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I'm having a bit of a conundrum. I bought a SIM800c development board off of ebay but the pins are not labeled at all and I have no idea which pins Rx and Tx are. I really want to hook it up to my COM port and test it out with putty. In case you weren't aware, these GSM modules communicate via the UART in AT commands.

Anyways I do have some helpful hints to solve this mystery. Here is a link to the item that I purchased.

https://www.ebay.com/itm/202041471362

You can see a picture of it on there. I also found what looks like my module along with a pinout->

https://www.itead.cc/wiki/RPI_SIM800_GSM/GPRS_ADD-ON_V2.0#Pinmap

Its not exactly the same as the one that I bought, but its the best thing I was able to dig up. I'm not sure if this is the correct pinout?

I tried hooking it up to some pins labeled "Test pins". There were pins for power and ground, along with two pins, one labeled R and the other labeled T. I thought those must be it, but when I test the AT commands in putty, there is no response from the GSM module. I'm pretty sure these can't be the right pins.

Please someone with knowledge, can you tell me which pins are Rx and Tx? If you have a schematic, please share.

One more thing I'd like to add is that I tried contacting the seller on ebay, but they told me "They are only a customer service and they don't know anything technical. Please research the internet."

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    \$\begingroup\$ A good lesson learned. Don't buy something if you can't get the manufacturer's datasheet. \$\endgroup\$ – Elliot Alderson Nov 19 '19 at 13:51
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This is probably not the answer you are looking for, but those 2 boards are not the same. There are many different versions of the SIM800c breakout boards, and that means you cannot say for sure whether pinouts are the same with these 2.

This bit of the answer you may enjoy less, although it is good news in that you can figure it out for yourself! You can look up the DATASHEET for the actual module and get a pinout of the module. Using a multimeter set to continuity mode, you can test each pin until you get a beep. You can then draw your own diagram of a pinout for that board. If you wanted, you could then upload it here as your own answer in case anybody else has a similar issue with this board.

If you are only after the Tx/Rx pins for now, then those are just pins 1 & 2 so shouldn't take too long!

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  • \$\begingroup\$ Thanks this was definitely helpful advice! I found out that this module has no connection between any of its pins and Rx and Tx. Its broken. Luckily, I have a spare module and this one definitely has Rx and Tx pins (confirmed with DMM). I will test that one later. Thanks! \$\endgroup\$ – user3308807 Nov 19 '19 at 22:47
  • \$\begingroup\$ @user3308807 No problem, it's always a better idea to get modules from somewhere that provides a datasheet or application guide, eBay can be cheap, but it's full of undocumented parts, faulty parts and even fake parts, so it is always a risk. If the answer helped you and solved your issue, don't forget to click the tick to mark it as accepted, or you can wait and see if anyone else comes in with a better answer. \$\endgroup\$ – MCG Nov 20 '19 at 8:22

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