Believe it or not, there are good reasons to use MODEMs on POTS lines to this day. One may want to have a MODEM connected to a router for out-of-band communications in case of disaster recovery.
A Typical MODEM Connection
[Host]–[MODEM]—-[Telco Switch]—-[Telco Switch]—-[MODEM]–[Host]
MODEM Signaling – Control
Pin 2 = Transmit Data
Pin 3 = Receive Data
Pin 4 = Request to send, Hardware Flow Control
Pin 5 = Clear to send, Hardware Flow Control
Pin 6 = Modem Control
Pin 7 = ground
Pin 8 = Carrier Ready
Pin 20 = DTR Data Terminal Ready
*Pins 6,8,20 are used to monitor connections
| Standard Modem Commands | |
|---|---|
| AT&F | Loads factory default settings |
| ATS0=n | Auto Answer on n rings |
| AT&C1 | CD truly reflects line state |
| AT&D3 | Hangs up on DTR low |
| ATS2=255 | Ignore “+++” (in-band signaling) |
| ATE0 | Echo off |
| ATM0 | Turn off speaker |
US Robotics Initialization String: at&fs0=1&c1&d3&h1&r2&b1&m4&k1&w
Discussion
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