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