Parallel Line Internet Protocol: Difference between revisions

Revision as of 19:45, 21 May 2009

Revision as of 19:45, 21 May 2009

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For most uses PLIP has been replaced by increasingly-common [[Ethernet]] [[Protocol (computing)|protocol]] based [[computer network|networking]] support and [[Ethernet crossover cable|cross-cable]] setups –– or other [[Link (telecommunications)#Point-to-point|point-to-point]] connections such as an [[USB]] host-to-host bridge/cable –– used to transfer files between two computers where a network is not necessary or available.

==Description==

Plip is the noise when you ;lknsf;jghasd jajyour pep pep pep pep pep pep pep pep i am huge

The [[laplink cable|null-printer cable]] connects five output pins of a parallel port to five input pins on the other port, and vice versa. Due to the lack of an internal timing in the parallel ports, synchronization is done via software handshaking: of the five input or output pins, four are used for data transfer and one is used for synchronization. The logical values at these pins can be read and written directly by the software via an io instruction.

Note that the method does NOT connect the bidirectional data lines from the two devices together, to avoid both lines being active at the same time. The control lines error, slct, papout, ack and busy on one device are connected to data pins d0 thru d4 respectively on the other.

Transmission of a byte is done by first breaking it into two [[nibble]]s of four bits each. Each nibble is then transmitted by first 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 can be send. Both hosts use a toggle on their acknowledge lines to indicate that the operation (read or write) has been performed; as a result, each host has to wait for a toggle from the other host before proceeding with a new read or write.

PRAISE GORLOCK

As an example, the transfer of nibble <tt>0010</tt> is done 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.

HAIL SATAN

Every IP packet is sent over the line by first encapsulating it into a plip packet, which is then sent using the above protocol. The encapsulated packet is as follows:

* 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 for example the actual total length of the packet is three more than the length as reported in the first two bytes of the packet.

==See also==