Sense Silica — RS485 Soil 7-in-1 (Modbus RTU)

This project reads an RS485 soil sensor (NPK + temperature, humidity, pH, conductivity) over Modbus RTU using a lightweight real-time scheduler (RIoS).

Parameters

Parameter	Value
Power supply	5-24V DC
Output signal	RS485
Temperature range	-40℃ to 80℃
Temperature accuracy	±0.5℃
Temperature resolution	0.1℃
Water measurement range	0-100%
Moisture accuracy	±3% (Within 0-53%); ±5% (Within 53-100%)
Water resolution	0.10%
Electrical conductivity range	0-10000 µS/cm
Conductivity resolution	10 µS/cm
pH measurement range	3-9 pH
pH measurement accuracy	±0.3 pH
pH resolution	0.01 pH
NPK measurement range	0-1999 mg/kg
NPK measurement accuracy	±2% F.S
NPK resolution	1 mg/kg (mg/L)
Storage environment	-20℃ to 60℃
Working pressure range	0.9-1.1 atm
Response time	<1s
Protection level	IP68

Architecture

• Hardware Layer: AVR + RS485 transceiver. RE/DE pins control half‑duplex.
• Protocol Layer: Modbus RTU helpers (modbus_rtu.h) for CRC, framing, and silent interval (≥ 4 character times).
• Transport Layer: Modbus client (modbus_client.h/.cpp) handles RE/DE, timeouts, retries, and generic Read Holding Registers (0x03).
• Device Layer: Soil sensor API (soil_sensor.h/.cpp) maps registers, converts units, and exposes typed reads.
• Application Layer: RIoS scheduler ticks tasks, e.g. LED toggle and sensor polling output.

Data Flow (Read Example)

1. Task triggers a read (e.g., moisture + temperature).
2. soil_sensor_read_moisture_temperature() calls modbus_client_read_holding() with address 0x01, start 0x0012, length 2.
3. Modbus client builds frame, waits ≥4 char times, enables DE/RE, writes, returns to RX.
4. Client reads response header, data, CRC; validates via modbus_rtu_parse_read_response().
5. Device layer converts raw to units (0.1 %RH, 0.1 °C signed) and returns typed values.
6. Application prints values (or can publish/store).

APIs

• modbus_client_init(cfg): Configure RE/DE pins and defaults.
• modbus_client_read_holding(cfg, address, regStart, qty, outValues): Reads qty registers.
• modbus_client_request_raw(cfg, req, reqLen, outBuf, maxOut, outLen): Diagnostic raw send/receive.
• modbus_client_probe_addresses(cfg, addrs[], addrCount, &outAddr): Probe for a responding device on the current baud.
• soil_sensor_read_ph(sensor, &ph) → float pH (0.01 pH per unit).
• soil_sensor_read_moisture_temperature(sensor, &pct, &tempC) → moisture %RH (0.1), temperature °C (0.1, signed).
• soil_sensor_read_conductivity(sensor, &uS) → µS/cm.
• soil_sensor_read_npk(sensor, &N, &P, &K) → mg/kg.
• soil_sensor_read_all(sensor, &SoilData) → aggregate.

Registers (subset)

• pH: 0x0006
• Moisture: 0x0012
• Temperature: 0x0013 (two’s complement for < 0)
• Conductivity: 0x0015
• Nitrogen/Phosphorus/Potassium: 0x001E/0x001F/0x0020
• Device Address: 0x0100
• Baud Rate: 0x0101 (2400/4800/9600)

Usage

See src/main.cpp for initialization and periodic polling. On startup, the app probes common baud rates (9600/4800/2400) and addresses (1–5); if a device responds, it logs and uses the detected pair. To adjust defaults, change NPK_RS485_DEFAULT.baudRate in include/rs485.h.

Notes

• The Modbus client enforces the initial silent interval and validates CRC. Add a post‑response silent interval if polling faster than ~100 ms.
• Temperature is signed: register value is 0.1 °C per unit; negative values are two’s complement.
• RE/DE polarity: ModbusClientConfig supports reActiveLow and deActiveHigh for MAX485 and similar. If wiring is inverted, adjust these flags accordingly.

LCD Wiring (JHD 16×2, HD44780‑compatible)

This project uses a JHD 16×2 character LCD in 4‑bit mode via the local lib/lcd driver. Connect as follows:

• Vss → GND
• Vdd → 5V
• Vee → 10k potentiometer wiper (pot ends to 5V and GND) for contrast
• RS → Arduino D12
• R/W → GND (write‑only)
• E → Arduino D11
• DB0–DB3 → not used (leave unconnected)
• DB4 → Arduino D10
• DB5 → Arduino D9
• DB6 → Arduino D8
• DB7 → Arduino D7
• LED+ → 5V through a 220Ω resistor (if not on module)
• LED− → GND

Notes

• Common ground: tie Arduino GND, LCD Vss/LED−, and sensor/RS485 grounds together.
• Start the 10k pot mid‑position; adjust Vee until characters are visible.
• Backlight current is typically ~15–20 mA; prefer a series resistor and avoid driving LED+ directly from an I/O pin.
• Only DB4–DB7 are used in 4‑bit mode; ensure R/W is tied to GND.

Code

• Initialization occurs in src/main.cpp with gLcd.begin() and periodic updates in Task_LcdUpdate().
• Pin mappings are defined in include/config.h under the pins namespace.

LCD Quick Test

To validate wiring independently of the sensor, upload this minimal sketch (pin map matches the project):

#include <Arduino.h>
#include "lcd.h"

LCD lcd(12, 11, 5, 4, 3, 2); // RS, E, D4–D7

void setup() {
  lcd.begin();
  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.print("Hello JHD 16x2");
  lcd.setCursor(0, 1);
  lcd.print("Pins RS12 E11");
}

void loop() {}

Build and upload with PlatformIO:

C:\Users\xvj1kor\.platformio\penv\Scripts\platformio.exe run --target upload

Reference

Arduino with HD44780 based Character LCDs