From owner-freebsd-hardware@FreeBSD.ORG  Wed Jan 20 05:11:57 2010
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In-reply-to: Your message of "Fri, 15 Jan 2010 08:34:17 +0200."
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Date: Tue, 19 Jan 2010 17:06:52 PST
From: Dieter <freebsd@sopwith.solgatos.com>
Subject: Re: Multiple serial consoles via null modem cable 
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>From another thread awhile back, Jeremy Chadwick wrote:

] Anything that
] uses a Prolific chip will work well (supports custom serial rates, and
] does not drop/lose characters).  The uplcom(4) driver is for this chip,
] and the man page lists off some consumer models/devices available.

Also from the thread awhile back, Bernd Walter wrote:

} This is because USB is absolutely crap for this purpose.
} RS232 terminals, especially with long cables, can produce several kind
} of spikes and ground loops, which USB is very very sensitive about.
} The upcoming new USB stack does a lot about handling transmission
} errors, but the underlying problem is USB and not FreeBSD.
} The best thing software can do about this is avoiding the panics.
} Nevertheless the new USB stack is likely not being merged into any alpha
} supporting OS release, so even that will not happen for alpha.
}
} My advise is to use a completely other technology to connect the terminals.
} A galvanic isolated USB device might work, but there are lot of PCI and
} Ethernet devices on the market which are more solid by design than USB.

If different systems are on different UPSs it might create a ground loop?
If necessary, you can optically isolate the RS-232 line, which should
fix the problem.

http://www.rs232-converters.com/opto-isolated_converters/Opto-Isolated_RS232_RS485_RS422_Isolators.htm

I have not actually tried this.  This is just an example of the isolators
available, not a recommendation for this specific product or vender.  Google
can find similar products.