Multiple serial consoles via null modem cable

Jeremy Chadwick freebsd at
Tue Jan 12 16:08:19 UTC 2010

On Tue, Jan 12, 2010 at 05:14:44PM +0200, Marin Atanasov wrote:
> Hello there,
> I'd like to ask you about the following - is it possible to have multiple
> serial consoles coming from a single host?
> What I am talking about is connecting multiple machines using a null modem
> cable - I know it is possible only to connect two machines and they need to
> be connected on sio0 (COM1).
> I'm thinking about the following situation - 1 system acting like a host
> with a serial port hub, each port of the hub is connected to a different
> machine on sio0, using null modem cables.
> This would make the first machine something like a cheaper kind of a
> terminal concentrator :)
> What do you think, have someone tried this and is it possible at all?

What you're describing is basically the concept of a serial console
server, where a FreeBSD box contains a multi-port serial card that's
connected to multiple other servers.  An individual would get on the
FreeBSD box with a multiport serial card (see below) and attach to the
serial port associated (wired to) whatever other box they want to log on

Yes this is possible with FreeBSD -- but you'll need to purchase a
multiport serial card that's supported natively by FreeBSD.  The two I'm
familiar with are Cyclades and DigiBoard, but this would've been back in
the day of FreeBSD 2.2.x and I've no idea what people use present-day.
I'm certain others here can recommend stuff that works.

But my personal/professional opinion is that you consider purchasing an
actual serial console server rather than "hack up" a FreeBSD box.

Most serial console servers these days (and even old Portmasters) can do
serial-to-TCP association, which allows you to literally do "telnet
consoleserver NNNN" where NNNN is a TCP port which is bound to a serial
port connected to each individual server.  E.g. port 2000 = box#1, port
2100 = box#2, etc..

As far as present-day devices go, the ones I can recommend are the
LX-series devices[1] from MRV Communications.  They're Linux-based, and
although may seem expensive (16-port w/out modem = ~US$1400), are a
one-time purchase.  Cyclades also makes identical devices, but they cost
something like US$3000-4000, and they wouldn't provide me with a
test/demo unit so I blew 'em off.

You can also consider looking for used hardware -- either Xyplex devices
(MRV bought Xyplex), or Portmaster[2] products.  I believe Cisco made
some devices as well.  WRT Portmasters, be aware that some of the
firmware images are spotty in quality (HELP commands missing,
undocumented commands, etc.).

There's two ports which can make interfacing/using these devices, or a
multiport serial card, much easier -- Conserver[3].  I work with the guy
who wrote it, so I'm biased.  :-)


I'm not sure what the first port is to be honest, but the 2nd port is
what I'm used to using with an MRV LX-4016S device.

If you'd like a bit more detail about "how it all works" (the software,
the hardware, configuration details, wiring, etc.), I can describe it in
greater detail.  Just ask.


| Jeremy Chadwick                                   jdc at |
| Parodius Networking              |
| UNIX Systems Administrator                  Mountain View, CA, USA |
| Making life hard for others since 1977.              PGP: 4BD6C0CB |

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