---
title: "Synchronous vs asynchronous transmission"
subject: "Instrumentation II"
chapter: 3
key: "semester_5/instrumentation_ii:sync-vs-async"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/sync-vs-async
site: BCT Engineering
---

# Synchronous vs asynchronous transmission

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

- Asked in: 2068 Baisakh (old course), 4 marks
- Video: [Synchronous Vs Asynchronous Serial Data Transmission - Communication Interface - Microprocessors](https://www.youtube.com/watch?v=ytlYG01IDss) — Ekeeda
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/sync-vs-async
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-3

## Question

Differentiate between synchronous and asynchronous data transmission.

## Short answer

Synchronous transmission sends blocks of characters using a shared clock and sync characters, has low overhead, high speed (Mbps), e.g., SPI. Asynchronous transmission sends one character at a time framed by start/stop bits, no common clock, higher overhead (~25–40%), lower speed (kbps), e.g., RS‑232.

## Answer

| Point | Synchronous transmission | Asynchronous transmission |
|---|---|---|
| Unit of transfer | Block of characters (frame) | One character at a time |
| Clock | Clock shared between sender and receiver (separate line or embedded in data) | No common clock; each side uses its own, with the same baud rate |
| Synchronisation | SYNC characters / flags at the start of the block | Start and stop bit per character |
| Overhead | Small (header and CRC for a long block) | Large: 2-3 extra bits per 7-8 data bits (about 25-40%) |
| Speed | High (Mbps) | Low (typically up to about 115 kbps for a UART) |
| Idle time | No gaps between characters | Gaps allowed between characters |
| Hardware | Complex: USRT/USART, clock recovery | Simple UART |
| Cost | Higher | Lower |
| Error detection | CRC on block | Parity per character |
| Example | HDLC, SDLC, Ethernet, SPI | RS-232, keyboard, serial mouse |

## Check yourself

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

1. Unit of transfer in synchronous?
   A. One character
   B. Byte with start bit
   C. Block of characters
   D. One bit only
   Answer: C. Synchronous sends blocks. (Key point: Unit)

2. Which has larger overhead per data bit?
   A. Neither
   B. Asynchronous (start/stop bits)
   C. Synchronous
   D. Equal
   Answer: B. Async adds 2-3 bits per character. (Key point: Overhead)

3. Clock in asynchronous?
   A. No common clock; same baud rate on both sides
   B. Shared clock
   C. Embedded in data
   D. Clock from host
   Answer: A. Each side uses its own clock. (Key point: Clock)

4. Asynchronous example?
   A. SPI
   B. Ethernet
   C. RS-232
   D. HDLC
   Answer: C. RS-232, keyboard, serial mouse. (Key point: Examples)

## Related in Instrumentation II

- [Error detection in serial data transmission](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aserial-error-detection) — Instrumentation II, chapter 3, asked 2×
- [Digital transmission system; analog vs digital](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aanalog-digital-transmission-compare) — Instrumentation II, chapter 5
- [Asynchronous serial transfer of a character](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aasync-7bit-ascii-2stop) — Instrumentation II, chapter 3
- [Data archiving vs data storage](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aarchiving-vs-storage) — Instrumentation II, chapter 5, asked 2×
- [Character transmission time at 1200 baud](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Abaud-time-1200) — Instrumentation II, chapter 3, asked 2×
- [Data logger with Bluetooth scatternet](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Abt-datalogger-scatternet) — Instrumentation II, chapter 5

## Related in other subjects

- [Synchronous vs asynchronous serial transmission](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Async-async-serial) — Microprocessor, chapter 4, asked 3×
- [Synchronous vs asynchronous circuits; SR to JK](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Async-async-sr-to-jk) — Digital Logic, chapter 6
- [Multiplexing; synchronous vs asynchronous TDM](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Amux-compare-sync-async-tdm) — Data Communication, chapter 6, asked 6×
- [Synchronous/asynchronous bus; LXI timing](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Async-async-bus-lxi-timing) — Microprocessor, chapter 2, asked 2×