Null modem cable pin-out for 25 to 9 1

[Sam92Legacy]

I'm not sure this is the right forum, but it seems that someone here will probably know the answer. I need to make a null modem cable to connect my voice mail system (25 pin) to my computer's 9 pin serial port. Does anyone know the pin-out for this situation? Also, I'm planning on doing a 50' run for this cable. Is that far enought where I would need an amplifier?

Thanks,

Sam

 

[daronwilson]

Well there are many ways to do it, depending on what is needed. In short, you need a transmit, a receive, and a ground. So pin 2 goes to pin 2, pin 3 to pin 3, and pin 7 on the DB-25 to pin 5 on the DB-9. That will get you there with no hardware flow control. If you find you need hardware flow control, it is a few more pins.

You should be able to get 50', though I would suggest a shielded cable, use the shield (drain wire) for your ground connection, and two wires for the transmit and receive.

Hope that helps, if you find you need hardware flow control, I think I've detailed that all out before in this forum, but if the simple cable doesn't do it, you may have to add some hardware flow control:


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

As you can see here, we fooling Clear to Send (CTS) and Request to Send (RTS) and using the Data Set Ready (DSR) and Data Terminal Ready (DTR) as they are intended to be used. The above is for a 9 pin null modem, so you would need to convert one end to DB25.

Hope this helps, if neither of these work...ask again and we'll find another configuration for you to try.

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

 

[Sam92Legacy]

Thanks, Doron. I'll try it out as soon as I can get the cable.