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Null Modem Cable.

A null modem cable is a form of RS-232 standard where the transmit and receive lines are cross linked. Normally a straight through serial cable is used with this kind of adapter. The adapter is pre configured with the necessary cross links. In some cables, handshake lines are also cross linked.

The null modem cable was designed with an intention to connect two teleprinter terminals directly without using modems. With the continuous growth of null modem and the adaptation of its RS-232 standard by other types of equipment, it was later improvised by developers to have DTE and DCE type  interfaces.

DB 9 null modem cable Accordingly, there exists two basic categories of Null modem. They are :

  • Female-to-female end null modem – They are mostly used in such  processes in which there is a requirement to interconnect two DTE systems.
  • Male to male end null modem – They are mostly used in such  processes in which there is a requirement to interconnect two DCEs. equivalent of this null modem would be used to connect two DCE systems.

Depending on the type of communication under use, this Null modem will consist extra connections. In this case, DB25 connector will be required. For example : Synchronous type of communication needs extra connection for timing signals.

The various signals, their pin configurations and direction is described below:

Null modem cable - signals It was a time, when null modem cables were commonly used for remote operations or file transfer between computers. However, now a days, virtual null modem is also being used in place of null modem cable. A virtual null modem is a method by which different computer applications can be connected directly with the help of a virtual serial port. In other words we can say it as a virtual null modem is a software application which simulates a hardware null modem within the computer.  A virtual null modem contains all features of a null modem cable in it. A virtual Null modem has following advantage over physical Null modem cable:

  • very high transmission speed of serial data
  • cable length restrictions no longer exist.
  • possibility of unlimited number of virtual connections.
  • computer’s physical serial ports remain free.

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