---
title: "Simplex, half and full duplex using RS-232"
subject: "Instrumentation II"
chapter: 3
key: "semester_5/instrumentation_ii:rs232-simplex-half-full"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/rs232-simplex-half-full
site: BCT Engineering
---

# Simplex, half and full duplex using RS-232

Instrumentation II · Chapter 3: Serial Interfacing With Microprocessor Based System

- Asked in: 2069 Chaitra, 7 marks; 2078 Bhadra, 4 marks (asked 2 times)
- Video: [Data Transmission Modes | Simplex, Half Duplex & Full Duplex](https://www.youtube.com/watch?v=ttBKCvczcIQ) — Learn Coding
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/rs232-simplex-half-full
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-3

## Question

Describe simplex, half duplex and full duplex operation using RS-232 port.

## Short answer

Simplex uses a one-way TXD-to-RXD connection. Half duplex shares lines bidirectionally but only one side transmits at a time, using RTS/CTS for turn-around. Full duplex uses separate TXD and RXD lines for simultaneous two-way transmission.

## Answer

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).
```text
 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).
```text
 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.
```text
 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).

| Mode | Direction | Simultaneous | Wires (minimum) | Example |
|---|---|---|---|---|
| Simplex | One way | - | 2 (TXD, GND) | PC to display |
| Half duplex | Both ways, alternately | No | 3 + RTS/CTS control | Master-slave polling |
| Full duplex | Both ways | Yes | 3 (TXD, RXD, GND) | PC-modem, null modem |

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

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is simplex?
   A. Both ways at once
   B. One direction only
   C. Both ways alternately
   D. Needs 25 wires
   Answer: B. Simplex sends in one direction only, e.g. computer to display board. (Key point: Simplex)

2. How are directions controlled in half duplex?
   A. Using RTS/CTS to claim the line and turn it around
   B. By a second ground
   C. By parity
   D. Not controlled
   Answer: A. A device raises RTS, waits for CTS, sends, and drops RTS to let the other side use the line. (Key point: Half duplex)

3. Why does RS-232 do full duplex naturally?
   A. It has separate TXD and RXD wires with a common ground
   B. It uses one wire
   C. It has RTS only
   D. It has a clock wire
   Answer: A. Separate transmit and receive paths let both directions work simultaneously. (Key point: Full duplex)

4. What is the minimum wiring for simplex?
   A. 2 (TXD, GND)
   B. 3 (TXD, RXD, GND)
   C. 4
   D. 7
   Answer: A. Simplex needs just sender TXD to receiver RXD plus GND, i.e. 2 lines per the table. (Key point: Minimum wires)

## Related in Instrumentation II

- [RS-232A criteria for simplex/half/full duplex](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ars232-simplex-half-full-criteria) — Instrumentation II, chapter 3
- [Connecting two DTE RS-232 devices](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Adte-dte-problem) — Instrumentation II, chapter 3, asked 2×
- [USB to RS-232 adapter design](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ausb-rs232-adapter) — Instrumentation II, chapter 3
- [TTL to RS-232 interfacing circuit](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Attl-rs232-interface) — Instrumentation II, chapter 3

## Related in other subjects

- [Short note: RS-232 standard](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Ashort-note-rs232) — Microprocessor, chapter 4, asked 3×
- [RS-232 signals, handshaking, DTE-DCE](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Ars232-signals-dte-dce) — Microprocessor, chapter 4, asked 3×
- [Full duplex on a single twisted pair](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atwisted-pair-full-duplex-converter) — Telecommunications, chapter 2