Simplex, half and full duplex using RS-232

RS-232 is a point-to-point serial interface. The three modes of operation differ in the direction in which data can flow at a given time.

1. Simplex

Data flows in one direction only; one side only transmits and the other only receives. Wires needed: one signal line and ground (TXD of sender -> RXD of receiver, plus GND).

 Transmitter                 Receiver
 TXD (2) ------------------> RXD (3)
 GND (7) ------------------- GND (7)

Example: a computer sending data to a display board or a printer without any reply; a sensor unit sending readings to a logger. Simple and cheap, but there is no acknowledgement or error recovery.

2. Half duplex

Data can flow in both directions, but not at the same time (one at a time). The RTS/CTS lines are used to control the direction: a device raises RTS to claim the line, waits for CTS, sends data, and drops RTS so that the other side may turn the line around. Only one data path is in use at a time (the same wires or a shared channel).

 Device A                        Device B
 TXD ---------------------------> RXD    (A sends, B listens)
 RXD <--------------------------- TXD    (B sends later)
 RTS --------------------------> CTS     (turn-around control)
 CTS <-------------------------- RTS

Example: a polling master-slave link where the master sends a command and then the slave replies; a modem on a two-wire line.

3. Full duplex

Data flows in both directions simultaneously. RS-232 does this naturally because it has separate wires for transmit (TXD) and receive (RXD), plus a common ground.

 Device A                        Device B
 TXD (2) -----------------------> RXD (3)    simultaneous
 RXD (3) <----------------------- TXD (2)
 GND (7) ------------------------ GND (7)

Example: PC to modem or PC to PC null modem link, where both can send and receive at the same time (UART transmitter and receiver work independently).

Throughput is highest in full duplex, but each mode uses the same RS-232 connector; the mode is chosen by wiring and software (handshaking).