Skip site navigation (1)Skip section navigation (2)

FreeBSD Man Pages

Man Page or Keyword Search:
Man Architecture
Apropos Keyword Search (all sections) Output format
home | help
LP(4)		       FreeBSD Kernel Interfaces Manual			 LP(4)

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 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.

     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 hand-
     shake 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 hand-
     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).

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 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	9.3			 March 4, 1996			   FreeBSD 9.3

NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | BUGS

Want to link to this manual page? Use this URL:
<http://www.freebsd.org/cgi/man.cgi?query=plip&sektion=4&manpath=FreeBSD+9.3-RELEASE>

home | help