I'd like to mention that it is possible to program most Hayes compatible 
modems to auto answer and auto dial as necessary, without using any 
Hayes commands at all. All control can be achieved using the DTR and RTS 
lines usually. F.ex. sending a character from the computer causes the 
modem to dial out, and the modem auto answers and sets CD when a 
carrier is detected. DTR resets the modem (and drops a connection if 
present at the time).

No actual handshaking needs to be used, the control signals can be 
'misused' to control dialout and reset. If necessary, XON/XOFF can be 
selected. The settings are programmed once (including the number to be 
dialed) and stored in NVRAM by the modem. The modem thereafter reverts 
to these settings after every power cycle or reset. Using ATDT... 
dialout is still possible in this mode.

The mode is extensively used for alarm systems, vending machines, and 
fixed service lines (like pump system remote control and the like). 
Details vary but they are covered in the relevant Hayes command manual 
(be sure to read the manual that applies to your modem, I have yet to 
see two that use the same settings).

In all cases the settings are achieved by ATxx commands and arcane S 
register settings. Sometimes flashing the modem with a certain firmware 
version is required.

Peter

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