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

# RS-232A criteria for simplex/half/full duplex

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

- Asked in: 2072 Kartik, 4 marks
- 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-criteria
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-3

## Question

What are the criteria which should be involved during the design of RS-232A in Simplex, Half Duplex and Full Duplex modes.

## Short answer

Simplex uses a one-way driver/receiver and GND. Half-duplex needs RTS/CTS for line turn-around, both ends with driver/receiver, and a master-slave protocol. Full-duplex has independent TX/RX with optional flow control. Common criteria: baud rate, ±3–15 V levels, under 15 m cable, and parity error detection.

## Answer

When designing an RS-232C link for simplex, half duplex or full duplex operation, the following criteria are considered.

| Criterion | Simplex | Half duplex | Full duplex |
|---|---|---|---|
| Direction of data | One way only | Both ways, one at a time | Both ways simultaneously |
| Signals/wires | TXD (one side), RXD (other), GND | TXD, RXD, GND plus RTS/CTS for line turn-around | TXD, RXD, GND (+ optional handshake) |
| Drivers and receivers | Driver at sender, receiver at receiver | Both ends have driver and receiver; direction control logic | Both ends have independent driver and receiver |
| Handshake | Not required (or one-way DSR) | Required: RTS/CTS to claim the channel | Optional hardware flow control |
| Protocol / software | Simple, no reply | Needs master-slave or token protocol to avoid collision | Independent transmit/receive tasks, buffers |
| UART | TX or RX only | TX and RX, but not used together | TX and RX used independently |
| Throughput | Lowest | Medium | Highest |
| Example | Computer to display board | Polled instrument network | PC to modem |

#### Common design criteria
1. **Baud rate** (up to 20 kbps) agreed by both ends, with same data bits, parity and stop bits.
2. **Voltage levels:** +/-3 to +/-15 V; use MAX232/MAX3232 for TTL conversion.
3. **Cable length** (about 15 m, load capacitance below 2500 pF) and shielding to reduce noise.
4. **Connector and pin assignment** (DB-9/DB-25, DTE/DCE; null modem if DTE-DTE).
5. **Common ground** between both devices.
6. **Flow control** (RTS/CTS, DTR/DSR or XON/XOFF) and buffers in half and full duplex.
7. **Error detection** with parity and error handling (framing, overrun).

## Check yourself

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

1. Which mode needs RTS/CTS to claim the channel?
   A. Simplex
   B. Half duplex
   C. Full duplex always
   D. None
   Answer: B. Half duplex needs RTS/CTS for line turn-around. (Key point: Half duplex handshake)

2. Which mode has the highest throughput?
   A. Simplex
   B. Half duplex
   C. Full duplex
   D. All equal
   Answer: C. Full duplex has independent TX and RX at once. (Key point: Throughput)

3. What common electrical criteria apply to RS-232C?
   A. +/-3 to +/-15 V, about 15 m cable, load below 2500 pF
   B. 0-5 V, 1200 m
   C. +/-2 V differential
   D. 10 Mbps
   Answer: A. Standard RS-232C levels and cable limits. (Key point: Voltage and cable)

4. What must both ends share?
   A. A common ground
   B. The same clock wire
   C. The same power supply
   D. Nothing
   Answer: A. Signal levels are referenced to a common ground. (Key point: Common ground)

## Related in Instrumentation II

- [Simplex, half and full duplex using RS-232](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ars232-simplex-half-full) — Instrumentation II, chapter 3, asked 2×

## Related in other subjects

- [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