FreeMODBUS

FreeMODBUS is a free implementation of the popular MODBUS protocol specially targeted for embedded systems. MODBUS is a popular network protocol in the industrial manufacturing environment. A MODBUS communication stack requires two layers. The Modbus Application Protocol which defines the data model and functions and a Network layer. In its current version FreeMODBUS provides an implementation of the Modbus Application Protocol v1.1a and supports RTU/ASCII transmission modes defined in the Modbus over serial line specification 1.0. Since version 0.7 FreeModbus also supports MODBUS TCP defined in Modbus Messaging on TCP/IP Implementation Guide v1.0a. It is licensed under the BSD[1] which permits its usage in commercial environments. The following MODBUS functions are currently supported:

• Read Input Register (0x04)
• Read Holding Registers (0x03)
• Write Single Register (0x06)
• Write Multiple Registers (0x10)
• Read/Write Multiple Registers (0x17)
• Read Coils (0x01)
• Write Single Coil (0x05)
• Write Multiple Coils (0x0F)
• Read Discrete Inputs (0x02)
• Report Slave ID (0x11)

The implementation is based upon the most recent standards and should be fully standard compliant. Receiving and transmitting of Modbus RTU/ASCII frames is implemented as a state machines which is driven by callbacks from the hardware abstraction layer. This makes porting to new platforms easy. If a frame is complete it is passed to the Modbus Application Layer where its content is inspected. Hooks are available in the Application Layer to add new Modbus functions.

[1]:This excludes the demo applications which might be licensed differently.