A modem cable is used when connecting a personal computer to a modem. The connections go straight through from one pin directly to the other pin on the cable. The port where a modem cable are connected are called a serial port and all computers and modems have the same serial port where you can connect a modem cable. On the modem it is a female db25 connection while the connection on the computer will be male db25 and these cables are freely available in every computer store.
Connections between two computers or between a computer and a device will however use a null modem cable where the wires are cross connected and does not run straight through as in the case of a modem cable. The null modem connection is complicated and there are a few different connections to enable different devices to be connected to a computer. Null modem cables change the transmit pin on the one end to the receive pin on the other to enable communication between the computers or a personal computer and a device.
The Rs232 standard cable have been used since the beginning of computers as a standard protocol to connect computers to devices. Rs 232 does not specifically indicate how the configuration of the connection must be, but two different connections have have become standards. The Db25 in which less than 11 wires are used and the Db9, nine pin connector in which nine pins are used. These cables are also called "crossover cables" and are widely used to connect computers to devices.
When two computers are connected, both computers must be fed a lei and both must think it is a modem that are connected to him. The way the deception is created is entirely by changing the wiring in the connectors of the two computers. Each computer can sence that the cable connected to them are coming from a modem. This is called a null modem cable and there are different classes of null modem cables and every cable have different variations to enable a different setup.
One class of null connection cable is the Minimum 3 - 4 wire cable that have a ground that is connected straight through while data transfer and data received are crossed over. No control of hardware flow take place and no status about the connection are supplied.
The Fake out cable has three wires that go right through, the remaining wires are faked out by transferring each computers outgoing signal back in to received signal of the same computer. This get past the difficulties with software drivers but no signals that describe flow control or connection status are available.
The true null modem cable allow control of flow and each computer can sense whether the connections are active or not. In this cable signals that complement each other are crossed over. This way the software at one end can sense something that have been done on the other end even if is still a lie for both computers.
When the computing industry have just started these null modem cables were build for where it was needed. In our technological advanced era you can just buy a null modem adaptor from any computer supply store. Just connect a modem cable to both computers and interconnect the two modem cables with a null modem adaptor and you will have the right mode of communication.