Documentation/networking/plip.rst - linux - Git at Google

 .. 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
	.. 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