add nxmodbus doc and examples

Documentation/applications/examples/nxmbserver/index.rst

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+======================================
+``nxmbserver`` NxModbus Server Example
+======================================
+
+The ``nxmbserver`` example demonstrates how to create a Modbus server (slave)
+using the NxModbus protocol stack. It supports RTU, ASCII, and TCP transports
+with simulated register data.
+
+Register Map
+============
+
+The example server provides the following simulated registers:
+
+Coils (read/write, FC01/FC05/FC15):
+
+- Address range: 1-100
+- Initial state: All zeros
+- Writable via FC05 (write single) and FC15 (write multiple)
+
+Discrete Inputs (read-only, FC02):
+
+- Address range: 1-100
+- Initial state: All zeros
+- Read-only (cannot be modified by client)
+
+Input Registers (read-only, FC04):
+
+- Address range: 1-100
+- Initial values: ``register[i] = i * 10`` (register 1 = 10, register 2 = 20,
+  register 10 = 100)
+
+Holding Registers (read/write, FC03/FC06/FC16/FC23):
+
+- Address range: 1-100
+- Initial values: ``register[i] = i * 100`` (register 1 = 100, register 2 = 200,
+  register 10 = 1000)
+- Writable via FC06 (write single), FC16 (write multiple), FC23 (read/write)
+
+Command-Line Options
+====================
+
+Transport Selection (required):
+
+- ``-t TYPE`` - Transport type: ``rtu``, ``ascii``, or ``tcp``
+
+Serial Transport Options (RTU/ASCII):
+
+- ``-d DEVICE`` - Serial device path (e.g., ``/dev/ttyS1``)
+- ``-b BAUD`` - Baud rate (default: 115200)
+- ``-p PARITY`` - Parity: ``none``, ``even``, or ``odd`` (default: none)
+
+TCP Transport Options:
+
+- ``-a ADDR`` - Bind address (default: 0.0.0.0 - all interfaces)
+- ``-P PORT`` - TCP port (default: 502)
+
+Modbus Options:
+
+- ``-u UNIT`` - Unit ID / slave address (default: 1)
+
+Usage Examples
+==============
+
+RTU Server on Serial Port::
+
+    nsh> nxmbserver -t rtu -d /dev/ttyS1 -b 115200
+    Starting Modbus RTU server on /dev/ttyS1 (baud=115200, unit=1)
+    Server running. Press Ctrl+C to stop.
+    Register map:
+      Coils:          1-100 (read/write)
+      Discrete:       1-100 (read-only)
+      Input regs:     1-100 (read-only, value=addr*10)
+      Holding regs:   1-100 (read/write, initial=addr*100)
+
+TCP Server on Default Port::
+
+    nsh> nxmbserver -t tcp -P 502
+    Starting Modbus TCP server on port 502 (unit=1)
+    Server running. Press Ctrl+C to stop.
+    Register map:
+      Coils:          1-100 (read/write)
+      Discrete:       1-100 (read-only)
+      Input regs:     1-100 (read-only, value=addr*10)
+      Holding regs:   1-100 (read/write, initial=addr*100)
+
+ASCII Server with Even Parity::
+
+    nsh> nxmbserver -t ascii -d /dev/ttyS1 -b 9600 -p even -u 5
+    Starting Modbus ASCII server on /dev/ttyS1 (baud=9600, unit=5)
+    Server running. Press Ctrl+C to stop.
+    ...
+
+Testing with nxmbclient
+=======================
+
+The server can be tested using the ``nxmbclient`` tool:
+
+Read Input Registers (initial values)::
+
+    nsh> nxmbclient -t tcp -h 127.0.0.1 read-input 1 5
+    Read 5 input registers from address 1:
+    [1]: 10
+    [2]: 20
+    [3]: 30
+    [4]: 40
+    [5]: 50
+
+Read Holding Registers (initial values)::
+
+    nsh> nxmbclient -t tcp -h 127.0.0.1 read-holding 1 5
+    Read 5 holding registers from address 1:
+    [1]: 100
+    [2]: 200
+    [3]: 300
+    [4]: 400
+    [5]: 500
+
+Write and Read Back Holding Register::
+
+    nsh> nxmbclient -t tcp -h 127.0.0.1 write-holding 10 9999
+    Wrote holding register at address 10: 9999
+
+    nsh> nxmbclient -t tcp -h 127.0.0.1 read-holding 10 1
+    Read 1 holding register from address 10:
+    [10]: 9999
+
+Configuration
+=============
+
+Enable the example in your NuttX configuration::
+
+    CONFIG_EXAMPLES_NXMBSERVER=y
+    CONFIG_INDUSTRY_NXMODBUS=y
+    CONFIG_NXMODBUS_RTU=y      # For RTU support
+    CONFIG_NXMODBUS_ASCII=y    # For ASCII support
+    CONFIG_NXMODBUS_TCP=y      # For TCP support
+
+Kconfig Options:
+
+- ``CONFIG_EXAMPLES_NXMBSERVER`` - Enable the nxmbserver example
+- ``CONFIG_EXAMPLES_NXMBSERVER_PROGNAME`` - Program name (default: "nxmbserver")
+- ``CONFIG_EXAMPLES_NXMBSERVER_PRIORITY`` - Task priority (default: 100)
+- ``CONFIG_EXAMPLES_NXMBSERVER_STACKSIZE`` - Stack size (default: DEFAULT_TASK_STACKSIZE)
+
+See Also
+========
+
+- :doc:`/applications/industry/nxmodbus/index` - NxModbus protocol stack
+- :doc:`/applications/system/nxmbclient/index` - NxModbus client tool