re-use cisco console cables

Posted 2025-11-01

Somehow, when you are in IT or into computers and computer-y things, you end up with more C13 to NEMA 5-15P power cables (for those of us in North America anyway) than you can shake a stick at (or whatever). Similarly, when you are a network engineer or are into network gear, you end up with a similar count of Cisco compatible console cables. You know, the cables that are 8P8C terminated on one end and DE-9 terminated on the other like this:

Well, I have a substantial stash of these, and finally decided that it would be nice to use these in other serial communications type situations. So, I decided to build a couple of connectors to allow me to do just that.

Here is how a Cisco compatible console cable is wired:

DE-9 Female    8P8C Male

   RXD 2─────────TXD 3
   TXD 3─────────RXD 6
   DTR 4─────────DSR 7
   GND 5───────┬─GND 4
               └─GND 5
   DSR 6─────────DTR 2
   RTS 7─────────CTS 8
   CTS 8─────────RTS 1
      

What I decided to do is use a pair of 8P8C keystone jacks to connect two cables together. This allows me to reconfigure which signals land on which wires to suit the application. For my specific needs, I mainly have to use cases.

Use Case One: Standard straight-through serial cable

This is pretty straight forward as two console cables just need to be connected 1 to 1. So you simply connect each position between the two keystones the same: pin 1 to pin 1, pin 2 to pin 2, so on and so forth. For this case, it may make sense to just buy pre-made 8P8C couplers, but I always have a stock of keystone jacks.

Use Case Two: Null modem cable

A lot of times, when you need to connect two devices together via RS232, you need what is known as a null modem cable. Now, unfortunately, Cisco compatible console cables are missing the Data Carrier Detect (DCD) line, so we cannot configure a pair as a true null modem cable, but we can get close. Simply join two 8P8C keystone jacks in the following configuration:

Keystone A     Keystone B

   RTS 1──────────CTS 8
   DTR 2──────────DSR 7
   TXD 3──────────RXD 6
   GND 4──────────GND 5
   GND 5──────────GND 4
   RXD 6──────────TXD 3
   DSR 7──────────DTR 2
   CTS 8──────────RTS 1

You may have noticed something here. All we've done is simply inverted the pinout. This is also known as "rolling" a cable.

The finished product may look something like this:

Back to that missing Data Carrier Detect (DCD) line. As I mentioned before *most* use cases are OK without this. However, if you run into a scenario where the cable is not working, it may be that a device is expecting to see DCD. The only device I have personally run into this with is the Nortel Meridian 1 Option 11C's Small System Controller. You will not get any response unless DCD is asserted.

One way you can "cheat" here is by simply bridging DCD to Ground (GND). However, a true null modem cable will tie DCD into the Data Set Ready (DSR) line on each side. Pretending we had DCD available, the coupler layout would look like this:

Keystone A    Keystone B

   RTS 1────────CTS 8
   DTR 2────────DSR 7─┐
   TXD 3────────RXD 6 │
   GND 4────────GND 5 │
   GND 5────────GND 4 │
   RXD 6────────TXD 3 │
 ┌─DSR 7────────DTR 2 │
 │ CTS 8────────RTS 1 │
 └─DCD X        DCD X─┘

Why is a Cisco compatible console cable missing Data Carrier Detect? Well, the Cisco compatible 8P8C serial pinout is designed in such a way that if a null modem cable is required, the cable simply needs to be rolled over. To keep GND aligned, the use of two wires was necessary. Because there are only 8 positions to work with, and DCD isn't required, it was sacrified.

If you *do* run into a situation where DCD is required, and you still want to put your stockpile of Cisco compatible console cables to work, there are various types of 9 pin D-sub kits you can buy to make your own connector. That will allow you to bridge DCD on the end that has the requirement. This is what I have done for the Meridian 1 Option 11C's SSC. In this case, I simply bridge DCD to GND.