---
title: "RS-232C handshaking signals"
subject: "Instrumentation II"
chapter: 3
key: "semester_5/instrumentation_ii:rs232-handshake-signals"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/rs232-handshake-signals
site: BCT Engineering
---

# RS-232C handshaking signals

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

- Asked in: 2073 Chaitra, 1+3 marks; 2068 Chaitra, 2 marks (asked 2 times)
- Video: [Unit 3 L14 | RS232 | RS232C in Microprocessor](https://www.youtube.com/watch?v=xoU-ymt4FlA) — Techno Tutorials ( e-Learning)
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/rs232-handshake-signals
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-3

## Question

Describe the functions of the RS-232C signals used in handshaking (DSR, DTR, RTS, CTS, TXD, RXD).

## Short answer

TXD carries data from DTE to DCE, RXD carries data from DCE to DTE. RTS requests to send, CTS grants permission; DTR indicates DTE ready, DSR indicates DCE ready, providing hardware flow control.

## Answer

RS-232C connects a **DTE** (data terminal equipment, e.g. computer) to a **DCE** (data communication equipment, e.g. modem). Handshaking signals control the flow so that data is sent only when the other side is ready.

| Signal | DB-25 pin | DB-9 pin | Direction (from DTE) | Function |
|---|---|---|---|---|
| TXD | 2 | 3 | Output | Transmitted data, serial data from DTE to DCE |
| RXD | 3 | 2 | Input | Received data, serial data from DCE to DTE |
| RTS | 4 | 7 | Output | Request To Send: DTE asks DCE for permission to send |
| CTS | 5 | 8 | Input | Clear To Send: DCE is ready to receive; DTE may send |
| DSR | 6 | 6 | Input | Data Set Ready: DCE is powered on and ready |
| DTR | 20 | 4 | Output | Data Terminal Ready: DTE is ready to communicate |
| GND | 7 | 5 | - | Signal ground |

#### Handshake sequence (DTE sending to a modem)
1. DTE raises **DTR**; DCE raises **DSR** (both ready).
2. DTE raises **RTS** to request to send.
3. DCE (when it can accept data) raises **CTS**.
4. DTE sends data on **TXD**; data coming from the DCE arrives on **RXD**.
5. After sending, DTE drops RTS and DCE drops CTS.

```text
 DTE                     DCE
 DTR (20) ------------>  DTR   : "I am ready"
 DSR (6)  <------------  DSR   : "I am ready"
 RTS (4)  ------------>  RTS   : "may I send?"
 CTS (5)  <------------  CTS   : "yes, send"
 TXD (2)  ------------>  data
 RXD (3)  <------------  data
```
Hardware flow control (RTS/CTS) prevents buffer overflow when a device is slower than the sender.

## Check yourself

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

1. What does RTS do?
   A. DCE is ready to receive
   B. DTE requests permission to send
   C. Carries received data
   D. Signal ground
   Answer: B. RTS is output from DTE asking the DCE for permission to send. (Key point: RTS)

2. What does CTS signal?
   A. DTE is powered
   B. DCE is ready to receive; DTE may send
   C. Data is transmitted
   D. Ring indicator
   Answer: B. CTS is the DCE reply that it is ready to receive. (Key point: CTS)

3. Which is the correct order of the handshake?
   A. RTS, then DTR, then CTS
   B. DTR/DSR, then RTS, then CTS, then data on TXD
   C. TXD, then RTS
   D. CTS, then DTR
   Answer: B. Both sides get ready (DTR/DSR), DTE raises RTS, DCE raises CTS, then data is sent. (Key point: Handshake order)

## Related in Instrumentation II

- [RS-422A vs RS-232C distance and speed](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ars422-vs-rs232-distance) — Instrumentation II, chapter 3, asked 3×
- [Compare RS-232C, RS-422A and RS-423A](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ars232-422-423-compare) — Instrumentation II, chapter 3
- [Importance and working of RS-232C](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ars232c-importance-signals) — Instrumentation II, chapter 3
- [RS-232C importance; time to transmit a string at 4800 baud](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ars232c-string-time-4800) — Instrumentation II, chapter 3
- [USB to RS-232 adapter design](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ausb-rs232-adapter) — Instrumentation II, chapter 3
- [ISA bus signals; replacement by PCI bus](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aisa-pci-bus) — Instrumentation II, chapter 2

## Related in other subjects

- [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×
- [I/O functions used with data files](https://www.bctengineer.com/question.md?key=semester_1%2Fcomputer_programming%3Afile-io-functions) — Computer Programming, chapter 10, asked 2×
- [Single and double handshaking](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Ahandshaking-single-double) — Microprocessor, chapter 4