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.\" Copyright (c) 1996 A.R.Gordon, andrew.gordon@net-tel.co.uk
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.\"	Id: man4.i386/lp.4,v 1.9 1999/02/14 12:06:16 nsouch Exp
.\" $FreeBSD: src/share/man/man4/lp.4,v 1.5.2.3 2000/12/29 10:18:00 ru Exp $
.\"
.Dd January 28, 2004
.Dt PLIP 4
.Os
.Sh NAME
.Nm plip
.Nd printer port Internet Protocol driver
.Sh SYNOPSIS
.Cd "plip* at ppbus?"
.Cd options PLIP_DEBUG
.Sh DESCRIPTION
The
.Nm
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.
.Pp
During the boot process, for each
.Xr ppbus 4
device which is attached and has an interrupt capability, a
corresponding
.Nm
device is attached.
The
.Nm
device is configured using
.Xr ifconfig 8
using the options for a point-to-point network interface:
.Pp
.Nm ifconfig
.Ar plip0
.Ar hostaddress destaddress
.Op Fl link0|link0
.Op up|down
.Op ...
.Pp
Configuring a
.Nm
device
.Dq up
with
.Xr ifconfig 8
causes the corresponding
.Xr ppbus 4
to be reserved for PLIP until the network interface is configured
.Dq down .
.Pp
The communication protocol is selected by the
.Cm link0
flag:
.Bl -tag -width Fl
.It Fl link0
(default)
Use
.Fx
mode (LPIP).
This is the simpler of the two modes and therefore slightly more
efficient.
.It Cm 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.
.El
.Pp
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
.Xr ifconfig 8
for details on configuring network interfaces.
.Ss Cable Connections
The cable connecting the two parallel ports should be wired as follows:
.Bd -literal
	Pin	Pin	Description
	2	15	Data0 -\*[Gt] ERROR*
	3	13	Data1 -\*[Gt] SLCT
	4	12	Data2 -\*[Gt] PE
	5	10	Data3 -\*[Gt] ACK*
	6	11	Data4 -\*[Gt] BUSY
	15	2	ERROR* -\*[Gt] Data0
	13	3	SLCT   -\*[Gt] Data1
	12	4	PE     -\*[Gt] Data2
	10	5	ACK*   -\*[Gt] Data3
	11	6	BUSY   -\*[Gt] Data4
	18-25	18-25	Ground
.Ed
.Pp
Cables with this wiring are widely available as
.Dq Tn Laplink
cables, and are often colored yellow.
.Pp
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.
.Ss FreeBSD LPIP mode
The signal lines are used as follows:
.Bl -tag -width dataxxxxXPinxxX
.It Em Data0 (Pin 2)
Data out, bit 0.
.It Em Data1 (Pin 3)
Data out, bit 1.
.It Em Data2 (Pin 4)
Data out, bit 2.
.It Em Data3 (Pin 5)
Handshake out.
.It Em Data4 (Pin 6)
Data out, bit 3.
.It Em ERROR* (pin 15)
Data in, bit 0.
.It Em SLCT (pin 13)
Data in, bit 1.
.It Em PE (pin 12)
Data in, bit 2.
.It Em BUSY (pin 11)
Data in, bit 3.
.It Em ACK* (pin 10)
Handshake in.
.El
.Pp
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.
.Pp
The packet format has a two-byte header, comprising the fixed values
0x08, 0x00, immediately followed by the IP header and data.
.Pp
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.
.Pp
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.
.Ss Crynwr/Linux CLPIP mode
The signal lines are used as follows:
.Bl -tag -width dataxxxxXPinxxX
.It Em Data0 (Pin 2)
Data out, bit 0.
.It Em Data1 (Pin 3)
Data out, bit 1.
.It Em Data2 (Pin 4)
Data out, bit 2.
.It Em Data3 (Pin 5)
Data out, bit 3.
.It Em Data4 (Pin 6)
Handshake out.
.It Em ERROR* (pin 15)
Data in, bit 0.
.It Em SLCT (pin 13)
Data in, bit 1.
.It Em PE (pin 12)
Data in, bit 2.
.It Em ACK* (pin 10)
Data in, bit 3.
.It Em BUSY (pin 11)
Handshake in.
.El
.Pp
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].
.Pp
Packet format is:
.Bd -literal
Length (least significant byte)
Length (most significant byte)
12 bytes of supposed MAC addresses (ignored by FreeBSD).
Fixed byte 0x08
Fixed byte 0x00
\*[Lt]IP datagram\*[Gt]
Checksum byte.
.Ed
.Pp
The length includes the 14 header bytes, but not the length bytes
themselves nor the checksum byte.
.Pp
The checksum is a simple arithmetic sum of all the bytes (again,
including the header but not checksum or length bytes).
.Fx
calculates outgoing checksums, but does not validate incoming ones.
.Pp
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).
.Pp
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).
.Sh SEE ALSO
.Xr atppc 4 ,
.Xr ppbus 4 ,
.Xr ifconfig 8
.Sh HISTORY
The
.Nm
driver was implemented for
.Xr ppbus 4
in
.Fx
and imported into
.Nx .
Crynwr packet drivers implemented PLIP for
.Tn MS-DOS .
Linux also has a PLIP driver.
The protocols are know as LPIP
.Pq Fx
and CLPIP (Crynwr/Linux) in the documentation and code of this
port.
LPIP originally appeared in
.Fx .
.Sh AUTHORS
This manual page is based on the
.Fx
.Nm lp
manual page.
The information has been updated for the
.Nx
port by Gary Thorpe.
.Sh 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.
.Pp
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.
.Pp
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.
.Pp
In the
.Fx
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.
.Pp
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
.Tn 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
.Dq Ic ping Fl f )
compared to CLPIP.