How to send and receive SMS with Raspberry Pi automation

The test of this post used a Raspberry Pi 4, the board inside the TouchBerry Pi 10.1" panel PC, and a SIM7600E-H 4G HAT (LTE Cat-4, 4G, 3G and 2G, with GNSS) to send an SMS to a mobile phone and read the answer. At the end you have a Python script that sends an alarm by SMS.

GSM, GPRS, LTE and SMS in short

An SMS needs only network coverage and a SIM with SMS service, not a data plan. That is why it is still used for alarms in remote sites.

Connect the SIM7600E-H 4G HAT to the Raspberry Pi

Insert the SIM card in the HAT, connect the LTE antenna, and connect the USB port of the HAT to a USB port of the Raspberry Pi. Set the jumpers of the HAT as its user manual says for USB use.

Over USB the HAT creates several /dev/ttyUSB ports, and one of them takes AT commands. In our test it was /dev/ttyUSB2. The serial port option of raspi-config is only needed if you connect the HAT through the UART pins of the GPIO header instead of USB.

Open the AT port and check the modem

Install screen, list the ports and open the AT port:

sudo apt update
sudo apt install screen
ls /dev/ttyUSB*
screen /dev/ttyUSB2 115200

Type each command and press Enter. The modem answers OK or ERROR. To leave screen, press Ctrl+A and then K.

Send an SMS with AT commands

Set text mode, give the number in international format and write the text after the > prompt. End the text with Ctrl+Z, not with Enter. The modem answers with +CMGS: and a reference number when the network accepts the message.

AT+CMGF=1
OK
AT+CMGS="+34600000000"
> Type your text here
+CMGS: 12

OK

Read the received SMS

AT+CMGL lists the messages stored in the SIM or in the modem. In text mode it takes a filter: "REC UNREAD", "REC READ", "STO UNSENT", "STO SENT" or "ALL".

AT+CMGF=1
OK
AT+CMGL="ALL"
+CMGL: 0,"REC READ","+346XXXXXXX6","","21/05/21,12:32:09+08"
HIGH Temperature

OK

Each message gives its index, its status, the sender, and the date and time. The last part of the time, +08, is the time zone in quarters of an hour: +08 is UTC+2.

• AT+CMGR=0 reads only the message with index 0.
• AT+CMGD=0 deletes it. Delete the messages that you have processed, because the SIM memory is small.
• AT+CNMI=2,1 makes the modem announce each new message with +CMTI: "SM",<index>, so a program does not need to poll.

Send an SMS alarm from a Python script

In an automation, a program sends the SMS when an alarm starts. This script uses pyserial, checks the SIM and the network, and sends one message:

import time

import serial  # pyserial

PORT = "/dev/ttyUSB2"     # AT port of the SIM7600 over USB, check with: ls /dev/ttyUSB*
NUMBER = "+34600000000"   # destination in international format
TEXT = "PLC alarm: high temperature in line 2"


def read_until(ser, tokens, timeout):
    """Read the modem answer until one of the tokens arrives or the time runs out."""
    end = time.monotonic() + timeout
    answer = b""
    while time.monotonic() < end:
        answer += ser.read(ser.in_waiting or 1)
        if any(token in answer for token in tokens):
            break
    return answer.decode(errors="replace").strip()


def command(ser, cmd, wait_for=b"OK", timeout=5):
    ser.write(cmd.encode() + b"\r")
    return read_until(ser, (wait_for, b"ERROR"), timeout)


with serial.Serial(PORT, 115200, timeout=0.2) as ser:
    print(command(ser, "AT"))
    print(command(ser, "AT+CPIN?"))    # +CPIN: READY means the SIM is unlocked
    print(command(ser, "AT+CREG?"))    # +CREG: 0,1 (home) or 0,5 (roaming) means registered
    print(command(ser, "AT+CMGF=1"))   # SMS text mode
    print(command(ser, f'AT+CMGS="{NUMBER}"', wait_for=b">"))
    ser.write(TEXT.encode() + b"\x1a")  # Ctrl+Z (0x1A) ends the text and sends it
    print(read_until(ser, (b"OK", b"ERROR"), 60))  # +CMGS: <reference> and OK

sudo apt install python3-serial
python3 send_sms.py

Output against our test modem:

OK
+CPIN: READY

OK
+CREG: 0,1

OK
OK
>
+CMGS: 12

OK

If the SMS does not go out

SMS and 4G in an industrial project

SMS is a good channel for a few alarms when there is no internet. For data, the usual way is MQTT or HTTPS over the 4G data connection. Industrial Shields controllers have cellular options of their own: the Raspberry PLC takes LTE Cat 4 or NB-IoT/LTE-M expansion boards, the TouchBerry Pi takes a 4G LTE expansion, and the ESP32 PLC can also connect over 4G. The SMS commands of 3GPP TS 27.005 are standard, so the sequence of this post is the same. The port name and the setup change with each module: check its user guide.

FAQ about SMS with a Raspberry Pi

Do I need a data plan to send SMS?

No. You need a SIM with SMS service and network coverage. A data plan is only needed for internet access.

Which ttyUSB port is the AT port?

In our test, /dev/ttyUSB2. If it does not answer, try the other ttyUSB ports with AT.

Why does the message not send when I press Enter?

Because in text mode the modem waits for Ctrl+Z (0x1A) to end the text. Enter only adds a new line to the message. Esc cancels it.

Can a received SMS start an action in the PLC?

Yes. A program reads the new messages with AT+CMGL or +CMTI, checks the sender and the text, and then acts. Accept commands only from known numbers.

Text mode or PDU mode?

Text mode is easier to read and to test. PDU mode is needed for special cases, such as long messages in several parts or alphabets outside the GSM alphabet.

More about cellular communication

• Node-RED tutorial: how to send AT commands with a Raspberry PLC
• How to work with the Raspberry Pi GPRS module
• How to use 4G with the ESP32 PLC
• Choosing an industrial controller

See the Raspberry PLC family and the TouchBerry Pi 10.1" panel PC.