Parallel Line Internet Protocol: Difference between revisions

Revision as of 18:52, 3 April 2007

Revision as of 22:50, 29 April 2007

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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.

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 toggletoggles 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.

As an example, the transfer of nibble <tt>0010</tt> is done as follows:

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t->r lines r->s lines operation

00010 0xxxx transmisttertransmitter sets data lines to 0010

10010 0xxxx transmisttertransmitter toggle ack line

recevierreceiver detects toggle and reads 0010

10010 1xxxx recevierreceiver toggle ack line

transmisttertransmitter detects toggle

When the transmisttertransmitter detectesdetects the toggle, this procedure is repeated for the next nibble.

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: