Hi @silvia ,
Thank you for your kind response.
@iamromulan @bean.wang-q @Kyson if you have any insights on the following please share.
Let’s get to the bottom of how the RM551E-GL presents itself to a host computer via PCI, what interfaces are available via PCI for network and AT commands, PCI-USB dual use, and if any UART adaptor card exists.
Presentation as PCI device how?
Regarding what PCI device the RM551-GL presents itself as to the host computer, I am 100% confused.
One suggestion from Google, AI, and some forum post, is that the RM551E-GL behaves like a Realtek 8125 2.5gbps Ethernet controller, i.e. so the host computer believes it’s a Realtek 8125 Ethernet controller that is connected to the M.2 PCIe slot. Screenshot of Google saying this:
I would guess more likely the above is not correct, but is an incorrect interpretation of the RM551E’s ability to be connected to a real Realtek 8125 Ethernet chip, but I am not sure. Someone needs to confirm. It would make all sense in the world that Quectel with the intention of reducing host computer driver requirements as much as possible, would present to the host computer as a Realtek 8125 chip. Then however comes the question, how do you make AT commands to this Quectel device, if the TCP port 1555 AT command interface has not been set up separately yet.
Some other source suggests that there will be a “wwan0” device on the Linux host computer, which derives from the QMI driver, plus a /dev/cdc-wdm0 “control device”.
The Linux kernel module name for this wwan function is qmi_wwan, and whether it is loaded is shown through lsmod | grep qmi_wwan.
Some sources appear to say that recent Linux versions will bundle a QMI driver that is good enough to drive the RM551E-GL already, and other sources say you need to download, build and install Quectel’s QMI module separately.
Some sources say that the RM551E-GL will presents itself to the host computer as a PCI-based USB controller chip, which then has one USB port, and on this USB port is a USB serial connection, which leads to the AT command interface on the RM551E-GL. Is that any real at all or misinformation?
Thanks for clarifying that RM551E-GL will not support MBIM over PCI.
Using the RM551E-GL concurrently via PCI and USB
You can’t use these at the same time.
Can you check with your expert team if it is even not possible to do AT commands via USB serial, while doing network via QMI PCI?
The AI thinks concurrent USB and PCI is possble and also the recommended way to use the modem. But it can be wrong of course.
URL https:// www.perplexity.ai/search/with-a-quectel-rm551e-gl-m-2-5-75AXNDfsSneg58PKvdTSRw . It says:
To issue AT commands to the Quectel RM551E-GL 5G modem connected via the PCIe M.2 slot in QMI mode on Linux, you generally need to use the USB interface of the modem for AT command communication, even when data networking is done through the PCIe interface.
Key points and setup approach:
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The RM551E-GL modem supports a dual interface mode where USB and PCIe are active simultaneously; USB is typically used for control/AT commands and PCIe for data (e.g., Ethernet-like WWAN device)
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The modem exposes a QMI PCI interface for data (handled by the Linux qmi_wwan driver, presenting wwanX network devices and /dev/cdc-wdmX device nodes for QMI communication). However, this interface does not directly support AT commands.
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AT commands are typically sent over USB serial ports created by the modem, such as /dev/ttyUSB0. This serial device is accessible when the USB interface of the Quectel module is recognized on Linux (visible via lsusb).
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To access AT commands when using the PCIe data interface:
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Connect to the modem’s USB serial port (over USB interface).
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Use a terminal program (e.g., minicom or screen) on /dev/ttyUSB0 or similar device.
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Send AT commands there.
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To enable and configure the QMI interface on Linux: …
^ Note the Perplexity response says clearly that the RM551E-GL MUST be connected via USB and PCI concurrently.
The dual bus USB-PCI card I show above, should only be sold because many Quectel models support the concurrent use of USB and PCI - if this use is not possible why do they sell it at all?
An alternative approach would be to connect the RM551E-GL via UART and PCI (M.2) concurrently, but I cannot find any adaptor cards available which have M.2 and UART. I find USB and UART, and I find gigabit Ethernet and UART, but no M.2.
The information I find says that the UART operates at 115200 bps and contains an AT command interface. With UART, there would be no need to be connected via USB.
AT commands interface via PCI available, how??
The information about how to get an AT commands interface via PCI is extremely unclear.
It is clear that AT commands can be processed via a custom protocol over TCP port 1555, but I think this might work only when the modem is connected to an actual Realtek 8125 chip and not when it’s connected to a host computer via PCI.
I have read some source that says Quectel’s Linux QMI driver contains functions to provide a virtual USB serial connection to the host computer, which serve an AT commands interface, but I’m not sure if this is real.
Furthermore I notice that there are reports of people accidentally bricking their Quectel M.2 modules when trying to make them work, for example because some AT commands are irreversible. Therefore I hope these points are clear before I configure, so I don’t brick the module one or many times due to unclarity.
Many thanks
