---
title: "Short note: serial and parallel interface"
subject: "Microprocessor"
chapter: 4
key: "semester_4/microprocessor:serial-parallel-interface"
source: https://www.bctengineer.com/q/semester_4/microprocessor/serial-parallel-interface
site: BCT Engineering
---

# Short note: serial and parallel interface

Microprocessor · Chapter 4: Microprocessor System

- Asked in: 2070 Magh, 4 marks
- Video: [Serial and Parallel Communication | Parallel Interface | Serial vs Parallel Communication](https://www.youtube.com/watch?v=mRTEsAVNhFM) — Embedded System Creations
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/microprocessor/serial-parallel-interface
- Whole chapter: https://www.bctengineer.com/md/semester_4/microprocessor/chapter-4

## Question

Write a short note on serial and parallel interface.

## Short answer

Parallel interface sends all bits of a byte simultaneously on separate lines; it is fast but needs many wires and suits short distances. Serial interface sends bits one after another on one line, needing few wires for long distances but slower. Examples: 8255 PPI and 8251A USART.

## Answer

An **interface** is the circuit that connects the microprocessor to a peripheral. By the way data is sent, it is either parallel or serial.

**Parallel interface:** all bits of a byte (or word) are sent together on separate lines, one line per bit, plus control (strobe/acknowledge) lines. It is fast and simple, but needs many wires, so it suits short distances (printer, keyboard, ADC). Chip: 8255 PPI.

**Serial interface:** bits are sent one after another on one line (plus ground, and control lines if needed). A parallel-to-serial shift register (UART/USART) does the conversion. It needs few wires and works over long distance, but is slower. Chip: 8251A USART. Standards: RS-232.

| Point | Parallel | Serial |
|---|---|---|
| Bits per clock | 8 or more | 1 |
| Lines needed | Many (8 data + control) | Few (TxD, RxD, GND) |
| Speed (short distance) | High | Lower |
| Distance | Short (a few metres) | Long |
| Cost of cable | High | Low |
| Crosstalk/noise | More | Less |
| Examples | Centronics printer, 8255 | RS-232, USB, 8251A |

```text
 Parallel: P->  D0 D1 D2 D3 D4 D5 D6 D7 ->   (8 wires)
 Serial  : P->  D0 D1 D2 D3 ... D7 (one wire, bit by bit)
```

## Check yourself

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

1. Why is the parallel interface used only over short distances?
   A. It is slower than serial
   B. It needs a USART
   C. It needs many wires, which are costly and prone to crosstalk over distance
   D. It can send only one bit at a time
   Answer: C. Parallel sends all bits on separate lines, so it needs many wires and suffers crosstalk, suiting only short distances. (Key point: Parallel interface limit)

2. What is the main advantage and drawback of serial transmission?
   A. Few wires and long distance, but slower
   B. Very fast but needs many wires
   C. Short distance and expensive cable
   D. Needs 8 data lines but is noise free
   Answer: A. Serial sends one bit at a time on a single line, so it needs few wires and works over long distances but is slower. (Key point: Serial advantage)

3. What converts the processor's parallel data to serial in a serial interface?
   A. A multiplexer
   B. An 8255 PPI
   C. A buffer such as 74LS244
   D. A parallel-to-serial shift register inside a UART/USART such as the 8251A
   Answer: D. UART/USART chips contain shift registers that convert parallel to serial; the 8255 is the parallel chip. (Key point: Serial conversion)

## Related in Microprocessor

- [Synchronous vs asynchronous serial transmission](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Async-async-serial) — Microprocessor, chapter 4, asked 3×

## Related in other subjects

- [Why serial transfer is preferred over parallel](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aserial-vs-parallel-preferred) — Instrumentation II, chapter 3
- [Series and parallel phase model; solids in hcp](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aseries-parallel-phase-model) — Concrete Technology and Masonry Structure, chapter 2