Is USB the new Serial Console?

I'm happy to report that I finally have access to a Cisco device with a USB console (a gently-used C1900 router), so I can experiment a bit with it. I also have had an interesting briefing about how these USB console interconnects came to be at Cisco, and some caveats.

As I understand it, essentially they have embedded a USB-to-EIA-232 bridge chip into the Cisco device. When you connect a PC, and you have suitable drivers, it simply appears to your machine to be a simple COM port. (I'm presuming that a MAC will see s suitable TTY and etc.)

When the serial bridge chip comes on-line, it steals the line console 0 data path from the RJ-45 console connection(!). I need to check if that means that some speed-shifting is also happening. That is, if my line console 0 is set for 9600 8-N-1, does it shift to 115,200 when it swings to the USB channel?

When the USB chip powers down (when you unplug the USB), the data path is supposed to drop back to the RJ-45 port. But, if you leave the USB connected, you could find yourself without access to the RJ45 console for remote administration. Fortunately, there is an idle timer to revert back to RJ45, but this wasn't implemented in hardware in the earliest versions of equipment to have the USB console. I'll summarize more of the details in the next couple weeks. I'm excited to finally get a chance to explore this, since many folks have asked me questions about it already.

This sounds like a great addition for quick desktop administration, or for jacking into equipment with your laptop in a data center. After all, fewer laptops have a built-in DE-9 connection these days, and everything has a USB port, right? With a USB console, you only need the single USB(A)-to-USBmini(B) cable. But, it hasn't (yet) become a ubiquitous connection, so we probably still also need to carry our USB-to-Serial dongle, and also our DE-9 to Cisco-RJ45 cable as well.

Check back in August to read about the results of my hacking.


Willie Sager said...

Nice post with awesome points! Can’t wait for the next one.

HP - 15.6" Refurbished Pavilion Laptop - 4GB Memory - 640GB Hard Drive - Dark Umber

summer lyra miles said...

Well, it is interesting more people uses USB to work on it. Anyway, thanks for sharing.