Modbus Protocol Guide

Modbus - The universal language of industrial automation

Modbus Protocol Tutorial

What is Modbus?

Modbus is one of the most widely used industrial communication protocols, developed by Modicon (now Schneider Electric) in 1979. CONPROSYS devices support both Modbus RTU (serial) and Modbus TCP (Ethernet).

Modbus communication architecture showing master-slave topology

Protocol Variants

Modbus RTU Configuration

RS-485 wiring diagram for Modbus RTU multi-drop network

Hardware Setup

RS-485 Wiring

1. Connect RS-485 cable with proper polarity:
   • A/D+: Data+ (typically green)
   • B/D-: Data- (typically yellow)
   • GND: Common ground (typically black)
2. Add 120Ω termination resistor at both network ends
3. Use twisted pair cable (22-24 AWG)
4. Maximum 32 devices on single bus (standard)

Serial Parameters Configuration

yaml
# Typical Modbus RTU Configuration
Port: COM1
Baud Rate: 9600 bps (common: 9600, 19200, 38400, 115200)
Data Bits: 8
Parity: None (common: None, Even, Odd)
Stop Bits: 1
Flow Control: None

# Character Timeout: 1.5 character times
# Frame Timeout: 3.5 character times

Device Discovery Script

python
# Python script for Modbus RTU device discovery
import minimalmodbus
import serial

def scan_modbus_devices(port='COM1', baudrate=9600, start_id=1, end_id=247):
    """
    Scan for Modbus RTU devices on specified slave ID range
    """
    found_devices = []
    
    print(f" Scanning Modbus RTU devices on {port} at {baudrate} baud...")
    print(f"   Slave ID range: {start_id} to {end_id}\n")
    
    for slave_id in range(start_id, end_id + 1):
        try:
            instrument = minimalmodbus.Instrument(port, slave_id)
            instrument.serial.baudrate = baudrate
            instrument.serial.timeout = 0.5
            
            # Try to read holding register 0
            value = instrument.read_register(0, 0)
            
            print(f" Device found at Slave ID {slave_id}")
            found_devices.append({
                'slaveId': slave_id,
                'responsive': True,
                'testValue': value
            })
            
        except Exception as e:
            # Device not responding at this ID
            pass
    
    print(f"\n Found {len(found_devices)} devices")
    return found_devices

# Run scan
devices = scan_modbus_devices('COM1', 9600, 1, 20)

Manual Configuration

Navigate to "Protocols" > "Modbus RTU" in web interface:

ini
[Device Settings]
Slave ID = 1
Baud Rate = 9600
Parity = None
Stop Bits = 1
Data Bits = 8
Port = COM1
Response Timeout = 1000ms
Retry Count = 3

Modbus TCP Configuration

Modbus TCP network topology with multiple devices

Network Setup

1. Ensure gateway and devices on same network/VLAN
2. Configure static IP addresses for devices
3. Default Modbus TCP port: 502
4. Enable port forwarding if needed

Device Configuration Example

javascript
// Node.js Modbus TCP client example
const ModbusRTU = require("modbus-serial");
const client = new ModbusRTU();

async function connectModbusTCP() {
    try {
        // Connect to Modbus TCP device
        await client.connectTCP("192.168.1.100", { port: 502 });
        client.setID(1); // Unit ID
        client.setTimeout(5000); // 5 second timeout
        
        console.log(" Connected to Modbus TCP device");
        
        // Read 10 holding registers starting at address 0
        const data = await client.readHoldingRegisters(0, 10);
        console.log(" Register values:", data.data);
        
        // Write single register
        await client.writeRegister(0, 12345);
        console.log(" Register written successfully");
        
        client.close();
    } catch (err) {
        console.error(" Error:", err.message);
    }
}

connectModbusTCP();

Connection Parameters

json
{
  "protocol": "modbus-tcp",
  "ipAddress": "192.168.1.100",
  "port": 502,
  "unitId": 1,
  "timeout": 5000,
  "reconnectInterval": 30000,
  "keepAlive": true
}

Register Mapping

Register Types & Address Ranges

Example Device Map

Device: Temperature/Humidity Sensor
Model: TH-2000

Register Map:
40001 (address 0)    - Temperature (INT16, 0.1°C resolution)
40002 (address 1)    - Humidity (UINT16, 0.1% resolution)
40003 (address 2)    - Dew Point (INT16, 0.1°C resolution)
40004 (address 3)    - Alarm Status (UINT16 bitmask)
40005 (address 4)    - Device ID (UINT16)
40006-40010          - Reserved

Coils:
00001 (address 0)    - Heater Enable (R/W)
00002 (address 1)    - Alarm Reset (W)

Configuration (Holding Registers):
40101 (address 100)  - Sample Rate (seconds)
40102 (address 101)  - Alarm Threshold High
40103 (address 102)  - Alarm Threshold Low

Function Codes

Standard Function Codes

Code    Name                        Description
────────────────────────────────────────────────────────────
0x01    Read Coils                  Read 1-2000 coils
0x02    Read Discrete Inputs        Read 1-2000 discrete inputs
0x03    Read Holding Registers      Read 1-125 registers
0x04    Read Input Registers        Read 1-125 registers
0x05    Write Single Coil           Write one coil
0x06    Write Single Register       Write one register
0x0F    Write Multiple Coils        Write multiple coils
0x10    Write Multiple Registers    Write multiple registers
0x17    Read/Write Multiple Regs    Combined read/write
0x2B    Read Device Identification  Get device info

Code Usage Example

python
# Python example using pymodbus
from pymodbus.client import ModbusTcpClient

client = ModbusTcpClient('192.168.1.100', port=502)
client.connect()

# Function 0x03: Read Holding Registers
result = client.read_holding_registers(address=0, count=10, slave=1)
if not result.isError():
    print(f"Registers: {result.registers}")

# Function 0x06: Write Single Register
client.write_register(address=0, value=100, slave=1)

# Function 0x10: Write Multiple Registers
values = [100, 200, 300, 400]
client.write_registers(address=0, values=values, slave=1)

# Function 0x01: Read Coils
coils = client.read_coils(address=0, count=8, slave=1)
if not coils.isError():
    print(f"Coil states: {coils.bits}")

client.close()

Data Types

Supported Data Types

Type        Size        Range                   Byte Order
──────────────────────────────────────────────────────────
BOOL        1 bit       0/1                     -
INT16       1 register  -32,768 to 32,767       AB
UINT16      1 register  0 to 65,535             AB
INT32       2 registers -2,147,483,648 to ...   ABCD
UINT32      2 registers 0 to 4,294,967,295      ABCD
FLOAT32     2 registers IEEE 754                ABCD
INT64       4 registers Large integers          ABCDEFGH
FLOAT64     4 registers IEEE 754 double         ABCDEFGH
STRING      N registers ASCII text              -

Byte Order Configuration

javascript
// Byte order conversion examples
const byteOrders = {
  'Big Endian': 'ABCD',       // Most significant byte first
  'Little Endian': 'DCBA',    // Least significant byte first  
  'Big Endian Swap': 'BADC',  // Swap 16-bit words
  'Little Endian Swap': 'CDAB' // Reverse order
};

// Example: Read 32-bit float with different byte orders
function readFloat32(buffer, byteOrder) {
  const bytes = new Uint8Array(buffer);
  let ordered;
  
  switch(byteOrder) {
    case 'ABCD': ordered = bytes; break;
    case 'DCBA': ordered = bytes.reverse(); break;
    case 'BADC': ordered = [bytes[1], bytes[0], bytes[3], bytes[2]]; break;
    case 'CDAB': ordered = [bytes[2], bytes[3], bytes[0], bytes[1]]; break;
  }
  
  const view = new DataView(new Uint8Array(ordered).buffer);
  return view.getFloat32(0);
}

Download Resources

• Modbus Protocol Specification - Complete protocol documentation (2.8MB)
• Device Register Map Template - Excel template (125KB)
• Modbus Testing Tools - ModScan, QModMaster (45MB)
• Troubleshooting Guide - Common issues (1.5MB)
• Sample Code Library - Python, Node.js, C# (2.2MB)

Best Practices

Performance Optimization

1. Batch Reads: Read multiple registers in one request
2. Optimal Polling: Don't poll faster than device can respond
3. Connection Pooling: Reuse TCP connections
4. Error Handling: Implement retry logic with backoff

Reliability

1. Timeouts: Set appropriate timeouts (1-5 seconds)
2. Watchdog: Monitor communication health
3. Buffering: Buffer data during network issues
4. Logging: Log all errors for diagnostics

Troubleshooting

Common Issues

No Response from Device

• Check physical wiring (RS-485 polarity)
• Verify correct slave ID
• Confirm baud rate matches device
• Check termination resistors
• Verify device is powered on

CRC/LRC Errors

• Check cable quality and shielding
• Reduce baud rate
• Check for electromagnetic interference
• Verify proper grounding

Timeout Errors

• Increase timeout value
• Check network latency
• Verify device is not overloaded
• Check for network congestion