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+Discrete Driver
+###############
+
+The Discrete driver is responsible for providing an
+interface to Discrete Input/Outputs.  The capabilities provided
+by this class of device driver are:
+
+- Initialize a Discrete I/O Board
+
+- Open a Particular Discrete Bitfield
+
+- Close a Particular Discrete Bitfield
+
+- Read from a Particular Discrete Bitfield
+
+- Write to a Particular Discrete Bitfield
+
+- Reset DACs
+
+- Reinitialize DACS
+
+Most discrete I/O devices are found on I/O cards that support many
+bits of discrete I/O on a single card.  This driver model is centered
+on the notion of reading bitfields from the card.
+
+There are currently no discrete I/O device drivers included in the
+RTEMS source tree.  The information provided in this chapter
+is based on drivers developed for applications using RTEMS.
+It is hoped that this driver model information can form the
+discrete I/O driver model that can be supported in future RTEMS
+distribution.
+
+Major and Minor Numbers
+=======================
+
+The *major* number of a device driver is its index in the
+RTEMS Device Address Table.
+
+A *minor* number is associated with each device instance
+managed by a particular device driver.  An RTEMS minor number
+is an ``unsigned32`` entity.  Convention calls for
+dividing the bits in the minor number down into categories
+that specify a particular bitfield.  This results in categories
+like the following:
+
+- *board* - indicates the board a particular bitfield is located on
+
+- *word* - indicates the particular word of discrete bits the
+  bitfield is located within
+
+- *start* - indicates the starting bit of the bitfield
+
+- *width* - indicates the width of the bitfield
+
+From the above, it should be clear that a single device driver
+can support multiple copies of the same board in a single system.
+The minor number is used to distinguish the devices.
+
+By providing a way to easily access a particular bitfield from
+the device driver, the application is insulated with knowing how
+to mask fields in and out of a discrete I/O.
+
+Discrete I/O Driver Configuration
+=================================
+
+There is not a standard discrete I/O driver configuration table but some
+fields are common across different drivers.  The discrete I/O driver
+configuration table is typically an array of structures with each
+structure containing the information for a particular board.
+The following is a list of the type of information normally required
+to configure an discrete I/O board:
+
+*board_offset*
+    is the base address of a board.
+
+*relay_initial_values*
+    is an array of the values that should be written to each output
+    word on the board during initialization.  This allows the driver
+    to start with the board’s output  in a known state.
+
+Initialize a Discrete I/O Board
+===============================
+
+At system initialization, the discrete I/O driver’s initialization entry point
+will be invoked.  As part of initialization, the driver will perform
+whatever board initializatin is required and then set all
+outputs to their configured initial state.
+
+The discrete I/O driver may register a device name for bitfields of
+particular interest to the system.  Normally this will be restricted
+to the names of each word and, if the driver supports it, an "all words".
+
+Open a Particular Discrete Bitfield
+===================================
+
+This is the driver open call.  Usually this call does nothing other than
+validate the minor number.
+
+With some drivers, it may be necessary to allocate memory when a particular
+device is opened.  If that is the case, then this is often the place
+to do this operation.
+
+Close a Particular Discrete Bitfield
+====================================
+
+This is the driver close call.  Usually this call does nothing.
+
+With some drivers, it may be necessary to allocate memory when a particular
+device is opened.  If that is the case, then this is the place
+where that memory should be deallocated.
+
+Read from a Particular Discrete Bitfield
+========================================
+
+This corresponds to the driver read call.  After validating the minor
+number and arguments, this call reads the indicated bitfield.  A
+discrete I/O devices may have to store the last value written to
+a discrete output.  If the bitfield is output only, saving the last
+written value gives the appearance that it can be read from also.
+If the bitfield is input, then it is sampled.
+
+*NOTE:* Many discrete inputs have a tendency to bounce.  The application
+may have to take account for bounces.
+
+The value returned is an ``unsigned32`` number
+representing the bitfield read.  This value is stored in the``argument_block`` passed in to the call.
+
+*NOTE:* Some discrete I/O drivers have a special minor number
+used to access all discrete I/O bits on the board.  If this special
+minor is used, then the area pointed to by ``argument_block`` must
+be the correct size.
+
+Write to a Particular Discrete Bitfield
+=======================================
+
+This corresponds to the driver write call.  After validating the minor
+number and arguments, this call writes the indicated device.  If the
+specified device is an ADC, then an error is usually returned.
+
+The value written is an ``unsigned32`` number
+representing the value to be written to the specified
+bitfield.  This value is stored in the``argument_block`` passed in to the call.
+
+*NOTE:* Some discrete I/O drivers have a special minor number
+used to access all discrete I/O bits on the board.  If this special
+minor is used, then the area pointed to by ``argument_block`` must
+be the correct size.
+
+Disable Discrete Outputs
+========================
+
+This is one of the IOCTL functions supported by the I/O control
+device driver entry point.  When this IOCTL function is invoked,
+the discrete outputs are disabled.
+
+*NOTE:* It may not be possible to disable/enable discrete output on all
+discrete I/O boards.
+
+Enable Discrete Outputs
+=======================
+
+This is one of the IOCTL functions supported by the I/O control
+device driver entry point.  When this IOCTL function is invoked,
+the discrete outputs are enabled.
+
+*NOTE:* It may not be possible to disable/enable discrete output on all
+discrete I/O boards.
+
+Reinitialize Outputs
+====================
+
+This is one of the IOCTL functions supported by the I/O control
+device driver entry point.  When this IOCTL function is invoked,
+the discrete outputs are rewritten with the configured initial
+output values.
+
+Get Last Written Values
+=======================
+
+This is one of the IOCTL functions supported by the I/O control
+device driver entry point.  When this IOCTL function is invoked,
+the following information is returned to the caller:
+
+- last value written to the specified output word
+
+- timestamp of when the last write was performed
+