Retractable Null Modem (RS-232C) Cable




About: Plane in my avatar is from Younkin Air Shows

Are you tired of never having a null modem serial cable when you need one...or tired of carrying around a bulky cable? I was, so I made one out of a retractable phone cable that I can easily carry in my pocket or toolkit.

By null modem cable I mean, female-to-female serial cross-over cable that uses DB-9 connectors. They use the RS-232C protocol. I mostly use null modem cables to upload firmware to TVs..and control them. Most TVs that have a RS-232C (Serial) port can be controlled using telnet (Like a wired remote, but more powerful).

I had a couple of retractable phone cables in the bottom of my drawer at work I thought I'd never use again, but I kept them anyway. As I was looking at them trying to figure out what I could do with them, I remembered that the RS-232C instructions in LG TV manuals say you only need 3 wires. I looked at one of the schematics and sure enough, inside the TV only the 2, 3, and 5 pins are connected. So I knew a retractable phone cable or Ethernet cable would work.

This will only work if your RS-232C devices use 4 or less wires. I haven't seen a retractable cable with more than 4 wires, but they may exist.

The second picture just shows two other types of retractable phone cables. I paid less than a $1 each for those. The wires in those are really small and hard to work with, so use the type I use in the instructions if you can.

Step 1: Needed Tools

Here are the main tools you need. Radio Shack number for the D-Sub connectors is 276-1428. The D-Sub Hoods is 276-1513. They have several different hoods at the shack, any will work. But..Avoid the metal ones, they're too heavy. You could even make your own hoods, just make sure you have something good to grab onto when removing the connectors from a device.

Step 2: Remove the Phone Jacks

First remove the phone jacks from the cable. I used pliers to pull back rubber stoppers before cutting off the jacks. I figured they would help with strain relief, but not required. Just snip off the jacks, but keep the cables the same length on both sides.

Step 3: Attach the Connector Pins

Strip the cables and then crimp on the connectors using pliers. Be sure to strip off enough to double-over the bare wire since it's so thin. You may also find it easier to lightly score the insulation with a knife before trying to strip it.

Step 4: Note the Wire Arrangement

After putting a pin on three wires, you're ready to add the connectors. This picture shows the wire layout. Note the #5 wire on both ends, this shows you how they're arranged since you can't tell with the cable wound up. Remember this is a cross-over cable, so the 2 and 3 wires are swapped on one end. You have to do this manually when inserting the pins into the connectors.

Step 5: Insert the Pins

Insert the pins into the proper connector holes. I'm using 2,3, and 5. You can change those if you are using the cable for something other than TVs. The pin holes are labeled on both sides of the connector. Insert a pin into the #2, #3, and #5 pin holes. Do the same on the other connector, but cross-over the #2 and #3 on one side. Use a small screw driver to press them into place.

If you did it correctly, the connectors will be orientated opposite of each right-side up and one up-side down. You can do a continuity test with a meter to check for correct pin layout if you want.

Step 6: Attach the Hoods

Now attach the hoods. You can squirt some silicone sealer in there to work as strain relief and to keep it from rattling. But, put it together first without the sealer and make sure it works.

Step 7: Screw in the Hoods

Put the screws in the hood. Use the two included nuts and the two screws that were fully threaded. I ignored the others. They are there to put through the sides of the connector and then you attach those little plates on the end. That keeps it from rattling and provides a more permanent connection to a device...but the silicone sealer mentioned before works better.

Step 8: The End

That's it. I hope you enjoy yours as much as I enjoy mine.



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    2 Discussions


    7 years ago on Introduction

    It's a great idea, but a lot of wasted work. Just Google "RJ45 to RS232" and you find items like this: then just connect it to a retractable Ethernet cable.

    1 reply

    Reply 7 years ago on Introduction

    Those aren't Null Modem. You'd still have to take them apart to cross-over 2 and 3. This is easier and local sourced parts. What you linked to is the exact reason i started making these and giving them out to people. We got tired of using those.