---
title: "I/O bus and interface modules"
subject: "Computer Organization and Architecture"
chapter: 7
key: "semester_5/computer_organization_and_architecture:io-bus-interface-modules"
source: https://www.bctengineer.com/q/semester_5/computer_organization_and_architecture/io-bus-interface-modules
site: BCT Engineering
---

# I/O bus and interface modules

Computer Organization and Architecture · Chapter 7: Input-Output Organization

- Asked in: 2067 Asar (old course), 4 marks
- Video: [Input Output Bus and Interface Modules||Types of Commands||Control | Status| Input Data| Output Data](https://www.youtube.com/watch?v=DtodJc_vDtU) — PPC Learning
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/computer_organization_and_architecture/io-bus-interface-modules
- Whole chapter: https://www.bctengineer.com/md/semester_5/computer_organization_and_architecture/chapter-7

## Question

Explain the I/O bus and interface modules.

## Short answer

The I/O bus carries data, address, and control lines linking the CPU to interfaces. Each interface module decodes its unique address, enabling only the addressed device to respond, and contains registers for control, status, and data transfers, also handling format and speed conversion.

## Answer

The **I/O bus** connects the CPU to all peripheral interfaces. It consists of **data lines**, **address lines** and **control lines** (read, write, etc.). The CPU puts a device address on the address lines; each interface module has an **address decoder** which recognises its own address, and only that interface is enabled to respond. Other interfaces stay disconnected from the bus.

```text
   CPU --+----------------------------- I/O bus
         | data | address | control
     +---+---+  +---+---+  +---+---+
     |Interf.|  |Interf.|  |Interf.|
     |  1    |  |  2    |  |  3    |
     +---+---+  +---+---+  +---+---+
         |          |          |
     Keyboard    Printer      Disk
```
An **interface module** sits between the bus and the peripheral. It contains a decoder for the address, data and control/status registers (ports), and logic that converts the data format, adjusts the speed and passes commands to the device. The CPU controls the device by reading and writing the interface's registers using I/O commands (control, status, data input, data output).

## Check yourself

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

1. What lines make up the I/O bus?
   A. Clock lines only
   B. Data, address and control lines
   C. Power lines
   D. Only data lines
   Answer: B. The I/O bus has data, address and control lines (read, write, etc.). (Key point: I/O bus lines)

2. What is the role of the address decoder in an interface module?
   A. Raises interrupts only
   B. Converts data to analog
   C. Recognises its own address so only that interface responds
   D. Stores the program
   Answer: C. Only the addressed interface is enabled; others stay disconnected. (Key point: Address decoder)

3. Which is NOT inside an interface module according to the answer?
   A. Data format and speed conversion logic
   B. Address decoder
   C. The main memory unit
   D. Data and control/status registers
   Answer: C. The interface has a decoder, registers and conversion logic; main memory is separate. (Key point: Interface module contents)

## Related in other subjects

- [Bus Interface Unit of 8086](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3A8086-bus-interface-unit) — Microprocessor, chapter 3
- [PCI bus; 8255 printer interface at 5500H](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Apci-printer-5500h) — Instrumentation II, chapter 2