---
title: "Rationale for microprocessor; open-loop MBI; DMA"
subject: "Instrumentation II"
chapter: 1
key: "semester_5/instrumentation_ii:rationale-open-loop-dma"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/rationale-open-loop-dma
site: BCT Engineering
---

# Rationale for microprocessor; open-loop MBI; DMA

Instrumentation II · Chapter 1: Microprocessor Based Instrumentation System

- Asked in: 2078 Kartik, 1+3 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/rationale-open-loop-dma
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-1

## Question

What is the rationale behind using a microprocessor in instrumentation systems? Describe two situations where an open-loop MBI system performs better than a closed-loop MBI system, illustrating your answer with block diagrams.

## Short answer

Rationale: microprocessor adds programmable intelligence, multi-channel handling, accuracy, and communication. Open-loop is better when the process is predictable (stepper positioning, traffic lights) or when feedback is difficult/delayed (timed heating) because it avoids sensor cost and loop stability problems.

## Answer

#### Rationale for using a microprocessor
A microprocessor makes instruments programmable, accurate and intelligent. It replaces fixed hardware logic with software, handles many channels, performs calculations (linearisation, PID, filtering), takes decisions, stores and displays data, and talks to other computers, all at low cost and size.

#### Situations where open loop is better than closed loop
1. **Process is predictable and the output follows the input exactly (e.g. stepper motor positioning, traffic signals, washing machine timer).** The result is known from the commands. A feedback sensor and ADC would add cost and complexity without improving accuracy.
2. **Measurement is difficult, slow or noisy, or feedback could cause instability (e.g. a drying or timed heating process, a fast process where the sensor delay is too long).** Open loop is faster and always stable, since there is no loop to oscillate.

```text
 Open loop (stepper motor positioning)
 Required      +----+  step pulses  +--------+  +-------+
 position ---> | uP |-------------->| Driver |->| Stepper|-> shaft
 (keypad)      +----+               +--------+  +-------+

 Closed loop (needs extra blocks)
 Set ->(+)-> uP -> DAC -> Actuator -> Process -+
        ^ -                                    |
        +---- ADC <- Conditioner <- Sensor <---+
```
Open loop gives lower cost, simpler software and no stability problems; it is chosen when disturbances are small and accuracy needs are moderate.

## Check yourself

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

1. Why is a microprocessor used in instrumentation?
   A. It makes instruments programmable, accurate and intelligent, replacing fixed hardware logic
   B. It makes instruments larger and fixed
   C. It removes the need for calibration hardware only
   D. It adds mechanical parts
   Answer: A. Software replaces hardware logic at low cost. (Key point: Rationale)

2. When is open loop better?
   A. When the process is predictable and output follows the input, such as stepper positioning
   B. When disturbances are large
   C. When high accuracy needs feedback
   D. When the process is unpredictable
   Answer: A. Feedback would add cost without improving accuracy. (Key point: Open loop better 1)

3. Why can open loop be better for fast processes with slow sensors?
   A. It oscillates less because of sensor delay
   B. It is faster and always stable since there is no loop to oscillate
   C. It uses feedback
   D. It is more costly
   Answer: B. No feedback means no delay-induced oscillation. (Key point: Open loop better 2)

4. Open loop is chosen when:
   A. Disturbances are large
   B. Very high accuracy is required
   C. Disturbances are small and accuracy needs are moderate
   D. Feedback is mandatory
   Answer: C. Open loop gives lower cost and simpler software in such cases. (Key point: Condition)

## Related in Instrumentation II

- [MBI features; open vs closed loop compared](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ambi-features-open-closed) — Instrumentation II, chapter 1
- [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×
- [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, advantages and DMA](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ambi-block-advantages-dma) — Instrumentation II, chapter 1
- [Types of microprocessor based instrumentation systems](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ambi-types) — Instrumentation II, chapter 1
- [MBI block diagram and advantages in industry](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ambi-block-advantages) — Instrumentation II, chapter 1

## Related in other subjects

- [Closed loop better than open loop (cruise)](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aclosed-loop-better-cruise-integral) — Embedded System, chapter 7
- [Open and closed loop systems with block diagram](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Aol-cl-bd-role) — Control Systems, chapter 1, asked 2×
- [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×
- [Why closed loop rejects disturbances better](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Adefine-cs-disturbance-rejection) — Control Systems, chapter 1