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+                       Fieldbus-Device Subsystem
+               ============================================
+
+Part 0 - What is a Fieldbus Device ?
+------------------------------------
+
+Fieldbus is the name of a family of industrial computer network protocols used
+for real-time distributed control, standardized as IEC 61158.
+
+A complex automated industrial system -- such as manufacturing assembly line --
+usually needs a distributed control system -- an organized hierarchy of
+controller systems -- to function. In this hierarchy, there is usually a
+Human Machine Interface (HMI) at the top, where an operator can monitor or
+operate the system. This is typically linked to a middle layer of programmable
+logic controllers (PLC) via a non-time-critical communications system
+(e.g. Ethernet). At the bottom of the control chain is the fieldbus that links
+the PLCs to the components that actually do the work, such as sensors,
+actuators, electric motors, console lights, switches, valves and contactors.
+
+(Source: Wikipedia)
+
+A "Fieldbus Device" is such an actuator, motor, console light, switch, ...
+controlled via the Fieldbus by a PLC aka "Fieldbus Controller".
+
+Communication between PLC and device typically happens via process data memory,
+separated into input and output areas. The Fieldbus then cyclically transfers
+the PLC's output area to the device's input area, and vice versa.
+
+Part I - Why do we need this subsystem?
+---------------------------------------
+
+Fieldbus device (client) adapters are commercially available. They allow data
+exchange with a PLC aka "Fieldbus Controller" via process data memory.
+
+They are typically used when a Linux device wants to expose itself as an
+actuator, motor, console light, switch, etc. over the fieldbus.
+
+The purpose of this subsystem is:
+a) present a general, standardized, extensible API/ABI to userspace; and
+b) present a convenient interface to drivers.
+
+Part II - How can drivers use the subsystem?
+--------------------------------------------
+
+Any driver that wants to register as a Fieldbus Device should allocate and
+populate a 'struct fieldbus_dev' (from include/linux/fieldbus_dev.h).
+Registration then happens by calling fieldbus_dev_register().
+
+Part III - How can userspace use the subsystem?
+-----------------------------------------------
+
+Fieldbus protocols and adapters are diverse and varied. However, they share
+a limited few common behaviours and properties. This allows us to define
+a simple interface consisting of a character device and a set of sysfs files:
+
+See:
+Documentation/ABI/testing/sysfs-class-fieldbus-dev
+Documentation/ABI/testing/fieldbus-dev-cdev
+
+Note that this simple interface does not provide a way to modify adapter
+configuration settings. It is therefore useful only for adapters that get their
+configuration settings some other way, e.g. non-volatile memory on the adapter,
+through the network, ...
+
+At a later phase, this simple interface can easily co-exist with a future
+(netlink-based ?) configuration settings interface.