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     lp -- printer port Internet Protocol driver


     ifconfig lp0 myaddress hisaddress [-link0]

     device ppbus
     device plip
     device ppc


     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
     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 cer-
     tain 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.
     shake.  [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 them-
     selves nor the checksum byte.

     The checksum is a simple arithmetic sum of all the bytes (again, includ-
     ing 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 sig-
     nalling the first byte of the packet (the length byte).

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


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


     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 con-
     sume 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.

FreeBSD 5.4			 March 4, 1996			   FreeBSD 5.4


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