subsys: add MODBUS subsystem

CODEOWNERS

@@ -574,6 +574,7 @@
 /subsys/mgmt/mcumgr/smp_udp.c             @aunsbjerg
 /subsys/mgmt/updatehub/                   @nandojve @otavio
 /subsys/mgmt/osdp/                        @cbsiddharth
+/subsys/modbus/                           @jfischer-no
 /subsys/net/buf.c                         @jukkar @jhedberg @tbursztyka @pfalcon
 /subsys/net/ip/                           @jukkar @tbursztyka @pfalcon
 /subsys/net/lib/                          @jukkar @tbursztyka @pfalcon

doc/reference/index.rst

@@ -23,6 +23,7 @@ API Reference
    logging/index.rst
    misc/index
    misc/data_structures.rst
+   modbus/index.rst
    networking/index.rst
    peripherals/index.rst
    power_management/index.rst

doc/reference/modbus/index.rst

@@ -0,0 +1,9 @@
+.. _modbus:
+
+MODBUS
+######
+
+.. toctree::
+   :maxdepth: 2
+
+   modbus.rst

doc/reference/modbus/modbus.rst

@@ -0,0 +1,43 @@
+.. _modbus_api:
+
+MODBUS
+######
+
+Modbus is an industrial messaging protocol. The protocol is specified
+for different types of networks or buses. Zephyr OS implementation
+supports communication over serial line and may be used
+with different physical interfaces, like RS485 or RS232.
+TCP support is not implemented directly, but there are helper functions
+to realize TCP support according to the application's needs.
+
+Modbus communication is based on client/server model.
+Only one client may be present on the bus. Client can communicate with several
+server devices. Server devices themselves are passive and must not send
+requests or unsolicited responses.
+Services requested by the client are specified by function codes (FCxx),
+and can be found in the specification or documentation of the API below.
+
+Zephyr RTOS implementation supports both client and server roles.
+
+More information about Modbus and Modbus RTU can be found on the website
+`MODBUS Protocol Specifications`_.
+
+Samples
+*******
+
+`modbus-rtu-server-sample` and `modbus-rtu-client-sample` give
+the possibility to try out RTU server and RTU client implementation with
+an evaluation board.
+
+`modbus-tcp-server-sample` is a simple Modbus TCP server.
+
+`modbus-gateway-sample` is an example how to build a TCP to serial line
+gateway with Zephyr OS.
+
+API Reference
+*************
+
+.. doxygengroup:: modbus
+   :project: Zephyr
+
+.. _`MODBUS Protocol Specifications`: https://www.modbus.org/specs.php

dts/bindings/misc/zephyr,modbus-serial.yaml

@@ -0,0 +1,27 @@
+# Copyright (c) 2020 PHYTEC Messtechnik GmbH
+# SPDX-License-Identifier: Apache-2.0
+
+description: Modbus over serial line device
+
+compatible: "zephyr,modbus-serial"
+
+include: uart-device.yaml
+
+properties:
+    de-gpios:
+      type: phandle-array
+      required: false
+      description: Driver enable pin.
+
+        Driver enable pin (DE) of the RS-485 transceiver.
+        If connected directly the MCU pin should be configured
+        as active high.
+
+    re-gpios:
+      type: phandle-array
+      required: false
+      description: Receiver enable pin.
+
+        Receiver enable pin (nRE) of the RS-485 transceiver.
+        If connected directly the MCU pin should be configured
+        as active low.