\title{PLIP: The Parallel Line Internet Protocol Device}\author{ Donald Becker (becker@super.org)}\affiliation{I.D.A. Supercomputing Research Center, Bowie MD 20715}%% At some point T. Thorn will probably contribute text,%% \author{ Tommy Thorn (tthorn@daimi.aau.dk)}\section{PLIP Introduction}This document describes the parallel port packet pusher for Net/LGX.This device interface allows a point-to-point connection between twoparallel ports to appear as a IP network interface.\chapter{PLIP hardware interconnection}PLIP uses several different data transfer methods.  The first (and theonly one implemented in the early version of the code) uses a standardprinter "null" cable to transfers data four bits at a time usingdata bit outputs connected to status bit inputs.The second data transfer method relies on both machines havingbi-directional parallel ports, rather than output-only ``printer''ports.  This allows byte-wide transfers and avoids reconstructingnibbles into bytes, leading to much faster transfers.\section{Parallel Transfer Mode 0 Cable}The cable for the first transfer mode is a standardprinter "null" cable which transfers data four bits at a time usingdata bit outputs of the first port (machine T) connected to thestatus bit inputs of the second port (machine R).  There are fivestatus inputs, and they are used as four data inputs and a clock (datastrobe) input, arranged so that the data input bits appear as contiguousbits 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 withtwo 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 endIf the cable you are using has a metallic shield it should beconnected to the metallic DB-25 shell at one end only.\section{Parallel Transfer Mode 1}The second data transfer method relies on both machines havingbi-directional parallel ports, rather than output-only ``printer''ports.  This allows byte-wide transfers, and avoids reconstructingnibbles into bytes.  This cable should not be used on unidirectional``printer'' (as opposed to ``parallel'') ports or when the machineisn't configured for PLIP, as it will result in output driverconflicts and the (unlikely) possibility of damage.The cable for this tranfer 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 endOnce again, if the cable you are using has a metallic shield it shouldbe connected to the metallic DB-25 shell at one end only.\section{PLIP Mode 0 tranfer protocol}The PLIP driver is compatible with the "Crynwr" parallel port transferstandard in Mode 0.  That standard specifies the following protocol:   send header nibble '8'   count-low octet   count-high octet   ... data octets   checksum octetEach octet is sent as	<wait for rx. '1'>	<send 0x10+(octet&0x0F)>	<wait for rx. '0'>	<send 0x00+((octet>>4)&0x0F)>To start a transfer the transmitting machine outputs a nibble 0x08.The raises the ACK line, triggering an interrupt in the receivingmachine.  The receiving machine disablesRestated:(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