Friday, July 30, 2010

Rollover cables for terminal server

In the previous post, I detailed configuration of a Lantronix SLC box.

The serial ports on this device are pinned exactly the same as a Cisco router's console port. You also get the same result with the chrome DB-25 and DE-9 connectors shipped by Sun, and the DE-9 adapters that Cisco used to ship.

They all look like this:
PIN Signal
1 Don't Care
2 Don't Care
3 Transmit
4 Ground
5 Ground
6 Receive

7 Don't Care
8 Don't Care

Those 'Don't Care' pins usually do something related to hardware flow control, but unless you're dealing with an old XL series Catalyst, you probably won't need to think about it.

For these devices to talk to one another, you'll need to connect Transmit to Receive, and Ground to Ground. Looking at the chart, it should be obvious why a "rollover" cable is important here.

I've seen customers use common straight-through Ethernet cables with lots of rollover dongles like this:
Those things work perfectly fine, but they're ugly (you might have 48 of them in 1RU), and never seem to be available when needed. And check out the price! Per-port, they cost almost as much as the SLC itself!

You could also use the traditional console cable like the ones that Cisco used to ship:

The problem with these is that they tend to retain their shape, and only bend in two directions. High-density cable management is awful.

My preference is to get a bag of 8P8C ends, a cheap crimper, and a big roll of this stuff. Then I leave all of it right alongside the terminal server.

It's cheap, it's super-easy to make a cable, because the conductors cooperate (not like making an Ethernet cable), the resulting cable is always exactly the right length. Also, the two-pair stuff is both small and flexible. It's a cable management dream.

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