Serial Port Concentrators vs. KVMs

Gary Stiehr gary at
Wed Apr 23 14:23:19 EDT 2003

	Having used both KVMs and serial port concentrators, I have my own 
opinions about the advantages and disadvantes of each.  I was hoping 
that list members might share their opinions as well.  My experience is 
with Belkin 8-port KVMs and with a Computone RAS2000 serial port 
concentrator.  Here are some of my opinions, please feel free to add to 
the list or correct me if I'm wrong.  In particular, any comments on 
scalability or some price comparisons would be interesting.

KVM Advantages
* Ease of Setup: usually you just run the keyboard/video/mouse cables to 
the KVM and then a set of keyboard/video/mouse cables from the KVM to 
some other node from which you can access the console for all of the 
nodes attached to the KVM.  There usually is nothing that needs to be 
done with the OS (although I've heard of some BIOSes having problems but 
I've never experienced this).  There is also usually nothing to set up 
with the KVM itself--just hook up the cables.

KVM Disadvantages
* Lots of cables: Even if you do not use a mouse cable, you still have 
two cables running from the back of each node.  I have heard of some 
KVMs lately that use an adapter to combine all three kvm cables into 
one.  I have not actually seen or used one but that would certainly help.

* No remote access: The only KVM switches that I have seen with remote 
access are "enterprise" KVM switches that have a high price tag.  I have 
no experience with this type of KVM switch but I would imagine it would 
be like a hybrid KVM/serial port concentrator.

Serial Port Concentrator (SPC) Advantages
* Remote access: Most SPCs that I looked at listed remote access as a 
feature.  And some, including the Computone RAS2000 that I use, allow 
you to access the them via ssh.

* Less cables: You only need to run one cable from the back of each node 
(from the serial port) to the SPC.

* Multiple access methods: As noted above, you can access a lot of SPCs 
via the network.  But if that is down, you can also access the SPC via a 
node that is attached via serial port to a special port on the SPC.

Serial Port Concentrator (SPC) Disadvantages
* Need to set up the SPC itself: In my case, this wasn't too bad. 
Unfortunately, I would think that each vendor would have its own set of 
procedures to follow for the setup of its own SPC.

* Somewhat of a learning curve:  If you have not had experience with 
serial ports (i.e., you know what they are but you've never done 
anything with them), there will be a lot of terms that are unfamiliar. 
You will also need to find out a lot of information about your hardware, 
OS and BIOS.  For instance, what speed do they support (9600 baud, 
115200 baud, etc.)? What terminal emulation do they support (vt100, 
vt102, ansi)? Is my serial port enabled in the BIOS?  Which serial port 
is which (For Linux: /dev/ttyS0, /dev/ttyS1, etc.)?  And so on.

* A significant number of small changes to OS: There are a number of 
changes that you need to make to the OS (in my case Linux) in order for 
the console messages to be sent to the serial port.  Thanks to various 
how-tos and other docs, I was able to make all of the appropriate 
changes but a lot of them were not very obvious (although once you read 
about them you can see why it would be necessary).

* Must access the BIOS on each system:  Unless your BIOS has serial port 
redirection enabled by default (if it has this feature at all), you will 
need to access each BIOS as you set the systems up (if you want to see 
console messages generated by the BIOS).

Thanks for reading this somewhat lengthy e-mail.  I would appreciate 
your comments.

Thank you,
Gary Stiehr
gary at

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