Null Modem adapter pinout from pc to pc

[abm086]

Hello,
I cannot seem to figure out how to wire up a null modem cable. I used a rolled cable with what I think is a null female db9 connector on each end. This did not work for me. Do I need one null connector and one straight through connector? If some one knows the pinout I need please let me know.
thanks
abm086

 

[franklin97355]

you need to swap pins so the signals from one computer arrive on the correct pins at the other computer. Pins 2 and 3 are the send and receive pins and pin 2 on one end needs to go to pin 3 at the other. pin 7 is ground at both ends. (this is for a 25 pin plug cause I can't remember the nine pin type.)

 

[daronwilson]

The data pins on a DB9 are 2 and 3 also, a basic null modem cable will need to swap these to pins at one end. Now...it isnt always that simple. In theory, you may get away with just having pins 2 and 3 connected (but swapped at one end) and the ground pin 5. However, depending on the requirements of the hardware and software, you may need to loop back or connect the hardware flow control.

So, if the regular 3 wire one doesnt work, give this a try:

DB-9 signal DB-9 signal
1 - DCD ---------------------|----- 7 - RTS
|----- 8 - CTS
2 - RXD --------------------------- 3 - TXD
3 - TXD --------------------------- 2 - RXD
4 - DTR --------------------------- 6 - DSR
5 - GND --------------------------- 5 - GND
6 - DSR --------------------------- 4 - DTR
7 - RTS -----|
8 - CTS -----|--------------------- 1 - DCD

The reason you may have problems is that the hardware and/or software may be implementing the two controls it can. The above cable connects Data Set Ready to Data Terminal Ready so that the devices can signal each other. Also, we are fooling the Clear to Send, Request to Send, and Carrier Detect.

There are more ways to configure thing if this still doesnt work. Depending on the equipment, I sometimes have to throw in my serial analyzer and see what is happening, then jumper accordingly.

Good Luck!

It is only my opinion, based on my experience and education...I am always willing to learn, educate me!
Daron J. Wilson, RCDD
daron.wilson@lhmorris.com