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Co-authored-by: Divya S Pillai <91891495+divipillai@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: 3 people

app/scripts/sm2_stop_cmux.sh

@@ -52,12 +52,12 @@ log_dbg() {
 	fi
 }
 
-AT_CMUX=$(ls /dev/gsmtty* 2>/dev/null | sort -V | head -n 1 || true)
+AT_CMUX=$(ls /dev/gsmtty* 2>/dev/null | sort -V | head -n 2 | tail -n 1 || true)
 
 # Stop modem trace collection if running
 start-stop-daemon --stop --pidfile $TRACE_PID_FILE --remove-pidfile --oknodo --retry 1
 
-# Send CMUX CLD frame via AT command on channel 1 if available
+# Send CMUX CLD frame via AT command on the AT command channel (DLC 2) if available
 if [ -n "$AT_CMUX" ] && [ -c "$AT_CMUX" ]; then
 	log_dbg "Sending AT#XCMUXCLD on $AT_CMUX"
 	chat $CHATOPT -t5 '' 'AT#XCMUXCLD' 'OK' >$AT_CMUX <$AT_CMUX || true

doc/app/sm_cellular_modem.rst

@@ -195,9 +195,9 @@ PPP cannot function while the modem is in NTN mode, so it must be stopped and re
 Option A manages CMUX and PPP as a single lifecycle: stopping PPP also issues ``AT+CFUN=0`` and tears down CMUX.
 For NTN scenarios, use the separate CMUX management scripts so that CMUX stays alive across PPP restarts:
 
-* :file:`sm2_start_cmux.sh` — starts CMUX only, then exits. The ``gsmtty`` channels remain open.
-* :file:`sm2_stop_cmux.sh` — stops CMUX and releases the serial port.
-* :file:`sm2_start_ppp.sh` with the ``-C`` flag — attaches PPP to an already-running CMUX. When PPP stops, the modem and CMUX are left untouched.
+* :file:`sm2_start_cmux.sh` - Starts CMUX only, then exits. The ``gsmtty`` channels remain open.
+* :file:`sm2_stop_cmux.sh` - Stops CMUX and releases the serial port.
+* :file:`sm2_start_ppp.sh` with the ``-C`` flag - attaches PPP to an already-running CMUX. When PPP stops, the modem and CMUX are left untouched.
 
 When PPP is not running, all CMUX channels (``gsmtty1`` and ``gsmtty2``) are available for AT commands.
 This allows the host to reconfigure the modem (for example, switch profiles) through the AT channel while PPP is inactive.