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Re: Serial port connection (null modem) not working

On Thu, Aug 23, 2007 at 03:37:00PM +0200, Fili Wiese wrote:
> I'm having trouble setting up a serial connection between two servers
> using a null modem cable.
> This link is needed as a redundant connection for Heartbeat.
> Both machines are indentical (both hardware and software).
> In my humble view, everything seems to be set up perfectly.
> However, an echo/cat test simply doesn't work:
> server1: echo test >/dev/ttyS0
> server2: cat </dev/ttyS0
> (no output)
> I'm really at a loss :S
> Does anybody have a clue where to look next?
> Maybe the (brand-new) null-modem cable is not wired properly but how do
> I find that out?

Since cat doesn't set up the port for use, it won't work.

I have yet to find a ready-made null-modem cable that is actually wired
correctly for use by UNIX.  How it should be wired can be found in
several places accessible as packages on Debian.

Modem-HOWTO, both in the doc-linux-HOWTO package
Hardware book, available in the hwb package.

Also, in book form:
Unix System Administration Handbook.  Nemeth, Evi, et.al.

The actual wireing of the cable can be verified with an ohm-meter if you
have both ends of the cable in your hand, or by putting a wrap plug on
one end.  The functioning of the link can be verified with minicom.  If
you put a wrap plug on one end, and the other end on one of the servers,
you can use minicom and talk to yourself.  The signal goes down the
wire, through the wrap plug and back you your box.

Then you can run minicom at both ends an talk.

Setting up ppp or something over the wire is a possibility but it won't
provide any further details if you don't need ppp.  If you _do_ need
ppp, then the PPP-HOWTO will tell you how to setup a link.  Its really
quite easy.  Basically you set up mgetty on one box and have ppp on the
other box access it.  Since there's no modem, there are no dialing


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