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  Null Modem

A Null Modem is used to connect two DTE's together. It is usually used as the cheapest way of managing multiplayer games and files transmission betwen the computers using Zmodem Protocol, Xmodem Protocol etc. This can also be used with many Microprocessor Development Systems.

Null Modem Connections
Null Modem Wiring Diagram

Above is the general way of connection with the help of Null Modem. This connection needs only three wires (TD, RD & SG) to effectively use a long cable for a reasonable price. The theory of operation is reasonably easy. The purpose is to use the connection of two computers without using modems. The main thing is that any data transmitted from the first computer must be received by the second computer, for this purpose TD is connected with RD output. The second computer must also meet this condition, for this purpose RD output must be connected with TD. Signal ground (SG) should also be connected as it is the framework for transmitting signals.

The Data Terminal Ready is looped back to Data Set Ready and Carrier Detect on both computers. When the Data Terminal Ready is declared active, then the Data Set Ready and Carrier Detect immediately become active. At this point the computer supposes the Virtual Modem to which it is connected is ready and has detected the carrier of the other modem.

All left to worry is the Request to Send and Clear To Send. As both computers communicate together at the same speed, flow control is not needed thus these two lines are also linked together on each computer. When the computer wishes to send data, it declares the Request to Send high and as it's hooked together with the Clear to Send, It immediately gets a reply that it is ok to send and does so.

Note that ring indicator output isn't connected with any pinouts, as this output is used to notify the computer that there's a telephone line ring signal, and as we do not use modems this pinout remains disconnected.

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