---
title: "MBI features; open vs closed loop compared"
subject: "Instrumentation II"
chapter: 1
key: "semester_5/instrumentation_ii:mbi-features-open-closed"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/mbi-features-open-closed
site: BCT Engineering
---

# MBI features; open vs closed loop compared

Instrumentation II · Chapter 1: Microprocessor Based Instrumentation System

- Asked in: 2079 Baisakh, 2+6 marks
- Video: [Open loop and Closed loop instrumentation system | Instrumentation II | IOE PU TU](https://www.youtube.com/watch?v=kuRXSXl_G88) — Biplav Paudel
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/mbi-features-open-closed
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-1

## Question

What are the basic features of MBI system? Compare open loop and closed loop microprocessor based system with suitable examples.

## Short answer

Features: programmable, intelligent decision, high accuracy, multi-channel, data storage and communication, self-test. Open loop has no feedback, is simpler, cheaper, always stable (e.g., washing machine, traffic light). Closed loop uses feedback for error correction, higher accuracy (e.g., furnace temperature control).

## Answer

#### Basic features of an MBI system
1. Programmable and flexible: functions change by software.
2. Intelligence: decision making, limit checks, alarms, linearisation.
3. High accuracy, stability and repeatability; software calibration.
4. Multi-channel handling by multiplexing.
5. Data storage, display, printing and communication links.
6. Self-test and diagnosis; low cost, size and power.

#### Open loop vs closed loop microprocessor systems
```text
 Open:   Set -> [uP] -> [DAC/Driver] -> [Actuator] -> Process
 Closed: Set ->(+)-> [uP] -> [DAC] -> [Actuator] -> Process
              ^-                                       |
              +---- [ADC] <- [Conditioner] <- [Sensor]-+
```

| Point | Open loop | Closed loop |
|---|---|---|
| Feedback | Absent | Present |
| Sensor and ADC | Not needed | Needed |
| Accuracy | Depends on calibration | High |
| Disturbance | Not corrected | Corrected |
| Cost, complexity | Low | Higher |
| Stability | Always stable | Needs tuning |
| Example | Automatic washing machine cycle, traffic light, stepper motor positioning | Room or oven temperature controller, motor speed control, liquid level control |

## Check yourself

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

1. How does an MBI system handle many channels?
   A. By multiplexing
   B. By one ADC per sensor only
   C. By hardware logic gates
   D. It cannot
   Answer: A. Multiplexing lets one processor serve many channels. (Key point: Multi-channel)

2. Which loop needs a sensor and ADC?
   A. Closed loop
   B. Open loop
   C. Both do not
   D. Neither
   Answer: A. Feedback requires sensing and conversion. (Key point: Sensor and ADC)

3. How are disturbances handled?
   A. Open loop corrects, closed loop does not
   B. Open loop does not correct them; closed loop does
   C. Neither corrects
   D. Both correct
   Answer: B. Feedback corrects disturbances. (Key point: Disturbance)

4. Which is a closed loop example?
   A. Traffic light
   B. Washing machine cycle
   C. Room or oven temperature controller
   D. Stepper motor positioning
   Answer: C. Those others are open loop. (Key point: Examples)

## Related in Instrumentation II

- [Rationale for microprocessor; open-loop MBI; DMA](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Arationale-open-loop-dma) — Instrumentation II, chapter 1
- [Closed loop MBI system](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aclosed-loop-mbi-define) — Instrumentation II, chapter 1, asked 2×
- [MBI block diagram and basic features](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ambi-block-features) — Instrumentation II, chapter 1, asked 4×
- [Open loop vs closed loop instrumentation](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aopen-closed-loop) — Instrumentation II, chapter 1, asked 2×
- [Features of microprocessor based instrumentation](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ambi-features) — Instrumentation II, chapter 1, asked 4×
- [Types of microprocessor based instrumentation systems](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ambi-types) — Instrumentation II, chapter 1

## Related in other subjects

- [Traffic light system of Kathmandu: open or closed loop](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Atraffic-light-justify) — Control Systems, chapter 1
- [Closed vs open loop step response analytically](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Acl-vs-ol-step-response) — Control Systems, chapter 1, asked 2×
- [Open loop versus closed loop control](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aopen-vs-closed-loop) — Embedded System, chapter 7, asked 6×
- [Why closed loop response is faster than open loop](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Acl-faster-than-ol) — Control Systems, chapter 1, asked 4×