Improving on the 5-in-1 Network Admin’s Cable

I recently discovered Michael Ossman’s 5-in-1 network admin’s cable. Before this discovery, I built serial cables with the same DB9/RJ45 modular adapters, but never with a standard pinout so they could be interchanged, never with the hardware flow control lines plumbed, and certainly never as part of a multifunction cable. Ossman’s system can even be expanded on; his article has an “Extras” section, and Jason McPherson wrote an article on building an ethernet and serial tap with the cable.

In this article, I describe how I changed the cable to meet my own needs better. I hope this is useful to others, too.

Why improve something already perfect???

I made two minimal improvements upon Ossman’s cable. First, in his cable, two identical RJ45-DB9 adapters are used with a mini-crossover cable, an RJ45 coupler and an ethernet cable to make a null-modem cable. In my environment, I find myself usually needing a null-modem cable to connect my laptop with a server’s serial port; the straight-through modem cable configuration is never needed.

My laptop is a ThinkPad X24, the ultra-portable X series, with no serial port on the back. In my toolkit, I throw in a USB<->serial adapter along with the 5-in-1 network admin’s cable. When used in the normal configuration, this is quite an unwieldy series of dongles:

To eliminate the two circled components above and infinitesimally reduce this mess, I built an RJ45-DB9 adapter with the crossover built in (note the label “X” below), eliminating the need for the mini-crossover cable and the RJ45 coupler in a normal PC-to-PC connection:

The second improvement is in the construction of the adapter, eliminating the need to purchase extra pins and a crimper. A soldering iron is still needed.

DB9/RJ45 Crossover Adapter Pinouts

If you’re reading this, you either have built or plan to build Ossman’s 5-in-1 network admin’s cable. You still need one DB9/RJ45 adapter exactly as described in his article, step 4. The second one is similar, but build it with the following pinouts:

    DB9 pin  signal  RJ45 pin    color
    -------  ------  --------    -----
      1       DCD      5         green
      2       RxD      1         blue
      3       TxD      3         black
      4       DTR      8         white
      5       SG       2 and 6   orange and yellow
      6       DSR      5         green
      7       RTS      7         brown
      8       CTS      4         red
      9       RI       none

If you compare this with the original adapter, you’ll notice that on the RJ45 connector side, pins 1 and 3, 5 and 8 (including the split), and 4 and 7 are swapped. This is exactly what the mini-crossover cable does in the original cable.

H3C switch adapter pinouts

This is the adapter for H3C switches (our company is too cheap for Cisco!). Note that this is a bit tricky since you must splice pins 4 and 7 together on the H3C side to provide both DCD and DSR to the switch.

I chose the convention of using the straight through DB9/RJ45 adapters on the equipment side, and the crossover adapter on the terminal side. The below chart has wiring diagrams for both crossover and straight; remember you need a crossover somewhere in the cable, so if you use the crossover adapter on the terminal side, then build the straight through adaptor, and vice-versa.

    RJ45 pin   RJ45 pin   signal  dir  RJ45 pin   color
    straight   X-over                  H3C side
    --------   --------   ------  ---  --------   -----
      3          1        RxD     <--   6         orange/white
      6          2        SG      ---   5         orange
      1          3        TxD     -->   3         green/white
      7          4        CTS     <--   8         blue
      8          5        DCD     <--   4,7       blue/white
      2          6        SG      ---   (none)	  (no connection)
      4          7        RTS     -->   1         brown/white
      5          8        DTR     -->   2         brown

Assembling the Adapter

I am a foreigner living in Beijing, and have no idea how to find things like the extra female pins for the DB9 connectors or their crimpers. Since this layout only uses 8 pins of the DB9 anyway, if you’re careful, you can build the adapter without the extra pins. Here are the steps I use.

1. I also happen to be colorblind, so instead of identifying the wires by color, I have to look into the connector to see their relative positions. The first thing I do is to separate the wires, pinching them into the slots on the edges of my connectors. Your connectors may or may not have these. You can see all the way to the bottom in this photo.
   
2. Cut and strip the wires
   1. Use a sharp knife (like an X-acto or utility knife) to strip a 3mm section from the middle of the orange wire. Try to do it as close to the RJ45 end as possible to give yourself room to work with later.
   2. Cut the yellow wire to the length of the end of DB9-side of the stripped section, and strip 3mm from the end of the yellow wire. Save the piece you cut off!
   3. Match the yellow piece cut off against the green wire (for the crossover adaptor) or the white wire (for the original adaptor), and mark its length. Then strip a 3mm section of the green or white wire as in the last step.
   4. Finally, strip 3mm off the end of the yellow wire you cut off.

   

3. Connect and solder wires
   1. Twist the orange and yellow wires (coming from the RJ45 side) together. Twist the green or white and the cut-off yellow end together.
   2. Solder the two places you twisted the wires together. Don’t hold the solder on the wire too long, or it will melt the wire’s insulation, and the solder will run inside the insulation and make the wire stiff and inflexible.
   3. If you have some small shrink tubing, put it over the wires and shrink it into place. I didn’t have any and used packing tape, which is ugly and doesn’t hold the wires together as well as shrink tubing, but it will prevent the two solder joints from contact and shorting the circuit.

   

4. Assemble the cable
   1. Finally, assemble the cable with the pinout from above.
   2. Add a small sticker with an “X” to indicate that this is the crossover adaptor.

   

5. Finally, twist the wires up a little and snap the assembly together! Don’t twist too tight, since your solder joints are probably the weak link.These are all the tools needed to create the entire 5-in-1 network admin’s cable.
   
6. When building these, it’s very handy to have an insertion/extraction tool:
   

Using the cable with the crossover adaptor

There are some slight differences in using the crossover adaptor. Here are the 5 cable combinations.

1. Ethernet cable:



2. Crossover cable:



3. Modem cable; this is straight-through (X + X = straight through):



4. Null-modem cable; this is good for PC-to-PC:



5. Cisco cable; use the original straight-through adaptor:

My commando cable kit

I love this so much, I have to show it off. Quit reading as soon as you can’t stand it!

My commando cable kit goes in my laptop bag, and consists of my 5-in-1 cable and the USB serial adaptor. The ethernet cable is a retractible, and amazingly enough, actually rated CAT-5 (the 4 twisted pairs are laid out into a flat cable rather than a round cable). The kit also includes a rectractable USB extension cable, and an adaptor for 4 of the most common USB connectors. And of course there’s a flash drive.

You should see my commando TOOL kit!