Null Modem Serial Cable or Non-Null Modem Serial Cable

[N8EUI]

Hi all,

When connecting a packet radio system and the TNC needs a cable with a male DB25 connector at one end and a DB9 female at the other, do I use a null modem serial cable or a non-null modem serial cable?

Thanks,
Tom, N8EUI

[W8JX]

Kinda depends on TNC. It is is a design from 90's or older, you will likely need one. If is is a newer design you will likely not need one.

[K0JEG]

TNCs are considered Data Communications Equipment (DCE), which have 9 or 25 pin female connectors. Computers are Data Terminal Equipment (DTE), which have 9 or 25 pin male connectors:

http://en.wikipedia.org/wiki/RS-232#Connectors

Straight through, 9 pin to 25 pin cables or adapters will work. A null modem cable is for connecting 2 computers together directly without a modem in between.

[W8JX]

While it is true that you need a null modem to connect old PC's i do recall needing it for some old modems and TNC's too long ago.

[N4CR]

Serial communications require a minimum of three signals to operate. More if you are going to use hardware flow control. If you assume you do need hardware flow control you can't go wrong.

Gnd, Tx and Rx. Get the pinouts of each end and determine if each end is DCE or DTE.

If the ends are different, you need a straight through cable. If both ends are the same (and it doesn't matter which both) you need a null cable. A little investigation of your connections will reveal which cable you need. Look specifically for the Tx and Rx pins.

There is an excellent description here: http://www.camiresearch.com/Data_Com_Basics/RS232_standard.html#anchor242192