> ## Documentation Index
> Fetch the complete documentation index at: https://docs.hologram.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Introduction to AT commands

> Learn AT command basics and operators (read, set, test) for modem control.

## AT command basics

AT (Attention) commands are used to communicate with cellular modems via a serial interface.

<Steps>
  <Step title="Connect to your modem">
    Use a terminal (e.g., Screen, Tera Term, PuTTY, GTKTerm) to open the modem's serial port. Send `AT` and expect `OK`.
  </Step>

  <Step title="Understand operators">
    * **? (read)**: Query current value, e.g., `AT+CFUN?`.
    * **= (set)**: Set value, e.g., `AT+CFUN=1`.
    * **=? (test)**: Show supported options, e.g., `AT+COPS=?`.
  </Step>
</Steps>

<Frame>
  <img src="https://mintcdn.com/hologram-b29df2a6/gBXuuJ66IPyI_nhw/images/troubleshooting/terminal-at-basics.png?fit=max&auto=format&n=gBXuuJ66IPyI_nhw&q=85&s=30fb33558de319d54949cd654f0069dd" alt="Terminal screenshot running AT basics" width="2608" height="1566" data-path="images/troubleshooting/terminal-at-basics.png" />
</Frame>

<Note>
  **Note:** You can also run the automated diagnostics script if preferred.
</Note>

<Card title="Automated modem diagnostics using Python" href="/guides/troubleshooting/automated-modem-diagnostics-using-python" horizontal />