---
title: "USB On-The-Go (OTG) protocol"
subject: "Instrumentation II"
chapter: 3
key: "semester_5/instrumentation_ii:usb-otg"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/usb-otg
site: BCT Engineering
---

# USB On-The-Go (OTG) protocol

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

- Asked in: 2073 Chaitra, 2 marks; 2072 Chaitra, 1 marks; 2070 Asar, 2 marks (asked 3 times)
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/usb-otg
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-3

## Question

Explain USB-OTG in brief.

## Short answer

USB On‑The‑Go supplements USB 2.0, letting a device act as both host and peripheral without a PC. A Micro‑AB receptacle and ID pin define the default role; Host Negotiation Protocol swaps roles, and Session Request Protocol wakes a host to save power.

## Answer

**USB On-The-Go (OTG)** is a supplement to the USB 2.0 standard that lets a portable device act as a **host** as well as a **peripheral**, so two devices (for example a camera and a printer, or a phone and a pen drive) can communicate without a PC.

- A device with OTG has a dual-role function and a **Micro-AB** receptacle; the **ID pin** of the plug decides the role: A-device (ID grounded) is the default host and supplies power; B-device is the default peripheral.
- **HNP (Host Negotiation Protocol)** allows the two devices to swap host and peripheral roles without changing the cable.
- **SRP (Session Request Protocol)** lets a B-device ask the A-device to turn on VBUS to save power.
- OTG hosts support a limited set of peripherals (targeted peripheral list) and supply a small current (at least 8 mA, typically 100 mA or more).

## Check yourself

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

1. What does USB OTG allow?
   A. Only faster speed
   B. A portable device to act as host as well as peripheral without a PC
   C. Wireless USB
   D. Power over 100 W
   Answer: B. OTG gives dual-role function so e.g. a phone can connect directly to a pen drive. (Key point: OTG idea)

2. How is the role decided in OTG?
   A. By the ID pin of the plug; ID grounded = A-device (default host)
   B. By cable length
   C. By the PC
   D. By D+ voltage only
   Answer: A. A-device (ID grounded) is the default host and supplies power; B-device is default peripheral. (Key point: ID pin)

3. Which protocol swaps host and peripheral roles without changing cable?
   A. SRP
   B. HNP
   C. SOF
   D. PING
   Answer: B. HNP (Host Negotiation Protocol) swaps roles; SRP asks the A-device to turn on VBUS. (Key point: HNP vs SRP)

## Related in Instrumentation II

- [Types of USB packets](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ausb-packet-types) — Instrumentation II, chapter 3, asked 3×
- [USB host, hub, device and packets](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ausb-host-hub-device-packets) — Instrumentation II, chapter 3
- [USB 1.1 vs 2.0 and device identification](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ausb11-vs-usb20-identify) — Instrumentation II, chapter 3

## Related in other subjects

- [Kyoto Protocol flexibility mechanisms](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Akyoto-flexibility-mechanisms) — Energy Environment and Society, chapter 2