223 lines
8.2 KiB
ReStructuredText
223 lines
8.2 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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================================================
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PLIP: The Parallel Line Internet Protocol Device
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================================================
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Donald Becker (becker@super.org)
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I.D.A. Supercomputing Research Center, Bowie MD 20715
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At some point T. Thorn will probably contribute text,
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Tommy Thorn (tthorn@daimi.aau.dk)
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PLIP Introduction
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-----------------
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This document describes the parallel port packet pusher for Net/LGX.
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This device interface allows a point-to-point connection between two
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parallel ports to appear as a IP network interface.
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What is PLIP?
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=============
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PLIP is Parallel Line IP, that is, the transportation of IP packages
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over a parallel port. In the case of a PC, the obvious choice is the
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printer port. PLIP is a non-standard, but [can use] uses the standard
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LapLink null-printer cable [can also work in turbo mode, with a PLIP
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cable]. [The protocol used to pack IP packages, is a simple one
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initiated by Crynwr.]
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Advantages of PLIP
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==================
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It's cheap, it's available everywhere, and it's easy.
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The PLIP cable is all that's needed to connect two Linux boxes, and it
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can be built for very few bucks.
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Connecting two Linux boxes takes only a second's decision and a few
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minutes' work, no need to search for a [supported] netcard. This might
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even be especially important in the case of notebooks, where netcards
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are not easily available.
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Not requiring a netcard also means that apart from connecting the
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cables, everything else is software configuration [which in principle
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could be made very easy.]
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Disadvantages of PLIP
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=====================
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Doesn't work over a modem, like SLIP and PPP. Limited range, 15 m.
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Can only be used to connect three (?) Linux boxes. Doesn't connect to
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an existing Ethernet. Isn't standard (not even de facto standard, like
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SLIP).
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Performance
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===========
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PLIP easily outperforms Ethernet cards....(ups, I was dreaming, but
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it *is* getting late. EOB)
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PLIP driver details
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-------------------
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The Linux PLIP driver is an implementation of the original Crynwr protocol,
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that uses the parallel port subsystem of the kernel in order to properly
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share parallel ports between PLIP and other services.
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IRQs and trigger timeouts
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=========================
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When a parallel port used for a PLIP driver has an IRQ configured to it, the
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PLIP driver is signaled whenever data is sent to it via the cable, such that
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when no data is available, the driver isn't being used.
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However, on some machines it is hard, if not impossible, to configure an IRQ
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to a certain parallel port, mainly because it is used by some other device.
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On these machines, the PLIP driver can be used in IRQ-less mode, where
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the PLIP driver would constantly poll the parallel port for data waiting,
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and if such data is available, process it. This mode is less efficient than
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the IRQ mode, because the driver has to check the parallel port many times
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per second, even when no data at all is sent. Some rough measurements
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indicate that there isn't a noticeable performance drop when using IRQ-less
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mode as compared to IRQ mode as far as the data transfer speed is involved.
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There is a performance drop on the machine hosting the driver.
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When the PLIP driver is used in IRQ mode, the timeout used for triggering a
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data transfer (the maximal time the PLIP driver would allow the other side
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before announcing a timeout, when trying to handshake a transfer of some
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data) is, by default, 500usec. As IRQ delivery is more or less immediate,
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this timeout is quite sufficient.
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When in IRQ-less mode, the PLIP driver polls the parallel port HZ times
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per second (where HZ is typically 100 on most platforms, and 1024 on an
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Alpha, as of this writing). Between two such polls, there are 10^6/HZ usecs.
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On an i386, for example, 10^6/100 = 10000usec. It is easy to see that it is
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quite possible for the trigger timeout to expire between two such polls, as
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the timeout is only 500usec long. As a result, it is required to change the
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trigger timeout on the *other* side of a PLIP connection, to about
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10^6/HZ usecs. If both sides of a PLIP connection are used in IRQ-less mode,
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this timeout is required on both sides.
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It appears that in practice, the trigger timeout can be shorter than in the
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above calculation. It isn't an important issue, unless the wire is faulty,
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in which case a long timeout would stall the machine when, for whatever
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reason, bits are dropped.
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A utility that can perform this change in Linux is plipconfig, which is part
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of the net-tools package (its location can be found in the
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Documentation/Changes file). An example command would be
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'plipconfig plipX trigger 10000', where plipX is the appropriate
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PLIP device.
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PLIP hardware interconnection
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-----------------------------
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PLIP uses several different data transfer methods. The first (and the
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only one implemented in the early version of the code) uses a standard
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printer "null" cable to transfer data four bits at a time using
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data bit outputs connected to status bit inputs.
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The second data transfer method relies on both machines having
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bi-directional parallel ports, rather than output-only ``printer``
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ports. This allows byte-wide transfers and avoids reconstructing
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nibbles into bytes, leading to much faster transfers.
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Parallel Transfer Mode 0 Cable
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==============================
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The cable for the first transfer mode is a standard
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printer "null" cable which transfers data four bits at a time using
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data bit outputs of the first port (machine T) connected to the
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status bit inputs of the second port (machine R). There are five
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status inputs, and they are used as four data inputs and a clock (data
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strobe) input, arranged so that the data input bits appear as contiguous
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bits with standard status register implementation.
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A cable that implements this protocol is available commercially as a
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"Null Printer" or "Turbo Laplink" cable. It can be constructed with
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two DB-25 male connectors symmetrically connected as follows::
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STROBE output 1*
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D0->ERROR 2 - 15 15 - 2
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D1->SLCT 3 - 13 13 - 3
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D2->PAPOUT 4 - 12 12 - 4
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D3->ACK 5 - 10 10 - 5
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D4->BUSY 6 - 11 11 - 6
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D5,D6,D7 are 7*, 8*, 9*
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AUTOFD output 14*
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INIT output 16*
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SLCTIN 17 - 17
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extra grounds are 18*,19*,20*,21*,22*,23*,24*
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GROUND 25 - 25
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* Do not connect these pins on either end
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If the cable you are using has a metallic shield it should be
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connected to the metallic DB-25 shell at one end only.
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Parallel Transfer Mode 1
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========================
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The second data transfer method relies on both machines having
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bi-directional parallel ports, rather than output-only ``printer``
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ports. This allows byte-wide transfers, and avoids reconstructing
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nibbles into bytes. This cable should not be used on unidirectional
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``printer`` (as opposed to ``parallel``) ports or when the machine
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isn't configured for PLIP, as it will result in output driver
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conflicts and the (unlikely) possibility of damage.
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The cable for this transfer mode should be constructed as follows::
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STROBE->BUSY 1 - 11
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D0->D0 2 - 2
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D1->D1 3 - 3
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D2->D2 4 - 4
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D3->D3 5 - 5
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D4->D4 6 - 6
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D5->D5 7 - 7
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D6->D6 8 - 8
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D7->D7 9 - 9
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INIT -> ACK 16 - 10
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AUTOFD->PAPOUT 14 - 12
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SLCT->SLCTIN 13 - 17
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GND->ERROR 18 - 15
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extra grounds are 19*,20*,21*,22*,23*,24*
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GROUND 25 - 25
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* Do not connect these pins on either end
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Once again, if the cable you are using has a metallic shield it should
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be connected to the metallic DB-25 shell at one end only.
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PLIP Mode 0 transfer protocol
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=============================
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The PLIP driver is compatible with the "Crynwr" parallel port transfer
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standard in Mode 0. That standard specifies the following protocol::
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send header nibble '0x8'
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count-low octet
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count-high octet
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... data octets
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checksum octet
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Each octet is sent as::
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<wait for rx. '0x1?'> <send 0x10+(octet&0x0F)>
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<wait for rx. '0x0?'> <send 0x00+((octet>>4)&0x0F)>
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To start a transfer the transmitting machine outputs a nibble 0x08.
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That raises the ACK line, triggering an interrupt in the receiving
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machine. The receiving machine disables interrupts and raises its own ACK
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line.
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Restated::
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(OUT is bit 0-4, OUT.j is bit j from OUT. IN likewise)
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Send_Byte:
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OUT := low nibble, OUT.4 := 1
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WAIT FOR IN.4 = 1
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OUT := high nibble, OUT.4 := 0
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WAIT FOR IN.4 = 0
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