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.