1 .. SPDX-License-Identifier: GPL-2.0 2 3 ================================================ 4 PLIP: The Parallel Line Internet Protocol Device 5 ================================================ 6 7 Donald Becker (becker@super.org) 8 I.D.A. Supercomputing Research Center, Bowie MD 20715 9 10 At some point T. Thorn will probably contribute text, 11 Tommy Thorn (tthorn@daimi.aau.dk) 12 13 PLIP Introduction 14 ----------------- 15 16 This document describes the parallel port packet pusher for Net/LGX. 17 This device interface allows a point-to-point connection between two 18 parallel ports to appear as a IP network interface. 19 20 What is PLIP? 21 ============= 22 23 PLIP is Parallel Line IP, that is, the transportation of IP packages 24 over a parallel port. In the case of a PC, the obvious choice is the 25 printer port. PLIP is a non-standard, but [can use] uses the standard 26 LapLink null-printer cable [can also work in turbo mode, with a PLIP 27 cable]. [The protocol used to pack IP packages, is a simple one 28 initiated by Crynwr.] 29 30 Advantages of PLIP 31 ================== 32 33 It's cheap, it's available everywhere, and it's easy. 34 35 The PLIP cable is all that's needed to connect two Linux boxes, and it 36 can be built for very few bucks. 37 38 Connecting two Linux boxes takes only a second's decision and a few 39 minutes' work, no need to search for a [supported] netcard. This might 40 even be especially important in the case of notebooks, where netcards 41 are not easily available. 42 43 Not requiring a netcard also means that apart from connecting the 44 cables, everything else is software configuration [which in principle 45 could be made very easy.] 46 47 Disadvantages of PLIP 48 ===================== 49 50 Doesn't work over a modem, like SLIP and PPP. Limited range, 15 m. 51 Can only be used to connect three (?) Linux boxes. Doesn't connect to 52 an existing Ethernet. Isn't standard (not even de facto standard, like 53 SLIP). 54 55 Performance 56 =========== 57 58 PLIP easily outperforms Ethernet cards....(ups, I was dreaming, but 59 it *is* getting late. EOB) 60 61 PLIP driver details 62 ------------------- 63 64 The Linux PLIP driver is an implementation of the original Crynwr protocol, 65 that uses the parallel port subsystem of the kernel in order to properly 66 share parallel ports between PLIP and other services. 67 68 IRQs and trigger timeouts 69 ========================= 70 71 When a parallel port used for a PLIP driver has an IRQ configured to it, the 72 PLIP driver is signaled whenever data is sent to it via the cable, such that 73 when no data is available, the driver isn't being used. 74 75 However, on some machines it is hard, if not impossible, to configure an IRQ 76 to a certain parallel port, mainly because it is used by some other device. 77 On these machines, the PLIP driver can be used in IRQ-less mode, where 78 the PLIP driver would constantly poll the parallel port for data waiting, 79 and if such data is available, process it. This mode is less efficient than 80 the IRQ mode, because the driver has to check the parallel port many times 81 per second, even when no data at all is sent. Some rough measurements 82 indicate that there isn't a noticeable performance drop when using IRQ-less 83 mode as compared to IRQ mode as far as the data transfer speed is involved. 84 There is a performance drop on the machine hosting the driver. 85 86 When the PLIP driver is used in IRQ mode, the timeout used for triggering a 87 data transfer (the maximal time the PLIP driver would allow the other side 88 before announcing a timeout, when trying to handshake a transfer of some 89 data) is, by default, 500usec. As IRQ delivery is more or less immediate, 90 this timeout is quite sufficient. 91 92 When in IRQ-less mode, the PLIP driver polls the parallel port HZ times 93 per second (where HZ is typically 100 on most platforms, and 1024 on an 94 Alpha, as of this writing). Between two such polls, there are 10^6/HZ usecs. 95 On an i386, for example, 10^6/100 = 10000usec. It is easy to see that it is 96 quite possible for the trigger timeout to expire between two such polls, as 97 the timeout is only 500usec long. As a result, it is required to change the 98 trigger timeout on the *other* side of a PLIP connection, to about 99 10^6/HZ usecs. If both sides of a PLIP connection are used in IRQ-less mode, 100 this timeout is required on both sides. 101 102 It appears that in practice, the trigger timeout can be shorter than in the 103 above calculation. It isn't an important issue, unless the wire is faulty, 104 in which case a long timeout would stall the machine when, for whatever 105 reason, bits are dropped. 106 107 A utility that can perform this change in Linux is plipconfig, which is part 108 of the net-tools package (its location can be found in the 109 Documentation/Changes file). An example command would be 110 'plipconfig plipX trigger 10000', where plipX is the appropriate 111 PLIP device. 112 113 PLIP hardware interconnection 114 ----------------------------- 115 116 PLIP uses several different data transfer methods. The first (and the 117 only one implemented in the early version of the code) uses a standard 118 printer "null" cable to transfer data four bits at a time using 119 data bit outputs connected to status bit inputs. 120 121 The second data transfer method relies on both machines having 122 bi-directional parallel ports, rather than output-only ``printer`` 123 ports. This allows byte-wide transfers and avoids reconstructing 124 nibbles into bytes, leading to much faster transfers. 125 126 Parallel Transfer Mode 0 Cable 127 ============================== 128 129 The cable for the first transfer mode is a standard 130 printer "null" cable which transfers data four bits at a time using 131 data bit outputs of the first port (machine T) connected to the 132 status bit inputs of the second port (machine R). There are five 133 status inputs, and they are used as four data inputs and a clock (data 134 strobe) input, arranged so that the data input bits appear as contiguous 135 bits with standard status register implementation. 136 137 A cable that implements this protocol is available commercially as a 138 "Null Printer" or "Turbo Laplink" cable. It can be constructed with 139 two DB-25 male connectors symmetrically connected as follows:: 140 141 STROBE output 1* 142 D0->ERROR 2 - 15 15 - 2 143 D1->SLCT 3 - 13 13 - 3 144 D2->PAPOUT 4 - 12 12 - 4 145 D3->ACK 5 - 10 10 - 5 146 D4->BUSY 6 - 11 11 - 6 147 D5,D6,D7 are 7*, 8*, 9* 148 AUTOFD output 14* 149 INIT output 16* 150 SLCTIN 17 - 17 151 extra grounds are 18*,19*,20*,21*,22*,23*,24* 152 GROUND 25 - 25 153 154 * Do not connect these pins on either end 155 156 If the cable you are using has a metallic shield it should be 157 connected to the metallic DB-25 shell at one end only. 158 159 Parallel Transfer Mode 1 160 ======================== 161 162 The second data transfer method relies on both machines having 163 bi-directional parallel ports, rather than output-only ``printer`` 164 ports. This allows byte-wide transfers, and avoids reconstructing 165 nibbles into bytes. This cable should not be used on unidirectional 166 ``printer`` (as opposed to ``parallel``) ports or when the machine 167 isn't configured for PLIP, as it will result in output driver 168 conflicts and the (unlikely) possibility of damage. 169 170 The cable for this transfer mode should be constructed as follows:: 171 172 STROBE->BUSY 1 - 11 173 D0->D0 2 - 2 174 D1->D1 3 - 3 175 D2->D2 4 - 4 176 D3->D3 5 - 5 177 D4->D4 6 - 6 178 D5->D5 7 - 7 179 D6->D6 8 - 8 180 D7->D7 9 - 9 181 INIT -> ACK 16 - 10 182 AUTOFD->PAPOUT 14 - 12 183 SLCT->SLCTIN 13 - 17 184 GND->ERROR 18 - 15 185 extra grounds are 19*,20*,21*,22*,23*,24* 186 GROUND 25 - 25 187 188 * Do not connect these pins on either end 189 190 Once again, if the cable you are using has a metallic shield it should 191 be connected to the metallic DB-25 shell at one end only. 192 193 PLIP Mode 0 transfer protocol 194 ============================= 195 196 The PLIP driver is compatible with the "Crynwr" parallel port transfer 197 standard in Mode 0. That standard specifies the following protocol:: 198 199 send header nibble '0x8' 200 count-low octet 201 count-high octet 202 ... data octets 203 checksum octet 204 205 Each octet is sent as:: 206 207 <wait for rx. '0x1?'> <send 0x10+(octet&0x0F)> 208 <wait for rx. '0x0?'> <send 0x00+((octet>>4)&0x0F)> 209 210 To start a transfer the transmitting machine outputs a nibble 0x08. 211 That raises the ACK line, triggering an interrupt in the receiving 212 machine. The receiving machine disables interrupts and raises its own ACK 213 line. 214 215 Restated:: 216 217 (OUT is bit 0-4, OUT.j is bit j from OUT. IN likewise) 218 Send_Byte: 219 OUT := low nibble, OUT.4 := 1 220 WAIT FOR IN.4 = 1 221 OUT := high nibble, OUT.4 := 0 222 WAIT FOR IN.4 = 0
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