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NAME

     lp — printer port Internet Protocol driver

SYNOPSIS

     ifconfig plip0 myaddress hisaddress [-link0]

     device ppbus
     device plip
     device ppc

DESCRIPTION

     The lp 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 plip device which is probed and has an interrupt assigned,
     a corresponding network device is created.

     Configuring an lp device with ifconfig(8) causes the corresponding parallel port bus 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.

   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 coloured
     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 signalled 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 signalling 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 signalled 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 signalled 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 signalling the first byte of the packet (the length byte).

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

SEE ALSO

     ppbus(4), ppc(4), ifconfig(8)

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.

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