Conversation Between Computer and Modem

DEVICE
SIGNAL
MEANING
Computer
DTR+
I want to call another system. Are you ready?
Modem
DSR+
Yes, I’m ready. Go ahead and dial.
Modem
DCD+
I’ve got your party, sir.
Computer
RTS+
Can I send data now?
Modem
CTS+
Sure. Go ahead.
Computer
TxD...
Data sent out.
Modem
...RxD
Data Received. Previous four steps may be repeated, with either device in the sending role.
Computer
DTR-
I’m done. Please hang up.
Modem
DCD-
Whatever you say.
All of the above sounds good in theory, but in practice it will not always work. Connecting a computer to a modem is generally easy, since a DTE to DCE connection is what RS-232 was made for. A straight-through cable connecting pins 1 through 8 and 20 (or all 25 pins) will usually do the trick.

Things can get quite a bit more complicated for a direct connection between two computers. Just as the function of pins 2 and 3 is asymmetrical between DTE and DCE devices, so too is the function of pins 6, 8 and 20. A DTE device (a computer or terminal) asserts DTR (pin 20) and expects to receive DSR (pin 6) and DCD (Data Carrier Detect). A DCE device (a modem) asserts DSR and DCD and expects to receive DTR. If two DTE devices are connected with a straight-through cable, no handshaking can occur.

To get around the handshaking problem, a null modem cable can cross some of the control lines as well as the data lines.