Re: [Gumstix-users] Repost - need help with modem!

[<no...@ho...>]

Thanks for the response!

I think I may have figured out the problem, but won't know for sure
until tonight when I am where I can try to test. After speaking with
the modem manufacturer a short while ago, they tell me also that yes,
you can disable flow control and DTR and just use TX/RX. I think that
what's messing me up is that I thought that the STUART port coming off
the robostix was sitting at 3.3v and TTL. I thought that the modem that
I got was supposed to be dual function and would work at either 3.3v TTL
or 5v RS232. I find however that the port is running 5v (not 3.3v) and
that the modem versions come in either 3.3 or 5v, but not one version
that auto-switches between the two. SO..my theory is that I have a
STUART port running TTL at 5v to a modem that is expecting 5v RS232
level signals and thus, I probably need to use a level shifter between
the STUART port and the modem?

What I really wanted to achieve was a 3.3v TTL serial port to embedded
modem at 3.3v TTL so as to not need the level shifter (or the extra
cost per unit built). So I'll contact the vendor and ask for another
sample to be sent out as 3.3v (such as I asked for in the first place)
and try level shifting this one in the meantime to see if that does
indeed fix the problem with the modem unit that I have.

Thanks again for the input and I'll post more as I know more.
--Pat

> 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 superseded by nvram switches set by AT commands).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: "no...@ho..." <no...@ho...>
> To: gum...@li...
> Sent: Tuesday, June 5, 2007 3:00:52 PM
> Subject: [Gumstix-users] Repost - need help with modem!
>
>
> All,
> 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
> 3.3v.
>
> 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:
> http://article.gmane.org/gmane.linux.distributions.gumstix.general/24539/match=)
>
> Thanks and...Help?!
>
> --Pat
>
>
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