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In article <3C32CF17.8030504@yahoo.com>
           finatronics@yahoo.com "Eric Hungerford" writes:

> I'm building a circuit that uses a modem. It's standard Hayes compatible.
> 
> How does the DTE (which is my circuit) tell the modem what baud rate to
> use to communicate BETWEEN THE DTE and MODEM (not the connection speed
> between the two modems)? It seems to be defaulting at 9600 baud, but I
> need another speed...

The DTE->modem link runs at whatever speed you configure the UART in your
COM port to be.  It sounds as if, by default, it's running at 9600Bd.
(This is the default speed of a PC's COM ports under DOS, possibly set as
such by the hardware reset and the PC's BIOS; under DOS one could change
this with the MODE command; under linux, there will probably be something
that one does with stty or getty, or something.)

> I tried to watch the data sent to the modem using a 'scope while loading
> minicom (in Linux) and it seems it's just sending Enter. Is that how it
> sets the communication speed? 

Yep; most modems manufactured since about 1985 have been able to
"auto-baud" on the DTE connection.  For instance, the manual for my
ancient 12000b/s Dataflex Rapier says "The Rapier modem will always
configure itself to match the speed you set in your communications
program, as long as you set this speed to 38,400 bits per second or less.
In other words, the modem will always be able to talk to the local
terminal it is connected to, regardless of the speed in use".

Obviously with more modern modems supporting faster telephone line
speeds, one nowadays runs the COM port at a faster speed; typically this
DTE speed will be at 115,200.

Modems will happily "auto-baud" like this when set as either eight data
bits with no parity, or seven data bits with odd parity.  Sometimes it
requires more than a single CR character; but usually it can work from
this, or from the A T of the standard Hayes protocol introducer.

>                                Does it change the levels of the control
> pins (DTR, DSR, etc) in any particular sequence to assist in this?

No.

Of course, really old modems work at a fixed DTE speed, usually set on
DIP switches.

-- 
Brian {Hamilton Kelly}                                          bhk@dsl.co.uk
    "We have gone from a world of concentrated knowledge and wisdom to one of
    distributed ignorance.  And we know and understand less while being incr-
    easingly capable."              Prof. Peter Cochrane, formerly of BT Labs