3 .\" Copyright (c) 1996 A.R.Gordon, andrew.gordon@net-tel.co.uk
4 .\" All rights reserved.
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16 .\" This product includes software developed by the University of
17 .\" California, Berkeley and its contributors.
18 .\" 4. Neither the name of the University nor the names of its contributors
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22 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
23 .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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34 .\" Id: man4.i386/lp.4,v 1.9 1999/02/14 12:06:16 nsouch Exp
42 .Nd printer port Internet Protocol driver
45 .Ar myaddress hisaddress
48 .Cd "controller ppbus0"
49 .Cd "device plip0 at ppbus?"
50 .Cd "controller ppc0 at isa? port? net irq 7
54 driver allows a PC parallel printer port to be used as a
55 point-to-point network interface between two similarly configured systems.
56 Data is transferred 4 bits at a time, using the printer status lines for
57 input: hence there is no requirement for special bidirectional hardware
58 and any standard AT-compatible printer port with working interrupts may be used.
60 During the boot process, for each
62 device which is probed and has an interrupt assigned, a corresponding
70 causes the corresponding
72 to be reserved for PLIP until the network interface is configured 'down'.
74 The communication protocol is selected by the
79 (default) Use FreeBSD mode (LPIP). This is the simpler of the two modes
80 and therefore slightly more efficient.
82 Use Crynwr/Linux compatible mode (CLPIP). This mode has a simulated ethernet
83 packet header, and is easier to interface to other types of equipment.
86 The interface MTU defaults to 1500, but may be set to any value. Both ends
87 of the link must be configured with the same MTU.
89 The cable connecting the two parallel ports should be wired as follows:
105 Cables with this wiring are widely available as 'Laplink' cables, and
106 are often coloured yellow.
108 The connections are symmetric, and provide 5 lines in each direction (four
109 data plus one handshake). The two modes use the same wiring, but make a
110 different choice of which line to use as handshake.
111 .Ss FreeBSD LPIP mode
112 The signal lines are used as follows:
113 .Bl -tag -width dataxxxx(Pinxx)
124 .It Em ERROR* (pin 15)
136 When idle, all data lines are at zero. Each byte is signalled in four steps:
137 sender writes the 4 most significant bits and raises the handshake line;
138 receiver reads the 4 bits and raises its handshake to acknowledge;
139 sender places the 4 least significant bits on the data lines and lowers
140 the handshake; receiver reads the data and lowers its handshake.
142 The packet format has a two-byte header, comprising the fixed values 0x08,
143 0x00, immediately followed by the IP header and data.
145 The start of a packet is indicated by simply signalling the first byte
146 of the header. The end of the packet is indicated by inverting
147 the data lines (ie. writing the ones-complement of the previous nibble
148 to be transmitted) without changing the state of the handshake.
150 Note that the end-of-packet marker assumes that the handshake signal and
151 the data-out bits can be written in a single instruction - otherwise
152 certain byte values in the packet data would falsely be interpreted
153 as end-of-packet. This is not a problem for the PC printer port,
154 but requires care when implementing this protocol on other equipment.
156 .Ss Crynwr/Linux CLPIP mode
157 The signal lines are used as follows:
158 .Bl -tag -width dataxxxx(Pinxx)
169 .It Em ERROR* (pin 15)
181 When idle, all data lines are at zero. Each byte is signalled in four steps:
182 sender writes the 4 least significant bits and raises the handshake line;
183 receiver reads the 4 bits and raises its handshake to acknowledge;
184 sender places the 4 most significant bits on the data lines and lowers
185 the handshake; receiver reads the data and lowers its handshake.
186 [Note that this is the opposite nibble order to LPIP mode].
190 Length (least significant byte)
191 Length (most significant byte)
192 12 bytes of supposed MAC addresses (ignored by FreeBSD).
199 The length includes the 14 header bytes, but not the length bytes themselves
200 nor the checksum byte.
202 The checksum is a simple arithmetic sum of all the bytes (again, including
203 the header but not checksum or length bytes). FreeBSD calculates
204 outgoing checksums, but does not validate incoming ones.
206 The start of packet has to be signalled specially, since the line chosen
207 for handshake-in cannot be used to generate an interrupt. The sender
208 writes the value 0x08 to the data lines, and waits for the receiver
209 to respond by writing 0x01 to its data lines. The sender then starts
210 signalling the first byte of the packet (the length byte).
212 End of packet is deduced from the packet length and is not signalled
213 specially (although the data lines are restored to the zero, idle
214 state to avoid spuriously indicating the start of the next packet).
220 Busy-waiting loops are used while handshaking bytes, (and worse still when
221 waiting for the receiving system to respond to an interrupt for the start
222 of a packet). Hence a fast system talking to a slow one will consume
223 excessive amounts of CPU. This is unavoidable in the case of CLPIP mode
224 due to the choice of handshake lines; it could theoretically be improved
225 in the case of LPIP mode.
227 Polling timeouts are controlled by counting loop iterations rather than
228 timers, and so are dependent on CPU speed. This is somewhat stabilised
229 by the need to perform (slow) ISA bus cycles to actually read the port.