A LapLink cable (also known as lablink or null-printer 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 cable is named after the Laplink software package, which was designed to make such a connection possible (many years before it became a Windows term and feature). It can also be used by the InterSvr program in MS-DOS 6.
A LapLink cable can be seen as a parallel equivalent to a serial null modem cable. Because of the higher bandwidth of the parallel port versus the serial port, a LapLink cable is able to shift data more quickly at the same clock speed, which was important at the time it was developed, since clock speeds then were relatively slow. This is still the reason it is used (rather than a RS-232 port) as a means to install Linux to machines without a CD-ROM/DVD station or Ethernet port built-in. And can also be used as an (additional) failover link between ha-cluster nodes.
However, the clock speed of any parallel interface is limited due to the need to synchronise all of the signals representing data bits (and, to a lesser extent, all of the other signals as well). The problem is that slight differences in the thickness, insulation, or spacing of the wires in a parallel cable can cause different signals--on different wires--to arrive at slightly different times (microseconds apart) at the far end of the cable, even when they were sent at exactly the same time from the near end. This is called skew. At very high clock speeds, these very small time differences become significant, causing an upper limit on the usable clock speed of the parallel interface. By precisely controlling cable quality, it is possible to reduce skew enough to allow higher parallel clock rates, but this becomes prohibitively expensive beyond a certain point. (This was the primary reason that speed advancements ceased, and popularity declined, for the SCSI parallel peripheral interface.) For this reason, high speed asynchronous serial ports are capable of higher bandwidth than parallel ports. (High speed synchronous serial ports suffer from the same skew problem as high speed parallel ports.)
The wiring of the cable is as follows (the two connectors are DB25 males):
Vice versa, pins 15-11 on the first connector are wired to 2-6 of the second connector.