I'm looking to do something similar soon, most likely with a console board to an external modem, using pppd in buildroot to connect to a dial up internet connection.  The bottom line is, yes, you can do it with just tx, rx and gnd.  The settings to allow this will vary between modem manufacturers (US robotics used to have dip switches, most have been superseded by nvram switches set by AT commands).  Also, without any hardware handshaking your best results will occur at or below 4800baud.

----- Original Message ----
From: "nospam@horseapple.net" <nospam@horseapple.net>
To: gumstix-users@lists.sourceforge.net
Sent: Tuesday, June 5, 2007 3:00:52 PM
Subject: [Gumstix-users] Repost - need help with modem!

I posted a couple of weeks back and got no response at all,
so forgive me, but I'm trying again.

I need to hook a serial modem (pots) up to the robostix on
the STUART port. I have a console up and going on the FFUART
port and don't want to use a tweener board.

If I turn off all flow control, etc. on the port, is it possible
to use a dial-up modem with just TX/RX/G/5v TTL via the STUART
port? If so, what am I missing?

I keep thinking that I should be able to do this without the
modem control lines, etc..somehow.

If this isn't possible, is there a way to route modem control
signals through some of the GPIO's on the robostix or some other
method I could use that would be sensible?
I have read about folks using the tweener with a modem and I don't
see that it really has more signals than the STUART. Just that
it's raised to RS/232. This modem will work with RS/232 or TTL at

Also, I notice when I put a meter on the STUART that instead of
3.3v TTL I am seeing 5v on the Vcc pin?? I'm confused as I thought
it was supposed to be at 3.3v. :-)

(original post here:

Thanks and...Help?!


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