Default Firmware (SLM)

Overview

SLM allows the LTE-M module to be used like a serial-based cellular modem using standard AT commands. This enables quick and easy cellular connectivity from a PC, MCU, or SBC (e.g. Raspberry Pi) without implementing a full LTE protocol stack.

What is Serial LTE Modem?

Serial LTE Modem (SLM) is firmware that turns the LTE module into a UART / USB controlled modem.

Key characteristics:

• AT command-based control

• No SDK or LTE stack required on the host

• Works with PC and external MCU

• Supports common networking features such as TCP, UDP, and MQTT

Getting Started

1. Hardware Connection

• Connect via USB or UART

  • SLM Commands

    • nRF9151-SOM: TX: GPIO 21, RX: GPIO 22

    • SOM Carrier Board: USB port

  • Debug Logs

    • nRF9151-SOM: TX: GPIO 10, RX: GPIO 11

    • SOM Carrier Board: TX: GPIO 10, RX: GPIO 11

• Default baud rate: 115200 bps

2. TCP and UDP AT Commands

289KB

nrf_connect_sdk_-_3_1_1_socket_at_commands.pdf

PDF

Open

You can find the TCP and UDP AT commands at this pdf file.

3. Testing Scenarios

Technical Documentationdocs.nordicsemi.com

You can find the testing scenarios at this link.

4. Practical Testing Scenarios

Initial output of SLM Commands Uart

Initial output of Debug Logs Uart

This is the initial screen displayed when SLM is powered on.

AT+COPS=1,2,"45005" // plmn select to SKT
AT+CFUN=1 // Set the device to full functionality
AT+CGATT? // Check the packet service attach status of the module
AT#XSOCKET=1,2,0 // Open IPv4 UDP Socket
AT#XCONNECT="echo.cocolinx.com",7777 // Connect to "echo.cocolinx.com" udp echo server
AT#XSEND="Hello World" // Send data to "echo.cocolinx.com"
AT#XRECV=3 // Receive data over the connection with timeout 3s

In this test, the following commands are executed in order.

As shown in the image above, after connecting to the SKT network, a socket is opened and a connection is made to a UDP echo server. When “Hello World” is sent, the same “Hello World” message is received in return.

If an error occurs, the following message will appear in the debug log.