---
title: "Types of bus arbitration"
subject: "Computer Organization and Architecture"
chapter: 1
key: "semester_5/computer_organization_and_architecture:bus-arbitration"
source: https://www.bctengineer.com/q/semester_5/computer_organization_and_architecture/bus-arbitration
site: BCT Engineering
---

# Types of bus arbitration

Computer Organization and Architecture · Chapter 1: Introduction

- Asked in: 2078 Bhadra, 6 marks
- Video: [Daisy Chain, Polling & Independent Request bus contention methods | Bus Contention & Bus Arbitration](https://www.youtube.com/watch?v=YSt_47u6rvc) — Engineering Funda
- 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/bus-arbitration
- Whole chapter: https://www.bctengineer.com/md/semester_5/computer_organization_and_architecture/chapter-1

## Question

Explain the different types of bus arbitration and compare them.

## Short answer

Centralized arbitration uses a single arbiter: daisy chaining passes a grant signal along a chain (fixed priority, simple but slow), polling cycles through addresses (programmable priority), and independent requests give each device its own lines (fastest). Distributed arbitration has no central controller; modules place priority numbers on shared lines and the highest wins, avoiding a single point of failure.

## Answer

Bus arbitration is the method of deciding which of several modules requesting the bus becomes the bus master. Types:

#### 1. Centralised arbitration (one arbiter)
- **Daisy chaining:** all devices share a Bus Request line. The controller sends Bus Grant to the first device, which passes it along the chain if it does not need the bus. The device nearest the controller has the highest priority.
- **Polling:** the controller sends a device address on poll lines (counter). The requesting device whose address matches takes the bus. Priority can be changed by changing the counting order.
- **Independent request:** each device has its own request and grant lines. The arbiter chooses by priority logic, so it is the fastest.

#### 2. Distributed arbitration (no central arbiter)
Each device has an arbitration number. On a request, devices place their numbers on shared lines and compare; the highest number wins.

#### Comparison
| Point | Daisy chain | Polling | Independent request | Distributed |
|---|---|---|---|---|
| Control lines | 3 | log₂n + 2 | 2n | log₂n |
| Speed | Slow | Medium | Fast | Medium |
| Priority | Fixed by position | Programmable | Programmable | By number |
| Cost / complexity | Low | Medium | High | High |
| Failure | Chain break affects devices after it | Poller failure | Arbiter failure | No single point of failure |
| Expandable | Easy | Easy | Hard | Easy |

## Check yourself

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

1. Which centralised scheme is the fastest?
   A. Daisy chaining
   B. Polling
   C. Distributed
   D. Independent request
   Answer: D. Each device has its own request and grant lines. (Key point: Fastest scheme)

2. How is priority set in daisy chaining?
   A. Programmable by counter
   B. Fixed by position
   C. By arbitration number
   D. By request time
   Answer: B. Position in the chain fixes priority. (Key point: Daisy chain priority)

3. How can polling priority be changed?
   A. By moving the devices
   B. By changing the data bus
   C. It cannot be changed
   D. By changing the counting order
   Answer: D. The counter order is programmable. (Key point: Polling flexibility)

4. Which statement about distributed arbitration is true?
   A. It has a single arbiter that may fail
   B. It has no single point of failure
   C. It needs 2n lines
   D. It has fixed priority by position
   Answer: B. Without a central arbiter there is no single point of failure. (Key point: Distributed failure)

## Related in Computer Organization and Architecture

- [Bus hierarchy and bus arbitration](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Abus-hierarchy-arbitration) — Computer Organization and Architecture, chapter 1
- [Bus structure of a typical computer](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Abus-structure) — Computer Organization and Architecture, chapter 1

## Related in other subjects

- [Arbitration methods and I2C significance](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aarbitration-types-i2c) — Embedded System, chapter 5
- [Priority arbitration vs daisy chain](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Apriority-arbitration-vs-daisy) — Embedded System, chapter 5
- [Ring bus rearrangement and zones](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aring-bus-comparison) — Switchgear and Protection, chapter 1
- [Infinite bus and HVDC vs HVAC comparison](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Ainfinite-bus-hvdc-vs-hvac) — Power System Analysis I, chapter 1