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SeppoTakaloSeppoTakalo
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app: Allow CMUX to be set up separately from PPP
Add new scripts to manage CMUX * sm2_start_cmux.sh * sm2_stop_cmux.sh Add new parameter to PPP script '-C' that allow attaching to already running CMUX session. Then connection is teared down, the CMUX session is left running. This combination can be used with NTN as the CMUX does not need to be teared down when swithing between normal and NTN networks. Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>
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‎app/scripts/sm2_start_cmux.sh‎

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#!/bin/bash -eu
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#
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# Copyright (c) 2026 Nordic Semiconductor ASA
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#
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# SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
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#
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# Script to set up CMUX multiplexing on Serial Modem application 2.0.0 or
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# later using standard 3GPP AT commands (AT+CMUX=0).
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#
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# Use this script to initialize CMUX independently from PPP. Once CMUX is
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# running, start and stop PPP using sm2_start_ppp.sh -C and sm_stop_ppp.sh
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# without tearing down CMUX each time.
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#
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# Use sm2_stop_cmux.sh to fully close CMUX and release the serial port.
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#
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# Uses the following DLC channels in CMUX:
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# - 1: PPP data channel
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# - 2: AT commands channel
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# - 3: Modem trace collection (optional)
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#
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#
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# Default parameters
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#
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MODEM=/dev/ttyACM0
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BAUD=115200
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IPR_BAUD=0
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VERBOSE=0
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CHATOPT=""
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MODEM_TRACE_FILE="/var/log/nrf91-modem-trace.bin"
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TRACE_PID_FILE="/var/run/nrf91-modem-trace.pid"
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TRACE=0
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usage() {
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echo "Usage: $0 [-s serial_port] [-b baud_rate] [-B new_speed] [-T] [-v] [-h]"
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echo ""
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echo " -s serial_port : Serial port where the modem is connected (default: $MODEM)"
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echo " -b baud_rate : Current baud rate of Serial Modem (default: $BAUD)"
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echo " -B new_speed : Use AT+IPR to change baud rate to <new_speed>"
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echo " Start with current baud rate and switch to new_speed after modem"
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echo " is responsive. If not set, baud rate will not be changed."
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echo " -T : Enable modem trace collection (file: $MODEM_TRACE_FILE)"
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echo " -v : Enable verbose output"
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echo " -h : Show this help message"
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echo ""
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exit 0
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}
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# Parse command line parameters
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while getopts s:b:B:Tvh flag
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do
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case "${flag}" in
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s) MODEM=${OPTARG};;
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b) BAUD=${OPTARG};;
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B) IPR_BAUD=${OPTARG};;
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T) TRACE=1;;
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v) VERBOSE=1; CHATOPT="-v";;
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h|?) usage;;
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esac
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done
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log_dbg() {
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if [ $VERBOSE -eq 1 ]; then
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logger --id=$$ -s "$@"
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fi
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}
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if [[ ! -c $MODEM ]]; then
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echo "Serial port not found: $MODEM"
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exit 1
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fi
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# Remove stale trace PID file if process is not running
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if [ -f "$TRACE_PID_FILE" ]; then
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if ! kill -0 $(cat "$TRACE_PID_FILE" 2>/dev/null) 2>/dev/null; then
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log_dbg "Removing stale trace PID file: $TRACE_PID_FILE"
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rm -f "$TRACE_PID_FILE"
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fi
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fi
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if find /dev -type c -name 'gsmtty*' | grep -q . ; then
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echo "Error: existing CMUX devices found (/dev/gsmtty*)"
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exit 1
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fi
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if pgrep ldattach >/dev/null; then
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echo "Error: existing ldattach process found"
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exit 1
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fi
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cmux_close() {
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printf "\xF9\xF9\xF9\xF9\xF9\xF9\xF9\xF9" > $MODEM
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printf "\xF9\x03\xEF\x05\xC3\x01\xF2\xF9" > $MODEM
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sleep 2
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}
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cleanup() {
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set +eu
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start-stop-daemon --stop --pidfile $TRACE_PID_FILE --remove-pidfile --oknodo
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pkill ldattach
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cmux_close
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echo "Failed to start CMUX..."
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exit 1
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}
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trap cleanup ERR
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# Configure serial port
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stty -F $MODEM $BAUD pass8 raw crtscts clocal
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log_dbg "Wait modem to boot"
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if chat -t1 "Ready--" "AT" "OK" <$MODEM >$MODEM; then
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log_dbg "Modem is in AT mode"
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else
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log_dbg "Modem not responding, try CMUX close down..."
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cmux_close
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if ! chat -t1 "" "AT" "OK" <$MODEM >$MODEM; then
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echo "Error: Modem not responding"
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exit 1
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fi
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fi
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if [ $IPR_BAUD -ne 0 ]; then
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log_dbg "Set baud rate on modem to $IPR_BAUD"
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chat $CHATOPT -t1 '' "AT+IPR=$IPR_BAUD" "OK" >$MODEM <$MODEM
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# Reconfigure serial port
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stty -F $MODEM $IPR_BAUD pass8 raw crtscts clocal
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fi
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log_dbg "Attach CMUX channel to modem..."
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chat $CHATOPT -t1 '' "AT+CMUX=0" "OK" >$MODEM <$MODEM
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ldattach GSM0710 $MODEM
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sleep 3
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# AT_CMUX is the channel where setup commands will be sent and converted into PPP
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AT_CMUX=$(ls /dev/gsmtty* | sort -V | head -n 1)
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# AT_CMUX_USER is the AT channel that host can use after PPP is set up
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AT_CMUX_USER=$(ls /dev/gsmtty* | sort -V | head -n 2 | tail -n 1)
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echo "AT CMUX: $AT_CMUX_USER"
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echo "PPP CMUX: $AT_CMUX"
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MT_CMUX=""
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if [ $TRACE -gt 0 ]; then
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MT_CMUX=$(ls /dev/gsmtty* | sort -V | head -n 3 | tail -n 1)
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log_dbg "Trace CMUX: $MT_CMUX"
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log_dbg "Trace file: $MODEM_TRACE_FILE"
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stty -F $MT_CMUX raw clocal -icrnl -ixon -opost
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fi
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stty -F $AT_CMUX clocal
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test -c $AT_CMUX
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if [ $TRACE -gt 0 ]; then
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log_dbg "Starting trace collection..."
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chat $CHATOPT -t1 '' 'AT#XCMUXTRACE=3' 'OK' >$AT_CMUX <$AT_CMUX
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# Prefer to use socat, if installed.
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if command -v socat >/dev/null 2>&1; then
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start-stop-daemon --start --pidfile $TRACE_PID_FILE --make-pidfile \
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--background --exec $(command -v socat) -- -u $MT_CMUX,cfmakeraw,clocal=1 \
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CREATE:$MODEM_TRACE_FILE
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else
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start-stop-daemon --start --pidfile $TRACE_PID_FILE --make-pidfile \
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--background --exec /bin/dd -- if=$MT_CMUX of=$MODEM_TRACE_FILE bs=1024
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fi
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fi
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log_dbg "CMUX started"

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