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Synchronous vs asynchronous serial transmission
Short answer
A serial interface transmits data one bit at a time. Synchronous transmission uses a shared clock and sends blocks with sync characters, offering high speed and low overhead. Asynchronous transmission sends individual characters framed by start/stop bits, using separate clocks, simpler but slower with higher overhead.
What do you mean by serial interface? Differentiate between synchronous and asynchronous serial data transmission.
Serial interface
A serial interface transfers data one bit at a time over a single line, so only a few wires are needed (TxD, RxD, ground). The processor's parallel data is converted to serial by a shift register (e.g. USART 8251A) and back at the receiver. It is cheap and suits long distance, but slower than parallel.
Synchronous vs asynchronous transmission
| Point | Synchronous | Asynchronous |
|---|---|---|
| Clock | Transmitter and receiver share the same clock (sent with data or recovered) | Separate clocks; no clock sent |
| Unit sent | Block of characters (frame) | One character at a time |
| Framing | Sync characters at the start of a block | Start bit and stop bit(s) around each character |
| Idle gap | None between characters in a block | Any gap between characters |
| Overhead | Low (few sync characters per block) | High (2–3 extra bits per 8-bit character) |
| Speed | High (e.g. disk, networks) | Low to medium (keyboard, modem) |
| Cost/complexity | More complex hardware | Simple and cheap |
| Example | HDLC, SDLC, 8251A in sync mode | RS-232 terminal link |
Asynchronous frame (one character):
idle ‾|_| D0 D1 D2 D3 D4 D5 D6 D7 |P| ‾‾|
start data bits parity stop
Synchronous: SYNC SYNC | data data data ... | CRC
In asynchronous format the receiver detects the 1 → 0 transition of the start bit and samples each bit at the agreed baud rate (usually with a 16× or 64× clock).
✓ Answer checked by Claude Opus 5.5 (high effort)
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