3 .\" Copyright (c) 1996 A.R.Gordon, andrew.gordon@net-tel.co.uk
4 .\" All rights reserved.
6 .\" Redistribution and use in source and binary forms, with or without
7 .\" modification, are permitted provided that the following conditions
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16 .\" This product includes software developed by the University of
17 .\" California, Berkeley and its contributors.
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34 .\" Id: man4.i386/lp.4,v 1.9 1999/02/14 12:06:16 nsouch Exp
35 .\" $FreeBSD: src/share/man/man4/lp.4,v 1.5.2.3 2000/12/29 10:18:00 ru Exp $
36 .\" $DragonFly: src/share/man/man4/lp.4,v 1.2 2003/06/17 04:36:59 dillon Exp $
43 .Nd printer port Internet Protocol driver
47 .Ar myaddress hisaddress
52 .Cd "device ppc0 at isa? port? irq 7"
56 driver allows a PC parallel printer port to be used as a
57 point-to-point network interface between two similarly configured systems.
58 Data is transferred 4 bits at a time, using the printer status lines for
59 input: hence there is no requirement for special bidirectional hardware
60 and any standard AT-compatible printer port with working interrupts may be used.
62 During the boot process, for each
64 device which is probed and has an interrupt assigned, a corresponding
72 causes the corresponding
74 to be reserved for PLIP until the network interface is configured 'down'.
76 The communication protocol is selected by the
83 mode (LPIP). This is the simpler of the two modes
84 and therefore slightly more efficient.
86 Use Crynwr/Linux compatible mode (CLPIP). This mode has a simulated ethernet
87 packet header, and is easier to interface to other types of equipment.
90 The interface MTU defaults to 1500, but may be set to any value. Both ends
91 of the link must be configured with the same MTU.
93 The cable connecting the two parallel ports should be wired as follows:
109 Cables with this wiring are widely available as 'Laplink' cables, and
110 are often coloured yellow.
112 The connections are symmetric, and provide 5 lines in each direction (four
113 data plus one handshake). The two modes use the same wiring, but make a
114 different choice of which line to use as handshake.
115 .Ss FreeBSD LPIP mode
116 The signal lines are used as follows:
117 .Bl -tag -width dataxxxx(Pinxx)
128 .It Em ERROR* (pin 15)
140 When idle, all data lines are at zero. Each byte is signalled in four steps:
141 sender writes the 4 most significant bits and raises the handshake line;
142 receiver reads the 4 bits and raises its handshake to acknowledge;
143 sender places the 4 least significant bits on the data lines and lowers
144 the handshake; receiver reads the data and lowers its handshake.
146 The packet format has a two-byte header, comprising the fixed values 0x08,
147 0x00, immediately followed by the IP header and data.
149 The start of a packet is indicated by simply signalling the first byte
150 of the header. The end of the packet is indicated by inverting
151 the data lines (ie. writing the ones-complement of the previous nibble
152 to be transmitted) without changing the state of the handshake.
154 Note that the end-of-packet marker assumes that the handshake signal and
155 the data-out bits can be written in a single instruction - otherwise
156 certain byte values in the packet data would falsely be interpreted
157 as end-of-packet. This is not a problem for the PC printer port,
158 but requires care when implementing this protocol on other equipment.
159 .Ss Crynwr/Linux CLPIP mode
160 The signal lines are used as follows:
161 .Bl -tag -width dataxxxx(Pinxx)
172 .It Em ERROR* (pin 15)
184 When idle, all data lines are at zero. Each byte is signalled in four steps:
185 sender writes the 4 least significant bits and raises the handshake line;
186 receiver reads the 4 bits and raises its handshake to acknowledge;
187 sender places the 4 most significant bits on the data lines and lowers
188 the handshake; receiver reads the data and lowers its handshake.
189 [Note that this is the opposite nibble order to LPIP mode].
193 Length (least significant byte)
194 Length (most significant byte)
195 12 bytes of supposed MAC addresses (ignored by FreeBSD).
202 The length includes the 14 header bytes, but not the length bytes themselves
203 nor the checksum byte.
205 The checksum is a simple arithmetic sum of all the bytes (again, including
206 the header but not checksum or length bytes).
209 outgoing checksums, but does not validate incoming ones.
211 The start of packet has to be signalled specially, since the line chosen
212 for handshake-in cannot be used to generate an interrupt. The sender
213 writes the value 0x08 to the data lines, and waits for the receiver
214 to respond by writing 0x01 to its data lines. The sender then starts
215 signalling the first byte of the packet (the length byte).
217 End of packet is deduced from the packet length and is not signalled
218 specially (although the data lines are restored to the zero, idle
219 state to avoid spuriously indicating the start of the next packet).
225 Busy-waiting loops are used while handshaking bytes, (and worse still when
226 waiting for the receiving system to respond to an interrupt for the start
227 of a packet). Hence a fast system talking to a slow one will consume
228 excessive amounts of CPU. This is unavoidable in the case of CLPIP mode
229 due to the choice of handshake lines; it could theoretically be improved
230 in the case of LPIP mode.
232 Polling timeouts are controlled by counting loop iterations rather than
233 timers, and so are dependent on CPU speed. This is somewhat stabilised
234 by the need to perform (slow) ISA bus cycles to actually read the port.