Null Modem Cable

[pio]

I am looking for a 9 pin female to female Null Modem cable and the sales person at the store gave me a 9 pin female to female cross over cable...is this the same thing???


Thanks...

 

[Guest_imported]

null modem cable would usually F-M but yours is
a little strange. Cross over are not null modem
cables. Null modem cables are only extensions.
You may find yourself a F-F gender changer and use
it with your F-M null modem cable.

 

[fidleid]

Serial cables (modem cables) use pins three pins. One for transmit, one for receive and one for ground. Example...2-transmit, 3-receive, 7 ground. If you connect to standard serial ports together you will have transmit connect to transmit and receive connected to receive which will work. You must trans from on and receive at the other so in our example 2 on plug A must cross over and connect to 3 on plug B and 3 on plug A must crossover and connect to pin on plug B. The gender of the plugs has nothing to do with whether or they are null modem. Null Moden is the term used to signify that the transmit and receive are crossed over.

So the crossover cable you have might be ok....I have not seen the term cross over cable being used with serial connectors.

I repeat the plug genders have nothing to do with the null modem. I have them in every conceivable configuration .....

 

[edfair]

Modem cables use 7 wires. Serial cables for interconnecting between machines use 3 wires , but require jumpers to be added on each end.
A proper null modem cable will have the wires crossed TX/RX but will also have the DTR/DSR and CTS/RTS pairs crossed also.
For a 3 wire cable to talk between 2 machines you will need (on a 25 pin connection) pins 4&5 and 6&20 jumpered on both ends. One of these sets is critical (DTR I think) and the other is software dependent. I use both as a matter of course. Ed Fair
unixstuff@juno.com
Any advice I give is my best judgement based on my interpretation of the facts you supply. Help increase my knowledge by providing some feedback, good or bad, on any advice I have given.

 

[RayWilson]

Hey Buddy,

As you have a null modem cable I assume you want to connect two computers together so that they talk to each other. A null modem cable (or crossover cable - which I think is just different terminology for the same thing) will achieve this. The connector types can be different (they will be the same on both ends but can be in different form E.g. 9 pin or RJ45 etc).

Each connector end has a transmit and recieve pin (among others). What a null modem cable does is cross the transmit and recieve pins between the ends so that transmit sends to recieve. If you didn't do this you would have an ordinary straight through cable. This has no apparent use (to my knowledge) in 9 pin serial form but in RJ45 it can be used to connect a computer to a network connection point (otherwise known as a patch lead). RJ45 is the connector type you get on network cards (kinda like your phone). So, as an example, you could connect two PC's together via their network cards with a RJ45, null modem cable. You want to connect them via serial put the same applies. Please excuse the crude diagrams:

Patch Lead (Straight Through)

End1 End2
1 *************************** 1
T *************************** T
3 *************************** 3
R *************************** R
.
.
9 *************************** 9

Null Modem (Crossover)

End1 End2
1 *************************** 1
T --->--\                                   /----<- T
3 *********/---<---->--\********* 3
R ---<--/                                   \-->--- R
4 *************************** 4
5 *************************** 5
.
.
9 *************************** 9

HTH

R