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structure of link is 4 data connected to control pins |
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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.
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.
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