unixdev.net


Switch to SpeakEasy.net DSL

The Modular Manual Browser

Home Page
Manual: (NetBSD-6.1.5)
Page:
Section:
Apropos / Subsearch:
optional field

PLIP(4)                    Kernel Interfaces Manual                    PLIP(4)

NAME
     plip -- printer port Internet Protocol driver

SYNOPSIS
     plip* at ppbus?
     options PLIP_DEBUG

DESCRIPTION
     The plip driver allows a PC parallel printer port to be used as a point-
     to-point network interface between two similarly configured systems.
     Data is transferred 4 bits at a time, using the printer status lines for
     input: hence there is no requirement for special bidirectional hardware
     and any standard AT-compatible printer port with working interrupts may
     be used.

     During the boot process, for each ppbus(4) device which is attached and
     has an interrupt capability, a corresponding plip device is attached.
     The plip device is configured using ifconfig(8) using the options for a
     point-to-point network interface:

     ifconfig plip0 hostaddress destaddress [-link0|link0] [up|down] [...]

     Configuring a plip device ``up'' with ifconfig(8) causes the
     corresponding ppbus(4) to be reserved for PLIP until the network
     interface is configured ``down''.

     The communication protocol is selected by the link0 flag:

     -link0      (default) Use FreeBSD mode (LPIP).  This is the simpler of
                 the two modes and therefore slightly more efficient.

     link0       Use Crynwr/Linux compatible mode (CLPIP).  This mode has a
                 simulated ethernet packet header, and is easier to interface
                 to other types of equipment.

     The interface MTU defaults to 1500, but may be set to any value.  Both
     ends of the link must be configured with the same MTU.  See ifconfig(8)
     for details on configuring network interfaces.

   Cable Connections
     The cable connecting the two parallel ports should be wired as follows:

             Pin     Pin     Description
             2       15      Data0 -> ERROR*
             3       13      Data1 -> SLCT
             4       12      Data2 -> PE
             5       10      Data3 -> ACK*
             6       11      Data4 -> BUSY
             15      2       ERROR* -> Data0
             13      3       SLCT   -> Data1
             12      4       PE     -> Data2
             10      5       ACK*   -> Data3
             11      6       BUSY   -> Data4
             18-25   18-25   Ground

     Cables with this wiring are widely available as ``Laplink'' cables, and
     are often colored yellow.

     The connections are symmetric, and provide 5 lines in each direction
     (four data plus one handshake).  The two modes use the same wiring, but
     make a different choice of which line to use as handshake.

   FreeBSD LPIP mode
     The signal lines are used as follows:

     Data0 (Pin 2)    Data out, bit 0.

     Data1 (Pin 3)    Data out, bit 1.

     Data2 (Pin 4)    Data out, bit 2.

     Data3 (Pin 5)    Handshake out.

     Data4 (Pin 6)    Data out, bit 3.

     ERROR* (pin 15)  Data in, bit 0.

     SLCT (pin 13)    Data in, bit 1.

     PE (pin 12)      Data in, bit 2.

     BUSY (pin 11)    Data in, bit 3.

     ACK* (pin 10)    Handshake in.

     When idle, all data lines are at zero.  Each byte is signaled in four
     steps:  sender writes the 4 most significant bits and raises the
     handshake line; receiver reads the 4 bits and raises its handshake to
     acknowledge; sender places the 4 least significant bits on the data lines
     and lowers the handshake; receiver reads the data and lowers its
     handshake.

     The packet format has a two-byte header, comprising the fixed values
     0x08, 0x00, immediately followed by the IP header and data.

     The start of a packet is indicated by simply signaling the first byte of
     the header.  The end of the packet is indicated by inverting the data
     lines (i.e. writing the ones-complement of the previous nibble to be
     transmitted) without changing the state of the handshake.

     Note that the end-of-packet marker assumes that the handshake signal and
     the data-out bits can be written in a single instruction - otherwise
     certain byte values in the packet data would falsely be interpreted as
     end-of-packet.  This is not a problem for the PC printer port, but
     requires care when implementing this protocol on other equipment.

   Crynwr/Linux CLPIP mode
     The signal lines are used as follows:

     Data0 (Pin 2)    Data out, bit 0.

     Data1 (Pin 3)    Data out, bit 1.

     Data2 (Pin 4)    Data out, bit 2.

     Data3 (Pin 5)    Data out, bit 3.

     Data4 (Pin 6)    Handshake out.

     ERROR* (pin 15)  Data in, bit 0.

     SLCT (pin 13)    Data in, bit 1.

     PE (pin 12)      Data in, bit 2.

     ACK* (pin 10)    Data in, bit 3.

     BUSY (pin 11)    Handshake in.

     When idle, all data lines are at zero.  Each byte is signaled in four
     steps:  sender writes the 4 least significant bits and raises the
     handshake line; receiver reads the 4 bits and raises its handshake to
     acknowledge; sender places the 4 most significant bits on the data lines
     and lowers the handshake; receiver reads the data and lowers its
     handshake.  [Note that this is the opposite nibble order to LPIP mode].

     Packet format is:

     Length (least significant byte)
     Length (most significant byte)
     12 bytes of supposed MAC addresses (ignored by FreeBSD).
     Fixed byte 0x08
     Fixed byte 0x00
     <IP datagram>
     Checksum byte.

     The length includes the 14 header bytes, but not the length bytes
     themselves nor the checksum byte.

     The checksum is a simple arithmetic sum of all the bytes (again,
     including the header but not checksum or length bytes).  FreeBSD
     calculates outgoing checksums, but does not validate incoming ones.

     The start of packet has to be signaled specially, since the line chosen
     for handshake-in cannot be used to generate an interrupt.  The sender
     writes the value 0x08 to the data lines, and waits for the receiver to
     respond by writing 0x01 to its data lines.  The sender then starts
     signaling the first byte of the packet (the length byte).

     End of packet is deduced from the packet length and is not signaled
     specially (although the data lines are restored to the zero, idle state
     to avoid spuriously indicating the start of the next packet).

SEE ALSO
     atppc(4), ppbus(4), ifconfig(8)

HISTORY
     The plip driver was implemented for ppbus(4) in FreeBSD and imported into
     NetBSD.  Crynwr packet drivers implemented PLIP for MS-DOS.  Linux also
     has a PLIP driver.  The protocols are know as LPIP (FreeBSD) and CLPIP
     (Crynwr/Linux) in the documentation and code of this port.  LPIP
     originally appeared in FreeBSD.

AUTHORS
     This manual page is based on the FreeBSD lp manual page.  The information
     has been updated for the NetBSD port by Gary Thorpe.

BUGS
     Busy-waiting loops are used while handshaking bytes (and worse still when
     waiting for the receiving system to respond to an interrupt for the start
     of a packet).  Hence a fast system talking to a slow one will consume
     excessive amounts of CPU.  This is unavoidable in the case of CLPIP mode
     due to the choice of handshake lines; it could theoretically be improved
     in the case of LPIP mode.

     Regardless of the speed difference between hosts, PLIP is CPU-intensive
     and its made worse by having to send nibbles (4 bits) at a time.

     Polling timeouts are controlled by counting loop iterations rather than
     timers, and so are dependent on CPU speed.  This is somewhat stabilized
     by the need to perform (slow) ISA bus cycles to actually read the port.

     In the FreeBSD implementation, the idle state was not properly being
     restored on errors or when finishing transmitting/receiving.  This
     implementation attempts to fix this problem which would result in an
     unresponsive interface that could no longer be used (the port bits get
     stuck in a state and nothing can progress) by zeroing the data register
     when necessary.

     For unknown reasons, the more complex protocol (CLPIP) yields higher data
     transfer rates during testing so far.  This could possibly be because the
     other side can reliably detect when the host is transmitting in this
     implementation of CLPIP (this may not necessarily be true in Linux or
     MS-DOS packet drivers).  CLPIP gets about 70 KB/sec (the best expected is
     about 75 KB/sec) and LPIP get about 55 KB/sec.  This is despite LPIP
     being able to send more packets over the interface (tested with ``ping
     -f'') compared to CLPIP.

NetBSD 6.1.5                   January 28, 2004                   NetBSD 6.1.5