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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

                >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