LapLink cable
Encyclopedia
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 port
s 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 be used by the InterSvr program in MS-DOS
6, and by Linux.
The term "Laplink" is a registered trademark used by Laplink Software Inc.
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. It 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 synchronize 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 time difference in parallel communication lines 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 port
s are capable of higher bandwidth than parallel ports. (High speed synchronous serial ports suffer from the same skew problem as high speed parallel ports.)
Direct cable connection
Direct Cable Connection , is a feature of Microsoft Windows that allows a computer to transfer and share files with another computer, via a connection using either the serial port, parallel port or the infrared port of each computer...
. The connection is achieved via 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...
s on the two computers. No networking hardware such as a network interface card or a modem
Modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
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 be used by the InterSvr program in MS-DOS
MS-DOS
MS-DOS is an operating system for x86-based personal computers. It was the most commonly used member of the DOS family of operating systems, and was the main operating system for IBM PC compatible personal computers during the 1980s to the mid 1990s, until it was gradually superseded by operating...
6, and by Linux.
The term "Laplink" is a registered trademark used by Laplink Software Inc.
Comparison to null modem cables
A LapLink cable can be seen as a parallel equivalent to a serial null modemNull modem
Null modem is a communication method to connect two DTEs directly using an RS-232 serial cable. The name stems from the historical use of the RS-232 cable to connect two teleprinter devices to modems in order to communicate with one another; null modem communication was possible by instead using...
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
RS-232
In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
port) as a means to install Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
to machines without a CD-ROM
CD-ROM
A CD-ROM is a pre-pressed compact disc that contains data accessible to, but not writable by, a computer for data storage and music playback. The 1985 “Yellow Book” standard developed by Sony and Philips adapted the format to hold any form of binary data....
/DVD
DVD
A DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than Compact Discs while having the same dimensions....
station or 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....
port built-in. It can also be used as an (additional) failover
Failover
In computing, failover is automatic switching to a redundant or standby computer server, system, or network upon the failure or abnormal termination of the previously active application, server, system, or network...
link between ha-cluster
High-availability cluster
High-availability clusters are groups of computers that support server applications that can be reliably utilized with a minimum of down-time. They operate by harnessing redundant computers in groups or clusters that provide continued service when system components fail...
nodes
Node (networking)
In communication networks, a node is a connection point, either a redistribution point or a communication endpoint . The definition of a node depends on the network and protocol layer referred to...
.
However, the clock speed of any parallel interface is limited due to the need to synchronize 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 time difference in parallel communication lines 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
SCSI
Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...
parallel peripheral interface.) For this reason, high speed asynchronous serial port
Serial port
In computing, a serial port is a serial communication physical interface through which information transfers in or out one bit at a time...
s are capable of higher bandwidth than parallel ports. (High speed synchronous serial ports suffer from the same skew problem as high speed parallel ports.)
Wiring
The wiring of the cable is as follows (the two connectors are DB25 males):| 2 | 15 |
| 3 | 13 |
| 4 | 12 |
| 5 | 10 |
| 6 | 11 |
| 10 | 5 |
| 11 | 6 |
| 12 | 4 |
| 13 | 3 |
| 15 | 2 |
| 17 | 19 |
| 18 | 18 |
| 19 | 17 |
| 21 | 21 |
| 22 | 22 |
| 23 | 23 |
| 25 | 25 |
See also
- Parallel Line Internet ProtocolParallel Line Internet ProtocolThe Parallel Line Internet Protocol is a computer networking protocol for direct computer-to-computer communications using the parallel port normally used for connections to a printer....
(PLIP) - Direct cable connectionDirect cable connectionDirect Cable Connection , is a feature of Microsoft Windows that allows a computer to transfer and share files with another computer, via a connection using either the serial port, parallel port or the infrared port of each computer...
- Serial console