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diff --git a/Documentation/networking/plip.rst b/Documentation/networking/plip.rst new file mode 100644 index 000000000..0eda74505 --- /dev/null +++ b/Documentation/networking/plip.rst @@ -0,0 +1,222 @@ +.. SPDX-License-Identifier: GPL-2.0 + +================================================ +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 |