ @JackCreasey thx for the clarification, at first i didn't connect the gnd pins but then i read on a forum that they both need to have a common ground in order to be able to communicate properly. \$\endgroup\$
    – sMooDi
    Sep 17, 2021 at 15:29
  • \$\begingroup\$ as jack said the ground is probably ok, It was just things from the top of my head I would check in your position. Here is the datasheet : img.filipeflop.com/files/download/Datasheet_SIM800L.pdf \$\endgroup\$
    – ESD
    Sep 17, 2021 at 15:38
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SIM800L won't respond to AT commands

I'd suggest you do the following:

  1. Connect RxD and TxD on the USB-->serial adapter together (not connected to the SIM800L)
  2. Use whatever software you use on the PC to check the adapter operation. You have a loopback so anything you type in should be reflected on the screen.
  3. Ensure the power supply for the SIM800L is between 3.7 and 4.3V (it won't work at 3.3V and may catastrophically fail at 5V). The supply must be able to provide peak current of about 2A when connected to the network.
  4. Connect the serial connections (you appear to have them correct).
  5. Configure you serial connection to send CR/LF.
  6. Send atCR/F or ATCR/LF twice. You should get AT?CR/LF echoed back.

The modules are nearly universally shipped with autobaud turned on. It would be highly unlikely to see any other configuration. If you have EVER had the module supply connected to 5V, it is likely the module is burned. However, the serial pins are able to connect to both 3.3V and 5V serial pins without damage.

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  • \$\begingroup\$ I tried everything you mentioned, the loopback test, the power supply is 5v5a (I tried 5v2a but that wasn't enough current for some reason), this module is supposed to be able to handle 5v since it has built in regulator circuit, as for the commands you mentioned, those are the commands I've been testing with, and I tried sending both of them twice with no results. So basically the only two possible solutions I can think of are, first resetting the module through the reset pin, however I don't think that would do much.... \$\endgroup\$
    – sMooDi
    Sep 18, 2021 at 11:47
  • \$\begingroup\$ .... Second, buying a new one and testing if the current one is defective, but I'm not sure if only the tx and rx pins can suddenly start malfunctioning, I'm guessing if that happens it would affect the whole module and it wouldn't even turn on, but it's still a possibility. However there's a third situation that I can think of which is that I'm not connecting the module with the ttl USB adapter correctly, if anyone has any insight on that please elaborate. \$\endgroup\$
    – sMooDi
    Sep 18, 2021 at 12:05

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