Autodetect modem in Linux?

Question

Is there a way to autodetect modem devices in Linux and then symlink them to /dev/modem using a udev rule?

Currently, I wrote a program that probes each serial port, sending AT and waiting for OK. I have a udev script that runs this for every ttyS and ttyUSB device. However, I've now been informed that there's other devices, such as OBD-II diagnostic bridges, that also speak with AT commands.

Answer 1

You could play a little with cts, rts or other serial status lines and check if the modem responds to lowering/raising that signal correspondingly. But your modem may be configured to ignore these lines, or there might be other devices that also react to those changes.

I think psusi got the correct answer.

I remember I once debugged hylafax to get the idea how it derived the type of modem. Basically it did the same as you did - send AT-commands and check the "typical" answer.

Answer 2

Detect the properties of each serial port. Look at the vendor and model strings. For example,

# udevadm info --query="property" --name=/dev/ttyUSB0**

UDEV_LOG=3
DEVPATH=/devices/platform/orion-ehci.0/usb1/1-1/1-1:1.0/ttyUSB0 /tty/ttyUSB0
MAJOR=188
MINOR=0
DEVNAME=/dev/ttyUSB0
SUBSYSTEM=tty
ID_PORT=0
ID_PATH=platform-orion-ehci.0-usb-0:1:1.0
ID_VENDOR=FTDI
ID_VENDOR_ENC=FTDI
ID_VENDOR_ID=0403
ID_MODEL=FT232R_USB_UART
ID_MODEL_ENC=FT232R\x20USB\x20UART
ID_MODEL_ID=6001
ID_REVISION=0600
ID_SERIAL=FTDI_FT232R_USB_UART_A40135O1
ID_SERIAL_SHORT=A40135O1
ID_TYPE=generic
ID_BUS=usb
ID_USB_INTERFACES=:ffffff:
ID_USB_INTERFACE_NUM=00
ID_USB_DRIVER=ftdi_sio
ID_IFACE=00
ID_VENDOR_FROM_DATABASE=Future Technology Devices International, Ltd
ID_MODEL_FROM_DATABASE=FT232 USB-Serial (UART) IC
DEVLINKS=/dev/char/188:0 /dev/serial/by-path/platform-orion-ehci.0-usb-0:1:1.0-port0 /dev/serial/by-id/usb-FTDI_FT232R_USB_UART_A40135O1-if00-port0