Log In

Come Join Us!

Are you a
Computer / IT professional?
Join Tek-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Tek-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Modem Cables

Modem Cables

Modem Cables

can someone explain to me or point me to a site that has diagrams of the configuration for modem cables, I am having a problem finding two that are wired the same way but they all work on the machines or modems they were supplied with.

Is there not a universal method of wireing them?


RE: Modem Cables

What type of modem cables are you talking about? DB-9's, 25's, etc? Or regular old single pair?

Matt Wray

RE: Modem Cables

The RS-232 'standard' has enough variations so no one knows what pins may be in use, the only reason to hope for USB analog modems is to kill RS-232.  (on PCs, there is also the speed of the UART, but the real hope for USB is to kill RS-232)

I tried to remain child-like, all I acheived was childish.

RE: Modem Cables

Basic configurations:  All these are RS232-c standards


Pin outs - ie: what's what.
DB25         DB9           Signal Name       Direction
 1                           protective gnd

 2            3              TX Data           DTE->DCE

 3            2              Rx Data           DCE->DTE

 4            7              RTS               DTE->DCE

 5            8              CTS               DCE->DTE

 6            6              DSR               DCE->DTE

 7            5              Signal Ground (common)

 8            1              DCD               DCE->DTE

 20           4              DTR               DTE->DCE

 22           9              RI                DCE->DTE

 23                          DSRD              DCE->DTE

Bottom LINE.

All that is absolutely required is TX, RX, and Sig Gnd(common).  Everything else is gravy.  All wires are straight through connections, WITH ONE EXCEPTION.  TX AND RX MUST BE CROSSED. That is TX/RX pins 2/3 on DTE are RX/TX on DCE pins 3/2.

What's DTE and DCE mean.  DTE is data terminal equipment (a pc or printer)  DCE is data communication equipment (modem).

Hope this answers it all.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Tek-Tips Forums free from inappropriate posts.
The Tek-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Tek-Tips forums is a member-only feature.

Click Here to join Tek-Tips and talk with other members! Already a Member? Login

Close Box

Join Tek-Tips® Today!

Join your peers on the Internet's largest technical computer professional community.
It's easy to join and it's free.

Here's Why Members Love Tek-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close