---
title: "Short note: RS-232 standard"
subject: "Microprocessor"
chapter: 4
key: "semester_4/microprocessor:short-note-rs232"
source: https://www.bctengineer.com/q/semester_4/microprocessor/short-note-rs232
site: BCT Engineering
---

# Short note: RS-232 standard

Microprocessor · Chapter 4: Microprocessor System

- Asked in: 2073 Magh, 4 marks; 2072 Magh, 4 marks; 2071 Bhadra, 4 marks (asked 3 times)
- Video: [RS-232 Serial Communication Protocol Explained: DB9 and DB25 Connectors, and DTE/DCE](https://www.youtube.com/watch?v=Qx2d20elziw) — Engineering Funda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/microprocessor/short-note-rs232
- Whole chapter: https://www.bctengineer.com/md/semester_4/microprocessor/chapter-4

## Question

Write a short note on RS-232 standard.

## Short answer

RS-232 is a serial communication standard defining voltage levels (±3–15V), connectors (DB-9/DB-25), and signals like TxD, RxD, RTS, CTS. It connects DTE and DCE, requiring level converters (MAX232) for TTL compatibility, with limited speed and distance.

## Answer

**RS-232** (RS-232C) is an EIA standard for serial data communication between a **DTE** (Data Terminal Equipment, e.g. a computer) and a **DCE** (Data Communication Equipment, e.g. a modem). It defines the electrical levels, connector, signals and handshaking.

**Electrical specification**
- Logic 1 (mark): −3 V to −15 V; logic 0 (space): +3 V to +15 V; the range −3 V to +3 V is undefined. These are not TTL levels, so a converter such as the **MAX232** (or 1488/1489) is used.
- Maximum speed about 20 kbps (up to 115.2 kbps in practice) and maximum cable length about 15 m (50 ft).
- Connector: 25-pin DB-25 or 9-pin DB-9.

**Main signals (DB-9)**
| Pin | Signal | Meaning |
|---|---|---|
| 1 | DCD | Data carrier detect |
| 2 | RxD | Received data |
| 3 | TxD | Transmitted data |
| 4 | DTR | Data terminal ready |
| 5 | GND | Signal ground |
| 6 | DSR | Data set ready |
| 7 | RTS | Request to send |
| 8 | CTS | Clear to send |
| 9 | RI | Ring indicator |

**Features:** single-ended signalling (common ground), asynchronous or synchronous format, point-to-point link only. It is interfaced to the 8085/8086 through a USART such as the 8251A. Its limitations are low speed, short distance and sensitivity to noise, because the line is unbalanced.

## Check yourself

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

1. In RS-232, what voltage represents logic 1 (mark)?
   A. +3 V to +15 V
   B. 0 V to +5 V
   C. -3 V to -15 V
   D. Between -3 V and +3 V
   Answer: C. Logic 1 is -3 V to -15 V and logic 0 is +3 V to +15 V; the band -3 V to +3 V is undefined. (Key point: RS-232 voltage levels)

2. Why is a converter such as MAX232 needed when interfacing RS-232 to a microprocessor?
   A. RS-232 levels are not TTL levels
   B. RS-232 is parallel and must be made serial
   C. The cable is too long for TTL
   D. RS-232 uses balanced differential lines
   Answer: A. RS-232 uses bipolar levels (about +/-3 to 15 V) that differ from TTL, so a MAX232 or 1488/1489 translates them. (Key point: Level converter)

3. Why is RS-232 limited in speed, distance and noise immunity?
   A. It uses too many data lines
   B. It is a multi-drop bus shared by many devices
   C. It only works in synchronous mode
   D. It uses single-ended (unbalanced) signalling with a common ground
   Answer: D. The line is unbalanced (single-ended with common ground), so it is noise sensitive and limited to about 15 m and low speed. (Key point: RS-232 limitations)

## Related in Microprocessor

- [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×

## Related in other subjects

- [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×
- [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×
- [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