Cisco Console Terminal Cables

After doing some research on the subject, I was totally confused about Cisco terminal cables. These are just regular serial cables, with with all kinds of weird “cisco-y” features. The big tutorials on the web really confuse the issue by trying to cover every possible type of cable.

I’ll explain here how to create an adapter with a DB9 on one end, and a regular CAT5 ethernet cable stuck in the other end, that plugs into the Console or AUX port on a Cisco 2800 router (but should work with many routers).

First, get a DB-9 female to RJ45 8P8C modular adapter. This has the DB-9 on one end, and a RJ45 jack on the other, and a bunch of loose wires allowing you to connect the RJ45 pins to the DB9 pins.

I get mine from ITC Electronics because they’re close by. It costs $2.25.

The colored wires are usually like this:

1 blue
2 orange
3 black
4 red
5 green
6 yellow
7 brown
8 white

The Cisco RJ45 ports are wired using a weird Cisco scheme. [It’s not weird – it’s the Yost serial cable standard. It’s a good way to wire serial cables.] It’s not one of the standards schemes. It works like this:

TxD 3
RxD 6

The RS232 pins on a DB-9 are like this:

TxD 2
RxD 3

So, just wire them up like that.

RTS is blue – it goes into pin 8 of the DB9.
DTR is orange – it goes into pin 6 of the DB9.

The tutorials out there talk about “rolled cables” and whatnot, but you don’t need to worry about that. Just wire it straight like the above, and you’re OK. You can leave pin 5 of the RJ45 (the green one) disconnected. At least I did, and it worked.

(I think, normally, the little adapter you make is wired so a rolled cable is required — but that seems totally foolish. The rolled cable looks just like an ethernet cable from afar. That would get lost in the toolbox. It’s better to just use ethernet cables and arrange the pins to work with ethernet cables.)

After all this is set up, you can use minicom (in Linux) to connect. Settings are 9600 8N1, no hardware flow control.

[After learning about Yost, I have to rescind my comments. The rolled cables are unique but the allow you to pretty easily make a wide range of common serial connections. Ignore what I wrote, and just make a Yost rolled cable and a DTE adapter, and you’re set.]

[What makes the rolled cable nice is that if you connect two computers – that is, two DTE devices – you then use two DTE adapters, and the cable will act as a null modem. If you connect to a DCE device, you use the DCE adapter, and it works. It’s just un-doing the null-modem and making it a straight through connection. Now, the real clevernsess of the rolled cable emerges when you have two DCE devices in a row – then you put DCE adapters on everything, and use the rolled cable.]