How to Send AT Commands to EC21 Module While in QMI Mode? (quectel-cm vs AT mode)

IoT Modules GSM Module
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I am working with a Quectel EC21 module on a Linux system. I am currently facing an issue regarding AT command access depending on the modem mode:

  • When the module is operating in QMI mode (using quectel-cm for internet connection), I am unable to send AT commands to the module. It seems that the AT port is either unavailable or non-responsive during this mode.
  • When I switch the module to AT mode, I am able to send AT commands successfully, but I lose internet connectivity through quectel-cm.

My questions are:

  1. s it possible to send AT commands to the EC21 module while it is in QMI mode?*
  • If yes, what is the recommended method or procedure to do this?
  • Are there any specific ports or device nodes I should use for AT command access while QMI is active?
  1. Is it possible to disable QMI mode and use the module in AT command mode only, while still maintaining internet connectivity?
  • If so, how can this be achieved? Is there a way to configure the driver or the module for this?
  1. Is this limitation related to the Linux driver, or is it a hardware/firmware restriction on the EC21 module?

Any advice, experience, or documentation related to sending AT commands in QMI mode, or switching modes for full AT access, would be greatly appreciated.

1 Like
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What is the QMI mode you mean? I think you were using USB port as connection,correct?
Usually, EC21 has seperate port for network connection and AT command. There is no confliction. You mentioned quectel-cm, then you should use Linux OS for your host. The AT port should be /dev/ttyUSB2 if there is no other USB device is connected.

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Hi,

The situation you’re experiencing with the EC21 is a classic combination of the common “connecting to the wrong port / other services blocking the port / USB composition” trio.

Normally, it’s possible to send ATs while QMI is enabled. There are also separate ttyUSB ports for AT, while QMI goes through cdc-wdmX (qmi_wwan). You should use the correct AT port and ensure that services like ModemManager aren’t blocking the port.

While data is being transmitted via QMI, connect to the idle “AT” or “Modem” port, stop/blacklist ModemManager, and set the correct raw_ip setting.

QMI control channel: /dev/cdc-wdm0 (iface: qmi_wwan)

AT/Modem ports: typically /dev/ttyUSB2 (Primary AT), /dev/ttyUSB3 (Secondary AT); some FWs may have one or two additional ports.

GNSS/NMEA and DIAG ports may also appear as /dev/ttyUSB0/1/4.

If you want “QMI disabled, only internet via AT”: Use PPP (ATD*99#) or ECM/MBIM USB network modes, but not QMI and PPP simultaneously.

  • QMI: /dev/cdc-wdm0 (+ wwan0)
  • AT: /dev/ttyUSB2 (Primary) and/or /dev/ttyUSB3 (Secondary) – AT+QUSBPORT?