This python application manages a cellular modem. These are the supported modem modules:

• Simcom SIM5360E (3G European & some other areas)
• Simcom SIM5360? (3G North American & some other areas)
• Simcom SIM7600x (4G Cat 1, as well as 2G & 3G. The x is for the various models, see Simcom datasheet)

Those modules are embedded in two products offered by Victron, the GX GSM and the GX LTE 4G.

The Simcom is connected to the Venus device via USB. The module presents as multiple tty devices in Linux: two functionally identical AT command / data interfaces and one for GPS NMEA messages.

In Venus, one AT interface is managed by pppd to provide the data connection. The other is used by this dbus-modem application.

A fix in the kernel is required, see the commits titled USB: serial: option: blacklist sendsetup on SIM5218 in the various machine branches on https://github.com/victronenergy/linux. And see also venus-private/wiki/kernel-config.

To cope with lockups or other issues, there is a watchdog IC watching over the Simcom. Its configured such that both the startup delay and subsequent normal delay are 60s. Note that in practice its more, and probably varies. We've seen 90s. Its output is connected to the reset pin on the Simcom.

The reset is on a GPIO of the Simcom:

• WD_WDI to GPIO44

For both pins we store a default config to the modem eeprom, see below.

To wait, or not to wait, for the first edge? If anything goes wrong in between the modem powering up, and the script sending the first edge, the mechanism will be stuck. Hence we decided to have the hardware watchdog active from the start. Not waiting for the first edge.

The following read-only values are exported under the com.victronenergy.modem service:

The SimStatus value is either (if less than 1000) an error code as defined by 3GPP TS 27.007 section 9.2 or (1000 and higher) a status code per the table below.

The RegStatus value is the status code returned by the +CREG command defined by 3GPP TS 27.007 section 7.2.

The following localsettings values are used. These are monitored and changes acted upon.

When the data connection is active, it is configured with a high routing metric. This way, the Linux kernel prioritises Ethernet or Wifi when these are available. A dnsmasq proxy forwards DNS lookups to the correct name server on Ethernet/Wifi or mobile data. See various recipes in meta-victronenergy as well as serial-starter in meta-victronenergy-private, for details.

No preparation of the modems is needed. This script, dbus-modem, always configures it (GPIO44, the watchdog reset)