As your server closet gets more crowded with more and more servers, switches, routers and firewalls, the ability to monitor them becomes an increasingly difficult task. I started using KVM switches (Keyboard/Video/Mouse) to control my servers, but as the number of servers increased, I found that the tangle of thick KVM cables became a real bother. The back of my servers is already a web of Ethernet and power cables. The KVM cables made things a lot worse and in some cases would knock loose a power cable and bring down a server.
An alternative method for controlling equipment is via serial connections. Most networking devices come with some sort of serial port. In some cases the port looks like an Ethernet port and is often labelled "console" and in other cases the serial port looks like the typical serial port on the back of your computer. What many people may not know is that most servers have the ability to be controlled via the serial port. There are limitations, such as no mouse control and no graphical interface, but these are not limitations for servers controlled via the command line. While Windows can be controlled via the command line, most people don't do so.
Even if your computer does not have the ability to redirect your video and keyboard output to the serial port, Linux and Unix have utilities that make this possible. So what are the advantages? Price is a big one. KVM switches, especially ones that can be remotely administered, can be expensive. The PortMaster box shown above is under $200. You will not find a KVM device that can monitor 30 units for anything close to this price. Another advantage is cable management. Instead of a three-part KVM cable, you're only running a serial cable. And, with special connectors, you can run an Ethernet between devices and plug it into a serial port adapter. This means that your equipment can be hundreds of feet away.
The one downside to the PortMaster box is that it does not come with an encrypted access port. This means that you need to use either a modem to connect directly to the unit or connect securely to one of your computers via SSH and then telnet over your internal network to the box. That's not a big constraint, but it's one you need to be aware of.
As I get more experience with this unit I post on my experiences.