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The Hayes modem has a minimum and a maximum baud rate.
If you connect a terminal to the modem, make sure you
have the right pins connected in relation with the
dipswitches. Also if you have selected e.g. 108/2 mode,
be sure your terminal program support them. It is
possible to connect only the pins 2,3 and 7. So in this
case the DTR and the DCD are forced true. In general I
don't like this methode. With the Hayes-1200, RTS and
CTS are not used, so only full duplex is possible. With
Hayes-2400 you have to use these signals only if you use
the Half Duplex mode (Synchrone mode). Otherwise RTS
(pin 4) is neglected and CTS (pin 5) is set true.
So if you have connected the terminal properly, at the
time you give an 'A' or 'a', the modem decides what the
baudrate is and set the terminal interface UART
according to it. Hayes is very street forward. This
speed is maintained till after the <carriage return>,
which always finishes a command line. With the next A
from a new AT command line, a new speed can be defined.
There is one other possibility. In auto answer mode or
if a call is made, the baudrate can be else as the speed
in the last passed command mode. This on real modem
level, the baudrate detection selects another speed. The
Hayes reacts with the connect code on this last command
mode speed and than switches to the new speed. To give
an example. Suppose the terminalspeed is 2400 baud. A
dial is made towards a 1200 baud only modem. The Hayes
will start the connect phase as described in 3.3.2. but
finally will get a connection on 1200 baud. So he will
react towards the terminal with 2400 baud with
'CONNECT' or '1', switches back to 1200 baud, which the
terminal has to follow, otherwise it result in a
terminal/modem mismatch. Also a <silence>+++<silence> on
2400 baud will not be accepted. Only 1200 baud, from
this moment on. So suppose the established call at 1200
baud is broken. The Hayes will, after the carrier
disappear, fall back into command mode still in 1200
baud. The terminal has to give an AT<string> on 2400
baud if it wants the original 2400 baud connection
again.
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