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-topoint 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 bidirec

tional hardware
and any standard AT-compatible printer port with working in

terrupts may
be used.

During the boot process, for each plip device which is

probed and has an
interrupt assigned, a corresponding network device is creat

ed.

Configuring an lp device with ifconfig(8) causes the corre

sponding
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 effi

cient.

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 sig

nalled 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 hand

shake.

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 hand

shake signal and
the data-out bits can be written in a single instruction

otherwise certain byte values in the packet data would falsely be inter

preted as endof-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 sig

nalled in four
steps: sender writes the 4 least significant bits and raises

the handshake line; receiver reads the 4 bits and raises its hand

shake 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 in

terrupt 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 theoret

ically 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.

BSD March 4, 1996

BSD