null modem

A direct cable connection from computer to computer.

Usually, modems operate on the RS232 standard. In that standard, one line is for transmit and one line is for receive. (The only other required line is a common ground or node.)

Cabling two computers together directly, instead of supplying modems and a telephone line, requires a crossover cable so that the transmit and receive lines switch roles. Such cables are called null modems.

The same crossover technique can be used in CAT5 or other twisted-pair Ethernet cables as well. This removes the need for an intermediary hub between the two computers.

Here is how to construct your own null modem connector:

1) Connect TD of each end with the RD of the other
2) Connect CTS of each end with the RTS of the other
2) Connect DSR of each end with the CD of the other
3) Connect SG of each end with the SG of the other
4) Connect DTR of each end with the DSR on the same end
5) If both ends of the connection are to be 25-pin, then also connect pin 1 (Frame Ground) to pin 1 of the other.

Depending on whether you are using a 9-pin or 25-pin serial connector, the pin numbers will be different.
Here are the ones that are essential for a null modem connection.
A complete list can be found in the serial port pinout node.

25-Pin		9-pin
----------------------
2	TD	3
3	RD	2
5	CTS	8
4	RTS	7
6	DSR	6
8	CD	1
7	SG	5
20	DTR	4