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