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Diffstat (limited to 'drivers/staging/fieldbus/Documentation/fieldbus_dev.txt')
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diff --git a/drivers/staging/fieldbus/Documentation/fieldbus_dev.txt b/drivers/staging/fieldbus/Documentation/fieldbus_dev.txt new file mode 100644 index 000000000..89fb8e146 --- /dev/null +++ b/drivers/staging/fieldbus/Documentation/fieldbus_dev.txt @@ -0,0 +1,66 @@ + 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: +drivers/staging/fieldbus/Documentation/ABI/sysfs-class-fieldbus-dev +drivers/staging/fieldbus/Documentation/ABI/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. |