Null modem connection

A null modem connection joins two DTEs (e.g. two PCs) directly by a cable (or adapter) that crosses the transmit/receive and handshake lines, so each DTE sees the other as a DCE. No modem is needed.

1. Null modem without handshaking (three-wire)

Only TXD, RXD and ground are used. TXD of each is connected to RXD of the other; the handshake pins at each connector are shorted locally, so the software always sees "ready". In the sketches, the number after each arrow is the pin on DTE B.

 DTE A                               DTE B
 TXD  2 ---------------------------> 3  RXD
 RXD  3 <--------------------------- 2  TXD
 GND  7 --------------------------- 7  GND
 (local loops at each end:  RTS 4 - CTS 5 ;  DTR 20 - DSR 6 - DCD 8)

Communication: A sends on TXD and B receives on RXD; B sends on its TXD and A receives. Both directions work together (full duplex). The sender cannot know whether the receiver is ready, so software flow control (XON/XOFF) or a low speed is required, otherwise data may be lost when the receiver is slow.

2. Null modem with handshaking (full handshake)

Control lines are cross-connected so the hardware tells each side when the other is ready.

 DTE A                               DTE B
 TXD  2 ---------------------------> 3  RXD
 RXD  3 <--------------------------- 2  TXD
 RTS  4 ---------------------------> 5  CTS
 CTS  5 <--------------------------- 4  RTS
 DTR 20 -------------------------+-> 6  DSR
                                 +-> 8  DCD
 DSR  6 <-+------------------------- 20 DTR
 DCD  8 <-+
 GND  7 --------------------------- 7  GND

Sequence: A raises DTR, so B sees DSR and DCD high (A is on-line). When A is ready to receive, it raises RTS; B sees this on its CTS input and may send. If A's buffer becomes full, A drops RTS, B's CTS goes low and B stops sending until A is ready again. The same happens in the other direction (B's RTS to A's CTS). In this way hardware flow control prevents overflow.