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     ppbus — Parallel Port Bus system


     device ppbus

     device vpo

     device lpt
     device plip
     device ppi
     device pps
     device lpbb


     The ppbus system provides a uniform, modular and architecture-independent system for the
     implementation of drivers to control various parallel devices, and to utilize different
     parallel port chipsets.


     In order to write new drivers or port existing drivers, the ppbus system provides the
     following facilities:

              architecture-independent macros or functions to access parallel ports

              mechanism to allow various devices to share the same parallel port

              a user interface named ppi(4) that allows parallel port access from outside the
               kernel without conflicting with kernel-in drivers.

   Developing new drivers
     The ppbus system has been designed to support the development of standard and non-standard

     Driver    Description
     vpo       VPI0 parallel to Adaptec AIC-7110 SCSI controller driver.  It uses standard and
               non-standard parallel port accesses.
     ppi       Parallel port interface for general I/O
     pps       Pulse per second Timing Interface
     lpbb      Philips official parallel port I2C bit-banging interface

   Porting existing drivers
     Another approach to the ppbus system is to port existing drivers.  Various drivers have
     already been ported:

     Driver    Description
     lpt       lpt printer driver
     plip      lp parallel network interface driver

     ppbus should let you port any other software even from other operating systems that provide
     similar services.


     Parallel port chipset support is provided by ppc(4).

     The ppbus system provides functions and macros to allocate a new parallel port bus, then
     initialize it and upper peripheral device drivers.

     ppc makes chipset detection and initialization and then calls ppbus attach functions to
     initialize the ppbus system.


     The logical parallel port model chosen for the ppbus system is the PC's parallel port model.
     Consequently, for the i386 implementation of ppbus, most of the services provided by ppc are
     macros for inb() and outb() calls.  But, for an other architecture, accesses to one of our
     logical registers (data, status, control...) may require more than one I/O access.

     The parallel port may operate in the following modes:

              compatible mode, also called Centronics mode

              bidirectional 8/4-bits mode, also called NIBBLE mode

              byte mode, also called PS/2 mode

              Extended Capability Port mode, ECP

              Enhanced Parallel Port mode, EPP

              mixed ECP+EPP or ECP+PS/2 modes

   Compatible mode
     This mode defines the protocol used by most PCs to transfer data to a printer.  In this
     mode, data is placed on the port's data lines, the printer status is checked for no errors
     and that it is not busy, and then a data Strobe is generated by the software to clock the
     data to the printer.

     Many I/O controllers have implemented a mode that uses a FIFO buffer to transfer data with
     the Compatibility mode protocol.  This mode is referred to as "Fast Centronics" or "Parallel
     Port FIFO mode".

   Bidirectional mode
     The NIBBLE mode is the most common way to get reverse channel data from a printer or
     peripheral.  Combined with the standard host to printer mode, it provides a complete
     bidirectional channel.

     In this mode, outputs are 8-bits long.  Inputs are accomplished by reading 4 of the 8 bits
     of the status register.

   Byte mode
     In this mode, the data register is used either for outputs and inputs.  Then, any transfer
     is 8-bits long.

   Extended Capability Port mode
     The ECP protocol was proposed as an advanced mode for communication with printer and scanner
     type peripherals.  Like the EPP protocol, ECP mode provides for a high performance
     bidirectional communication path between the host adapter and the peripheral.

     ECP protocol features include:

           Run_Length_Encoding (RLE) data compression for host adapters

           FIFOs for both the forward and reverse channels

           DMA as well as programmed I/O for the host register interface.

   Enhanced Parallel Port mode
     The EPP protocol was originally developed as a means to provide a high performance parallel
     port link that would still be compatible with the standard parallel port.

     The EPP mode has two types of cycle: address and data.  What makes the difference at
     hardware level is the strobe of the byte placed on the data lines.  Data are strobed with
     nAutofeed, addresses are strobed with nSelectin signals.

     A particularity of the ISA implementation of the EPP protocol is that an EPP cycle fits in
     an ISA cycle.  In this fashion, parallel port peripherals can operate at close to the same
     performance levels as an equivalent ISA plug-in card.

     At software level, you may implement the protocol you wish, using data and address cycles as
     you want.  This is for the IEEE1284 compatible part.  Then, peripheral vendors may implement
     protocol handshake with the following status lines: PError, nFault and Select.  Try to know
     how these lines toggle with your peripheral, allowing the peripheral to request more data,
     stop the transfer and so on.

     At any time, the peripheral may interrupt the host with the nAck signal without disturbing
     the current transfer.

   Mixed modes
     Some manufacturers, like SMC, have implemented chipsets that support mixed modes.  With such
     chipsets, mode switching is available at any time by accessing the extended control

IEEE1284-1994 Standard

     This standard is also named "IEEE Standard Signaling Method for a Bidirectional Parallel
     Peripheral Interface for Personal Computers".  It defines a signaling method for
     asynchronous, fully interlocked, bidirectional parallel communications between hosts and
     printers or other peripherals.  It also specifies a format for a peripheral identification
     string and a method of returning this string to the host outside of the bidirectional data

     This standard is architecture independent and only specifies dialog handshake at signal
     level.  One should refer to architecture specific documentation in order to manipulate
     machine dependent registers, mapped memory or other methods to control these signals.

     The IEEE1284 protocol is fully oriented with all supported parallel port modes.  The
     computer acts as master and the peripheral as slave.

     Any transfer is defined as a finite state automaton.  It allows software to properly manage
     the fully interlocked scheme of the signaling method.  The compatible mode is supported "as
     is" without any negotiation because it is compatible.  Any other mode must be firstly
     negotiated by the host to check it is supported by the peripheral, then to enter one of the
     forward idle states.

     At any time, the slave may want to send data to the host.  This is only possible from
     forward idle states (nibble, byte, ecp...).  So, the host must have previously negotiated to
     permit the peripheral to request transfer.  Interrupt lines may be dedicated to the
     requesting signals to prevent time consuming polling methods.

     But peripheral requests are only a hint to the master host.  If the host accepts the
     transfer, it must firstly negotiate the reverse mode and then starts the transfer.  At any
     time during reverse transfer, the host may terminate the transfer or the slave may drive
     wires to signal that no more data is available.

     IEEE1284 Standard support has been implemented at the top of the ppbus system as a set of
     procedures that perform high level functions like negotiation, termination, transfer in any
     mode without bothering you with low level characteristics of the standard.

     IEEE1284 interacts with the ppbus system as little as possible.  That means you still have
     to request the ppbus when you want to access it, the negotiate function does not do it for
     you.  And of course, release it later.


   adapter, ppbus and device layers
     First, there is the adapter layer, the lowest of the ppbus system.  It provides chipset
     abstraction throw a set of low level functions that maps the logical model to the underlying

     Secondly, there is the ppbus layer that provides functions to:

           1.   share the parallel port bus among the daisy-chain like connected devices

           2.   manage devices linked to ppbus

           3.   propose an arch-independent interface to access the hardware layer.

     Finally, the device layer gathers the parallel peripheral device drivers.

   Parallel modes management
     We have to differentiate operating modes at various ppbus system layers.  Actually, ppbus
     and adapter operating modes on one hands and for each one, current and available modes are

     With this level of abstraction a particular chipset may commute from any native mode to any
     other mode emulated with extended modes without disturbing upper layers.  For example, most
     chipsets support NIBBLE mode as native and emulated with ECP and/or EPP.

     This architecture should support IEEE1284-1994 modes.


   The boot process
     The boot process starts with the probe stage of the ppc(4) driver during ISA bus (PC
     architecture) initialization.  During attachment of the ppc driver, a new ppbus structure is
     allocated, then probe and attachment for this new bus node are called.

     ppbus attachment tries to detect any PnP parallel peripheral (according to Plug and Play
     Parallel Port Devices draft from (c)1993-4 Microsoft Corporation) then probes and attaches
     known device drivers.

     During probe, device drivers are supposed to request the ppbus and try to set their
     operating mode.  This mode will be saved in the context structure and returned each time the
     driver requests the ppbus.

   Bus allocation and interrupts
     ppbus allocation is mandatory not to corrupt I/O of other devices.  Another usage of ppbus
     allocation is to reserve the port and receive incoming interrupts.

     High level interrupt handlers are connected to the ppbus system thanks to the newbus
     BUS_SETUP_INTR() and BUS_TEARDOWN_INTR() functions.  But, in order to attach a handler,
     drivers must own the bus.  Consequently, a ppbus request is mandatory in order to call the
     above functions (see existing drivers for more info).  Note that the interrupt handler is
     automatically released when the ppbus is released.

     Microsequences is a general purpose mechanism to allow fast low-level manipulation of the
     parallel port.  Microsequences may be used to do either standard (in IEEE1284 modes) or non-
     standard transfers.  The philosophy of microsequences is to avoid the overhead of the ppbus
     layer and do most of the job at adapter level.

     A microsequence is an array of opcodes and parameters.  Each opcode codes an operation
     (opcodes are described in microseq(9)).  Standard I/O operations are implemented at ppbus
     level whereas basic I/O operations and microseq language are coded at adapter level for

     As an example, the vpo(4) driver uses microsequences to implement:

              a modified version of the NIBBLE transfer mode

              various I/O sequences to initialize, select and allocate the peripheral


     lpt(4), plip(4), ppc(4), ppi(4), vpo(4)


     The ppbus manual page first appeared in FreeBSD 3.0.


     This manual page was written by Nicolas Souchu.