---
title: "Case study: proposed system design"
subject: "Instrumentation II"
chapter: 10
key: "semester_5/instrumentation_ii:cs-proposed-system-design"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/cs-proposed-system-design
site: BCT Engineering
---

# Case study: proposed system design

Instrumentation II · Chapter 10: Case Study

- Asked in: 2075 Asoj, 6 marks
- Video: [Case Study Instrumentation II (Uncut Video just for help)](https://www.youtube.com/watch?v=YXfgNRoyonM) — IOE Syllabus
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/cs-proposed-system-design
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-10

## Question

Design a proposed system using microprocessor/microcontroller input/output devices, interfacing process, communication protocols, data converters and handshake signals with neatly labeled block diagram.

## Short answer

The design uses a microcontroller with 12-bit ADC reading PT100 (0–150°C to 0–5V, resolution 0.037°C), pulse flow sensor, and keypad. Outputs control steam valve and pump via opto-isolated relays; RS-485 Modbus RTU communicates with SCADA; hysteresis control maintains 72.5–74°C.

## Answer

**Assumed case study:** a milk pasteurisation and packaging section of a dairy plant (HTST process: raw milk tank, heat exchanger, steam-heated holding tube, cooling, filling). The existing system uses analog panel indicators, bimetal thermostats, a manual steam valve and operator logging. (If your own visit was to another plant, replace the process names; the structure of the answer stays the same.)

#### Block diagram
```text
 PT100 --> Amp/SigCond --+
 Level sensor ----------+--> MUX/ADC (12-bit)
 Flow sensor ---(pulse)-+         |
 Keypad ----------------+---> [Microcontroller] <--> LCD 16x2
                               |  ARM/AVR + flash   |
   Opto + SSR/relay <----------+  watchdog, timers  +--> RS-485
     |                         |                          (Modbus)
 Pump / steam valve / alarm    +--> EEPROM log         PC / SCADA
 Power supply: 24 V + 5 V + UPS
```

#### Design details
- **Processor:** a microcontroller with an on-chip 12-bit ADC, UART, timers and 32 KB flash.
- **Inputs:** PT100 with amplifier (0-150 C to 0-5 V), level switch, flow sensor (pulse), keypad.
- **Outputs:** LCD, buzzer, and the pump and steam valve through an optocoupler, transistor and relay/SSR.
- **Interfacing process:** ADC channel 0 is read every 100 ms; the control routine compares the temperature with the set-point; the output pin drives the relay.
- **Data converter:** 12-bit ADC, resolution 150/4096 = 0.037 C per count.
- **Communication protocol:** RS-485, Modbus RTU, 9600 baud, 8N1, CRC-16.
- **Handshake signals:** ADC start/EOC, LCD E strobe, RS-485 DE/RE direction control.
```c
/* Assumed: 8-bit MCU, ADC on channel 0 (PT100 amplifier), relay on PORTB.0 */
unsigned int read_adc(void){            /* 12-bit conversion */
    ADC_START(); while(!ADC_DONE());   /* start, wait for EOC */
    return ADC_DATA;                    /* 0..4095 */
}
void control(void){
    float t = read_adc() * 150.0 / 4096; /* temperature in C */
    if (t < 72.5) STEAM_ON();           /* open steam valve */
    else if (t > 74.0) STEAM_OFF();     /* hysteresis control */
    wdt_reset();                        /* watchdog */
}
```
The control band (72.5 to 74 C) stays above the 72 C minimum holding temperature.

## Check yourself

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

1. How does the control routine work in the interfacing process?
   A. The ADC is read every 100 ms, temperature is compared with the set-point and the output pin drives the relay
   B. The relay is read and the ADC drives the LCD
   C. The keypad sets the baud rate
   D. The PC directly drives the steam valve
   Answer: A. The loop reads channel 0, compares to set-point and switches the relay output. (Key point: Interfacing process)

2. Which communication settings are specified?
   A. I2C at 400 kHz
   B. Modbus RTU over RS-485, 9600 baud, 8N1, CRC-16
   C. SPI with 16-bit CRC
   D. RS-232 at 115200 baud without CRC
   Answer: B. RS-485 with Modbus RTU, 9600 baud, 8N1 and CRC-16. (Key point: Communication protocol)

3. How are the pump and steam valve driven from the MCU?
   A. Directly from the port pin
   B. Through the LCD
   C. Through optocoupler, transistor and relay/SSR
   D. Through the ADC
   Answer: C. The optocoupler isolates the MCU and the driver supplies the current the relay needs. (Key point: Output isolation)

## Related in Instrumentation II

- [Case study: workflow, factors, proposed design, benefits](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Acs-2076chaitra) — Instrumentation II, chapter 10
- [Case study: existing vs proposed system](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Acs-2074asoj) — Instrumentation II, chapter 10
- [Case study: revamping MBI system for CEO](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Acs-ceo-revamp) — Instrumentation II, chapter 10
- [Double handshake data transfer with 8255-PPI](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Adouble-handshake-8255) — Instrumentation II, chapter 2
- [Case study: critical factors in process control](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Acs-critical-factors) — Instrumentation II, chapter 10, asked 2×

## Related in other subjects

- [Analog communication block diagram; FDM of three signals](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aanalog-block-fdm-three-signals) — Communication Systems, chapter 5