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I am new to raspberry pi zero and I want to connect it with gsm module for gprs connectivity . I bought sim800a gsm module but its led not even blinking single time I am able to turn on the UART on pi zero and coonect it over usb cable through ssh . Can anyone help me out how to achieve what I want and also suggest a better gsm module for gprs if sim800a is not good enough

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  • This is not answerable unless you explain how you connected everything together. For instance, are you sure your SIM800A is powered? – Dmitry Grigoryev 16 hours ago
  • yes I am using 12v-2A power adapter to sim800a and without making any connection to my pi zero it’s network led should blink , SIM is working in cellphone fine. If led is not blinking then there is no point in checking it through pi zero – M.Shah 9 hours ago
  • @M.Shah, Ah, let me see. (1) Last question first. SIM800A is an old, but matured product. As such, it is a very good tool to learn, especially to build a solid background, before moving to 5G (coming next year to my city!). It is popular, therefore you can find forum discussions of user problems etc. Even if you have not already bought the thing, I would encourage you NEVER GIVE UP!!! – tlfong01 1 hour ago   
  • @M.Shah, your following comment is not very clear, “… I am able to turn on the UART on pi zero and connect it over usb cable through ssh”. WARNING: SIM800 USB socket is for firmware upgrade and debugging, NOT for normal operation using AT commends etc. As you have already set up RpiZ USB (to serial UART adapter/cable, I assume) So perhaps we can start and focus to the serial AT command thing first. I am drafting an answer for newbies. Let me know if there is anything you could not follow, and I will try to explain in more details. – tlfong01 1 hour ago   



How to use RpiZ USB to serial adapter/cable to connect to SIM800A, and starting talking AT?


(1) Introduction and Functional Block Diagram

First thing first, let us look at the functional block diagram, and make sure we know which “function” we are messing around. It is吖good idea to start with the simplest thing first, in our case, UART. Forget for now the newbie scary things I2C, SPI! 🙂

sim800 block diagram

(2) How can Rpi send AT serial commands to SIM800?

I read that SIM800 can talk with Rpi using serial AT commands. So perhaps we can start looking at the UART setup and any Hello World AT command shall we use.

sim800 rpi serial

(3) SIM800 Serial Communication Specification

Now I am read the manual to see how SIM800 serial looks like. It seems that SIM800 can do auto baudrate detect. In other words, if Rpi is set to No Parity, 1 stop bit (8N1) any speed, SIM800 should at power up, detect Rpi 9600 N81 “AT” command and responds with something like “OK” (See Ref 6).

sim800 serial spec

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(1) SIM800A Hardware Designer Guide V1.02 (English) – Shanghai SimCom Wireless Solutions 2016jun30

(2) SIM800A Hardware Designer Guide V1.02 (Simplified Chinese) – Shanghai SimCom Wireless Solutions 2016

(3) Amazon Raspberry Pi SIM800 GSM GPRS Module for SIM800 Quad-Band GSM/GPRS/BT Module – US$30

(4) LinkWare Pilot (3GL, 3G/HSPA, PPP/CDC-ECM/CDC-NCM) – £95

(5) LinkWare Pilot (3GL, 3G/HSPA) Forum Discussion

(6) SIM800 Serial Communication Problem Forum Discussion

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Appendix A – SIM800A Hardware Design Guide V1.02 (English) – Shanghai SimCom Wireless Solutions 2016jun30


Dual-band GSM/GPRS module that works on frequencies EGSM 900MHz and DCS 1800MHz

GPRS multi-slot class 12/ class 10

GPRS coding schemes CS-1, CS-2, CS-3 and CS-4.

Hardware interfaces

One UART port

One USB port for debugging and firmware upgrading

Audio channel which includes a microphone input and a receiver output

One SIM card interface

Support up to 4*5 Keypads

One display interface

One I2C master interface for peripheral management

Programmable GPIO

Two PWM output

One ADC input

Bluetooth antenna interface

GSM antenna interface

Communication Protocol

UART, TCP/IP, extended TCP/IP AT commands

Frequency bands

EGSM 900, DCS 1800

Can search the 4 frequency bands automatically.

Frequency bands can be set by AT command “AT+CBAND”.

GSM Phase 2/2+

GPRS connectivity

Multi-slot class 12 (default) Multi-slot class 1~12 (option)


Downlink transfer: max. 85.6 kbps

Uplink transfer: max. 85.6 kbps

Coding scheme: CS-1, CS-2, CS-3 and CS-4

Integrate the TCP/IP protocol.

Support Packet Broadcast Control Channel (PBCCH)


MT, MO, CB, Text and PDU mode

SMS storage: SIM card

Serial port

Full modem serial port

Can be used for AT commands or data stream

Support RTS/CTS hardware handshake

Comply with GSM 07.10 Multiplexer Protocol

Support auto baud detect from 1200 bps to 115,200bps


For debugging and upgrading firmware

Appendix B – LinkWare Pilot ((3GL, 3G/HSPA, PPP/CDC-ECM/CDC-NCM)


The PiloT is a HAT compliant board which is compatible with all versions of Raspberry Pi with 40 pin header, including Pi 2,3 and Zero models.

Fully controllable with I/O pins, the PiloT can communicate with the Pi over USB or serial ports.

Two versions are available – the PiloT 3GL, which offers hexaband 3G/HSPA for global coverage, with fallback to EDGE/GPRS, and SiRF V GNSS technology, and the PiloT 4G-1,which is an LTE CAT-1 device with fallback to 2G for Europe.

The PiloT can be fully controlled from the Raspberry Pi, and can support data communication using either PPP or CDC-ECM (PiloT 3GL) or CDC-NCM (PiloT 4G-1). Scripts to automate communication setup are available. (Recommend use of USB ports for Pi 3 as the main serial port is used for Bluetooth / Wi-Fi communication)

The PiloT is currently compatible with the following devices: Rpi2/3, B/B+, RpiZ/ZW

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