Null Modem Cable

DESCRIPTION: A Null Modem is used to connect two DTE's together. This is commonly used as a cheap way to network games or to transfer files between computers using Zmodem Protocol, Xmodem Protocol etc.
          This can also be used with many Microprocessor Development Systems.

          Above is my preferred method of wiring a Null Modem. It only requires 3 wires (TD, RD & SG) to be wired straight through thus is more cost effective to use with long cable runs. The theory of operation is reasonably easy. The aim is to make to computer think it is talking to a modem rather than another computer. Any data transmitted from the first computer must be received by the second thus TD is connected to RD. The second computer must have the same set-up thus RD is connected to TD. Signal Ground (SG) must also be connected so both grounds are common to each computer.

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

          All left to worry about now 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 asserts 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.

          Notice that the ring indicator is not connected to anything of each end. This line is only used to tell the computer that there is a ringing signal on the phone line. As we don't have a modem connected to the phone line this is left disconnected.

          Or you can use standard modem cable to connect to a modem decsribed on schematic below.

Modem Cable

          The shell is connected to the chassis ground.

LoopBack Plug

          This loopback plug can come in extremely handy when writing Serial / RS232 Communications Programs. It has the receive and transmit lines connected together, so that anything transmitted out of the Serial Port is immediately received by the same port. If you connect this to a Serial Port an load a Terminal Program, anything you type will be immediately displayed on the screen.

          Please note that this is not intended for use with Diagnostic Programs and thus will probably not work. For these programs you require a differently wired Loop Back plug which may vary from program to program.