Capstone 5: Industrial Monitoring System

System Requirements

Hardware Architecture

flowchart TD
    A["24VDC
DIN Rail Power"] --> B["DC-DC Isolated
5V + 3.3V"]
    B --> C["STM32F4
Main Controller"]
    C -->|SPI| D["ADS1248
24-bit ADC × 2"]
    D --> E["Analog Front-End
8× 4-20mA / 0-10V"]
    C -->|UART| F["RS-485
Modbus RTU"]
    C -->|SPI| G["W5500
Modbus TCP"]
    C -->|SPI| H["HART Modem
AD5700"]
    C --> I["EEPROM
Calibration Data"]
    C --> J["Watchdog
+ Reset IC"]
    style C fill:#3B9797,color:#fff
    style D fill:#16476A,color:#fff
                            

Analog Input Design

# 4-20mA analog input front-end design calculator
# Converts loop current to ADC input voltage

import numpy as np

# Design parameters
i_min_ma = 4.0       # mA (live zero)
i_max_ma = 20.0      # mA (full scale)
r_sense = 249.0      # Ohm precision sense resistor (0.1%)
v_ref = 2.5          # ADC reference voltage
adc_bits = 24        # ADS1248 resolution

# Voltage across sense resistor
v_min = i_min_ma * 1e-3 * r_sense
v_max = i_max_ma * 1e-3 * r_sense

# ADC codes
lsb = v_ref / (2**adc_bits)
code_min = int(v_min / lsb)
code_max = int(v_max / lsb)

print("4-20mA Analog Input Design")
print("=" * 50)
print(f"Sense resistor:     {r_sense} Ω (0.1%)")
print(f"Voltage at  4mA:    {v_min:.3f} V")
print(f"Voltage at 20mA:    {v_max:.3f} V")
print(f"ADC reference:      {v_ref} V")
print(f"ADC resolution:     {adc_bits} bits ({2**adc_bits:,} codes)")
print(f"LSB size:           {lsb*1e6:.3f} µV")
print(f"Code at  4mA:       {code_min:,}")
print(f"Code at 20mA:       {code_max:,}")
print(f"Usable dynamic range: {code_max - code_min:,} codes")
print(f"Resolution per µA:  {(code_max-code_min)/16000:.1f} codes/µA")

# Wire break detection: current < 3.5mA
wire_break_v = 3.5e-3 * r_sense
print(f"\nWire break threshold: {wire_break_v:.4f} V ({3.5} mA)")
print(f"Over-range detect:   {22e-3 * r_sense:.4f} V ({22.0} mA)")

Modbus RTU Implementation

/* Modbus RTU slave — register map for 8-channel monitor
   STM32F4 UART + RS-485 transceiver (MAX3485) */

#include <stdint.h>

/* Modbus holding registers (40001-40032) */
typedef struct {
    uint16_t channel_raw[8];     /* 40001-40008: Raw ADC counts */
    uint16_t channel_scaled[8];  /* 40009-40016: Scaled value × 100 */
    uint16_t channel_status[8];  /* 40017-40024: Status flags */
    uint16_t alarm_flags;        /* 40025: Alarm bitmap */
    uint16_t device_status;      /* 40026: Device health */
    uint16_t sample_rate;        /* 40027: Samples/sec */
    uint16_t uptime_hours;       /* 40028: Hours since boot */
    uint16_t fw_version;         /* 40029: Firmware version */
    uint16_t serial_hi;          /* 40030: Serial number (high) */
    uint16_t serial_lo;          /* 40031: Serial number (low) */
    uint16_t modbus_addr;        /* 40032: Modbus slave address */
} modbus_regs_t;

/* Channel status bits */
#define CH_STATUS_OK          0x0000
#define CH_STATUS_WIRE_BREAK  0x0001  /* Current < 3.5 mA */
#define CH_STATUS_OVER_RANGE  0x0002  /* Current > 22 mA */
#define CH_STATUS_UNDER_RANGE 0x0004  /* Current < 3.8 mA (warning) */
#define CH_STATUS_CAL_ERROR   0x0008  /* Calibration invalid */
#define CH_STATUS_DISABLED    0x8000  /* Channel disabled */

/* Scale raw ADC to engineering units:
 * val = (raw - zero_offset) * span / (full_scale - zero_offset) + eng_min
 * Stored as integer × 100 (e.g., 25.50°C = 2550)
 */

Channel Configuration Planner

Conclusion

Industrial monitoring requires precision analog design (24-bit ADC with galvanic isolation), robust communication protocols (Modbus RTU/TCP, HART), and compliance with safety standards (ATEX, IEC 61010). This capstone bridges consumer embedded systems knowledge with professional process control engineering.