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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.
Debian March 4, 1996 LP(4)