Parallel Line Internet Protocol
Encyclopedia
The Parallel Line Internet Protocol (PLIP) is a computer network
ing protocol for direct computer-to-computer communications using the parallel port
normally used for connections to a printer.
The Parallel Line Internet Protocol provides Link Layer
services for the Internet Protocol
, the protocol used for forming small local area network
s and large computer networks, such as the Internet
, enabling computers without standard dedicated networking hardware, such as Ethernet
, but with older parallel port devices, to communicate.
is the standards-based networking model and software specification for forming small and large computer networks, from local area networks to global communication systems, such as the Internet. It is usually implemented by software and hardware features that use Ethernet
network interface cards, cabling, and networking switches or hubs.
Early personal computer
s did not have Ethernet hardware included in their design and bus adapters were initially expensive. A solution to was to use the, at the time, standard parallel port
, typically used for connection to a printer or similar output device. The ports on two computers are connected with a so-called null-printer cable, sometimes called a LapLink cable
.
The laplink cable connects five output pins of a parallel port to five input pins on the opposing port, for each direction. Due to the lack of an internal timing in the parallel ports, synchronization is implemented via software handshaking: four of the five pins are used for data transfer and one is used for synchronization. The logical values at these pins are read and written directly by the software via an input or output instruction.
This method does not connect the bidirectional data lines of the two devices, in order to avoid both lines being active at the same time. The status lines ERROR, SLCT, PAPOUT, ACK and BUSY on one device are connected to data pins d0 through d4 respectively on the other.
Transmission of a byte
is accomplished by dividing it into two nibble
s of four bits each. Each nibble is transmitted by setting the four data lines according to the four nibble bits and then toggling the acknowledge line. This toggle indicates the receiving host that the nibble is ready to be read. Once the receiving host has read the nibble, it toggles its synchronization line to tell the transmitter that the nibble has been read and that a new one may be sent. Both hosts use a toggle on their acknowledge lines to indicate that the read or write operation has been performed. As a result, each host has to wait for a toggle from the other host before proceeding with a new operation.
As an example, the transfer of nibble 0010 proceeds as follows:
When the transmitter detects the toggle, this procedure is repeated for the next nibble.
Internet Protocol
packets are sent over the line after encapsulating them into PLIP packets before transmission over the line. The encapsulated packet has the following structure:
The length and checksum are calculated over the second and third field only, so that the actual total length of the packet is three more than the length as reported in the first two bytes of the packet.
(SLIP), using null-modem cables, but allows transfer of four bits at a time rather than one. It generally works at higher bitrate
s. The method is based on the "Crynwr" standard devised by Russ Nelson
.
Ethernet may also be used as a direct computer-to-computer communications method using an Ethernet crossover cable
.
Other point-to-point connections, such as USB host-to-host bridges or cables are also used to transfer files between two computers where a network is not necessary or available.
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
ing protocol for direct computer-to-computer communications using the parallel port
Parallel port
A parallel port is a type of interface found on computers for connecting various peripherals. In computing, a parallel port is a parallel communication physical interface. It is also known as a printer port or Centronics port...
normally used for connections to a printer.
The Parallel Line Internet Protocol provides Link Layer
Link Layer
In computer networking, the link layer is the lowest layer in the Internet Protocol Suite , the networking architecture of the Internet . It is the group of methods or protocols that only operate on a host's link...
services for the Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
, the protocol used for forming small local area network
Local area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
s and large computer networks, such as the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
, enabling computers without standard dedicated networking hardware, such as Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
, but with older parallel port devices, to communicate.
Operation
The Internet Protocol SuiteInternet protocol suite
The Internet protocol suite is the set of communications protocols used for the Internet and other similar networks. It is commonly known as TCP/IP from its most important protocols: Transmission Control Protocol and Internet Protocol , which were the first networking protocols defined in this...
is the standards-based networking model and software specification for forming small and large computer networks, from local area networks to global communication systems, such as the Internet. It is usually implemented by software and hardware features that use Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
network interface cards, cabling, and networking switches or hubs.
Early personal computer
Personal computer
A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...
s did not have Ethernet hardware included in their design and bus adapters were initially expensive. A solution to was to use the, at the time, standard parallel port
Parallel port
A parallel port is a type of interface found on computers for connecting various peripherals. In computing, a parallel port is a parallel communication physical interface. It is also known as a printer port or Centronics port...
, typically used for connection to a printer or similar output device. The ports on two computers are connected with a so-called null-printer cable, sometimes called a LapLink cable
LapLink cable
A LapLink cable is a cable that allows one to connect two computers together to establish a direct cable connection. The connection is achieved via the parallel ports on the two computers. No networking hardware such as a network interface card or a modem are required...
.
The laplink cable connects five output pins of a parallel port to five input pins on the opposing port, for each direction. Due to the lack of an internal timing in the parallel ports, synchronization is implemented via software handshaking: four of the five pins are used for data transfer and one is used for synchronization. The logical values at these pins are read and written directly by the software via an input or output instruction.
This method does not connect the bidirectional data lines of the two devices, in order to avoid both lines being active at the same time. The status lines ERROR, SLCT, PAPOUT, ACK and BUSY on one device are connected to data pins d0 through d4 respectively on the other.
Transmission of a byte
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
is accomplished by dividing it into two nibble
Nibble
In computing, a nibble is a four-bit aggregation, or half an octet...
s of four bits each. Each nibble is transmitted by setting the four data lines according to the four nibble bits and then toggling the acknowledge line. This toggle indicates the receiving host that the nibble is ready to be read. Once the receiving host has read the nibble, it toggles its synchronization line to tell the transmitter that the nibble has been read and that a new one may be sent. Both hosts use a toggle on their acknowledge lines to indicate that the read or write operation has been performed. As a result, each host has to wait for a toggle from the other host before proceeding with a new operation.
As an example, the transfer of nibble 0010 proceeds as follows:
t->r lines r->s lines operation
00010 0xxxx transmitter sets data lines to 0010
10010 0xxxx transmitter toggle ACK line
receiver detects toggle and reads 0010
10010 1xxxx receiver toggle ACK line
transmitter detects toggle
When the transmitter detects the toggle, this procedure is repeated for the next nibble.
Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
packets are sent over the line after encapsulating them into PLIP packets before transmission over the line. The encapsulated packet has the following structure:
- packet length: 2 bytes, little endian
- ethernet header (mostly used for backward compatibility)
- the IP packet
- checksum: 1 byte, sum modulo 256 of bytes in the packet
The length and checksum are calculated over the second and third field only, so that the actual total length of the packet is three more than the length as reported in the first two bytes of the packet.
Similar methods
An analogous feature for serial communications ports is the Serial Line Internet ProtocolSerial Line Internet Protocol
The Serial Line Internet Protocol is an encapsulation of the Internet Protocol designed to work over serial ports and modem connections. It is documented in RFC 1055...
(SLIP), using null-modem cables, but allows transfer of four bits at a time rather than one. It generally works at higher bitrate
Bitrate
In telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time....
s. The method is based on the "Crynwr" standard devised by Russ Nelson
Russ Nelson
Russell "Russ" Nelson is an American computer programmer. He was a founding board member of the Open Source Initiative and briefly served as its president in 2005.-Career:...
.
Ethernet may also be used as a direct computer-to-computer communications method using an Ethernet crossover cable
Ethernet crossover cable
An Ethernet crossover cable is a type of Ethernet cable used to connect computing devices together directly. Normal straight through or patch cables were used to connect from a host network interface controller to a network switch, hub or router.A cable with connections that "cross over" was used...
.
Other point-to-point connections, such as USB host-to-host bridges or cables are also used to transfer files between two computers where a network is not necessary or available.
External links
- TLDPLinux Documentation ProjectThe Linux Documentation Project is an all-volunteer project that maintains a large collection of GNU and Linux-related documentation and publishes the collection online. It began as a way for hackers to share their documentation with each other and with their users, and for users to share...
: PLIP-Install-HOWTO - A description of the PLIP protocol by Alessandro Rubini