- Qt 5.15
- Qt Serial Bus
- Qt Modbus
- Qt 5.15.19 Reference Documentation

# Qt Modbus

Modbus is a request/reply protocol that has one Modbus client in its network and may have many Modbus servers. Servers can control only their own internal states and clients can read and write data in them. Modbus is not restricted by the network type, in theory it can use any QIODevice. However, not every plugin supports all classes derived from QIODevice.

Each Modbus server has four tables in which it can store data.

name | data range in one field | permissions for client
--- | --- | ---
Discrete Inputs | 0-1 | Read
Coils | 0-1 | Read/Write
Input Registers | 0-65535 (0xffff) | Read
Holding Registers | 0-65535 (0xffff) | Read/Write

The Modbus API provides some common API to access the Modbus devices:

- QModbusDevice provides an API for common functionality with client and server.
- QModbusClient provides an API for direct access to Modbus client.
- QModbusServer provides an API for direct access to Modbus server.
- QModbusDataUnit represents a data value.
- QModbusReply is created by QModbusClient as a handle for write/read operation.