· Eduardo Vieira · Industrial Automation · 1 min read
Modbus Data Model - Coils, Discrete Inputs, Holding Registers, Input Registers
A deep dive into Modbus data types, addressing, and how to interact with coils, discrete inputs, and registers in your automation systems.
Modbus Data Model: Coils, Discrete Inputs, Holding Registers, Input Registers
Overview
Modbus defines four primary data types mapped to function codes:
| Data Type | Function Codes | Access | Size |
|---|---|---|---|
| Coils | 1 (Read), 5, 15 | Read/Write | 1 bit |
| Discrete Inputs | 2 | Read-only | 1 bit |
| Holding Registers | 3 (Read), 6, 16 | Read/Write | 16 bits |
| Input Registers | 4 | Read-only | 16 bits |
Coils (Discrete Outputs)
- Represent binary outputs such as relays, LEDs, and actuators.
- Operate with single-bit resolution.
- Function Code 1 reads coil status; 5 writes single coil; 15 writes multiple coils.
Example: Reading Coils
from pymodbus.client.sync import ModbusSerialClient
client = ModbusSerialClient(method='rtu', port='/dev/ttyUSB0', baudrate=9600)
result = client.read_coils(address=0, count=10, unit=1)
print(result.bits)Discrete Inputs (Binary Inputs)
- Represent binary inputs like switches or sensors.
- Function Code 2 reads discrete inputs.
Holding Registers (Read/Write 16-bit)
- Store configuration parameters, setpoints, and control variables.
- Function Code 3 reads; 6 writes single register; 16 writes multiple registers.
Example: Writing Multiple Registers
from pymodbus.client.sync import ModbusTcpClient
client = ModbusTcpClient('192.168.0.10')
client.write_registers(address=10, values=[100, 200], unit=1)Input Registers (Read-only 16-bit)
- Represent sensor readings like temperature, pressure, or analog inputs.
- Function Code 4 reads input registers.
Addressing
- Addresses start at 0 in Modbus frames but are often shown as 1-based in documentation.
- Use unit IDs to differentiate slave devices on a shared bus.
Conclusion
Understanding the Modbus data model is foundational for effective industrial communication. Properly leveraging coils, discrete inputs, and registers enables robust control and monitoring in automation systems.
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