I am new to Raspberry Pi Zero and I want to connect it with a GSM module for GPRS connectivity. I bought the SIM800A GSM module but its LED is not even blinking one single time.

I am able to turn on the UART on RpiZero and connect 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?

  • This is not answerable unless you explain how you connected everything together. For instance, are you sure your SIM800A is powered? Commented Oct 11, 2019 at 11:28
  • 1
    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
    Commented Oct 11, 2019 at 18:36
  • @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
    Commented Oct 12, 2019 at 2:34
  • @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
    Commented Oct 12, 2019 at 2:41
  • Where are you supplying the from? Commented Oct 24, 2019 at 9:47

1 Answer 1



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

(4) Do not use VNC or SSH (Ref 7, 8)

If you are using SSH with Win10, you do't need to use puTTY. But if you are using an old PC, you might need to use puTTY. This might confuse things, for two reasons:

(a) You might use Win10 puTTY to SSH your Rpi, but then you need to use another Rpi puTTY to send serial AT commands to SIM800. This is very error prone,

(b) When sending/receiving AT commands, you might see control charatcters such as "Carriage Return, Line Feed (CR/LF), or "NewLine" etc. This control code might not pass through SSH wires. So I suggest for now you do NOT use SSH, and use keyboard/mouse/mon directly connected to rpi, whose UART directly connected to SIM800. This way troubleshooting should be easier, especially for newbies! :)

(5) Very Slow 2G/GSM/GPRS, or fast 3G/4G/LTE, or super fast 5G/NR?

The OP asks if he should buy a new card. My immediate comment is that if he has already bought the old SIM800 GSM/GPRS card, he should at least try it and learn the basic things. But then afterwards, he might like to consider the newer 4G/LTE modules, or like me, wait until 5G coming to town. I guess 4G/LTE/5G modules should also use the good old UART serial AT commands, so the tricks discussed here should still be useful in 5G times.

cellular network

gsm lte speed

(6) Setting up Rpi stretch python for testing SIM8000 AT command

SIM800 communicates using I2C, SPI, USB, and UART protocol. This answer only takes care of the Rpi serial port AT command set part (Warning: SIM800 has an USB connector which is only for firmware update and debugging, should not be used for send/received AT commands).

I usually recommend to test the SIM800 UART AT command set operation by two big steps:

(1) Use a terminal emulator, such as Win10 puTTY, TeraTerm, or RealTerm to talk to SIM800. Only when terminal emulator AT/OK/Ready command testing is OK, then go to second step.

(2) Use Rpi UART (WARNING: may need to convert to 5V signal, IF not using USB to TTL 5V adapter/cable (See Ref 15). Appendix C below has a fully debugged, complete, minimal, verifiable, plug and play, with sample outputs, python test program to test Rpi UART set up. It is only after Rpi loop > back testing is good then real Rpi to SIM800 should begin.

/ to continue, ...


(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

(7) SSH: Remote control your Rpi from Win - The MagPi 2017

(8) VNC: Remote Access A Rpi - MagPi 2017

(9) Cellular Network 1G/2G/3G/4G/5G Generations

/ to continue, ...


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

Appnedix C - Testing Rpi3B+ UART TxD/Rxd 9600 baud 8N1 loopback/repeat send bytes for SIM800 and similar UART modules with AT Command Set

# Rpi3B+ serial port loop back test v2.0 tlfong01 2019may27hkt2137 ***

# Rpi3B+ Raspbian stretch 9 (full version 2019april), IDLE python 3.5.3

# Program name = serial_loopback_2019may2702.py

# Description

# This program tests serial port loop back of
#   (1) Rpi built in UART/Serial Port, or
#   (2) USB/TTL Serial Adapter

# Rpi built in UART/Serial Port:
#   Devcie name = '/dev/serial0'
#   TxD = Rpi UART/serial port output pin = BCM GPIO pin #15  ( = Rpi 40 pin connector pin position #  8)
#   RxD = Rpi UART/serial port  input pin = BCM GPIO pin #16  ( = Rpi 40 pin connector pin position # 10)

# UART/TTL Serial Adapter/Cable ($ ls /dev/ttyUSB* to list USB/TTYserial port names)
#   Device name = '/dev/ttyUSB0'
#   Pins = Gnd, Tx, Rx, 3V3, 5V0 (3V3, 5V0 are power pins, for powering external devices) 

# Loopback hardwareware setup
#   Use a connecting wire / jumper wire / female to female DuPont connector to connect TxD pin to RxD pin

# Program execution example
# 1. Desktop GUI python 3 IDLE
#    Copy program to any directory under Rpi home directory, eg /home/python_programs  
#    File menu > save as > serial_loopback_2019may26x1.py (or Ctrl+S, or Ctrl+Shift+S)
#    Run  menu > run > run module (or F5)
# 2, Terminal mode
#    To add later

# Sample output
 RESTART: /home/pi/Python Programs/Python_Programs/test1200/serial_loopback_2019may2702.py 
        Begin serialPortLoopBack() [Remember to connect Tx to Rx!] , ...
        bytes written =  b'AT\r\n'
        bytes read    =  b'AT\r\n'
        End   serialPortLoopBack(), ...

from   time import sleep
import serial

# *** Setup serial port and set baud rate functions ***

def setSerialPortBaudRate(serialPort, baudRate):
    serialPort.baudrate = baudRate

# *** Serial port write and read bytes functions ***

def serialPortWriteBytes(serialPort, writeBytes):

def serialPortReadBytes(serialPort, maxBytesLength):
    readBytes = serialPort.read(maxBytesLength)
    return readBytes

def serialPortWriteWaitReadBytes(serialPort, writeBytes, maxBytesLength, waitSeconds):
    readBytes = serialPortReadBytes(serialPort, maxBytesLength)
    print('        bytes written = ', writeBytes) 
    print('        bytes read    = ', readBytes)
    return readBytes

# *** Test functions ***

def testSerailPortRepeatWriteBytes(serialPort, writeBytes, betweenBytePauseSeconds, repeatCount):
    print('       Begin repeatWriteOneByte(), ...')   
    for i in range(repeatCount):
        serialPortWriteBytes(serialPort, writeBytes)                
    print('       End   repeatWriteOneByte().')

def testSerialPortLoopBack(serialPort, writeBytes, maxBytesLength, waitSeconds): 
    print('        Begin serialPortLoopBack() [Remember to connect Tx to Rx!] , ...')
    serialPortWriteWaitReadBytes(serialPort, writeBytes, maxBytesLength, waitSeconds)     
    print('        End   serialPortLoopBack(), ...')

# *** Setup serial port and set baud rate ***

deviceName = '/dev/serial0' # For Rpi built UART/Serial (BCM GPIO pins 14, 15)
# deviceName = '/dev/ttyUSB0' # For USB/UART adapter/cable

serialPort0 = serial.Serial(port = deviceName,
        baudrate = 9600,
        parity = serial.PARITY_NONE,
        stopbits = serial.STOPBITS_ONE,
        bytesize = serial.EIGHTBITS,
        timeout= 1)

setSerialPortBaudRate(serialPort = serialPort0, baudRate = 9600)

# *** Main Tests ***

#Test serial port repeat write bytes ***
#testSerailPortRepeatWriteBytes(serialPort = serialPort0, writeBytes = b'0x55', \
#                               betweenBytePauseSeconds = 0.005, repeatCount = 200000000)

# Test serial loop back ***
testSerialPortLoopBack(serialPort = serialPort0, writeBytes = b'AT\r\n', maxBytesLength = 32, waitSeconds = 0.01)

# End

/ to continue, ...

Not the answer you're looking for? Browse other questions tagged or ask your own question.