NAME
plip —
printer port Internet Protocol
driver
SYNOPSIS
plip* at ppbus?
options PLIP_DEBUG
DESCRIPTION
The
plip 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
ppbus(4) device which is attached
and has an interrupt capability, a corresponding
plip device
is attached. The
plip device is configured using
ifconfig(8) using the options
for a point-to-point network interface:
ifconfig plip0
hostaddress destaddress
[
-link0|link0]
[
up|down] [
...]
Configuring a
plip device “up” with
ifconfig(8) causes the
corresponding
ppbus(4) 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. See
ifconfig(8) for details on
configuring network interfaces.
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 colored 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 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.
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 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.
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 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].
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 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).
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).
SEE ALSO
atppc(4),
ppbus(4),
ifconfig(8)
HISTORY
The
plip driver was implemented for
ppbus(4) in
FreeBSD and imported into
NetBSD. Crynwr packet drivers implemented PLIP for
MS-DOS. Linux also has a PLIP driver. The protocols are know as LPIP
(
FreeBSD) and CLPIP (Crynwr/Linux) in the
documentation and code of this port. LPIP originally appeared in
FreeBSD.
AUTHORS
This manual page is based on the
FreeBSD
lp manual page. The information has been updated for the
NetBSD port by Gary Thorpe.
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.
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.
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.
In the
FreeBSD 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.
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 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 “
ping
-f”) compared to CLPIP.