Building upon the efforts undertaken to implement a primitive Modbus protocol interface for the IEC 61131-3 virtual machine environment, I have spent some time consolidating this development and have released an embedded Modbus slave protocol library. This protocol library implements support for the most common Modbus operations including:
- Read coils (function code 1);
- Read discrete inputs (function code 2);
- Read holding registers (function code 3);
- Read input registers (function code 4);
- Write single coil (function code 5);
- Write single register (function code 6);
- Write multiple coils (function code 15);
- Write multiple registers (function code 16);
- Write mask register (function code 22); and
- Read write multiple holding registers (function code 23).
In this initial implementation, this library primary supports Modbus TCP operations – aligned with the initial implementation of Modbus operations on the IEC 61131-3 virtual machine – but there is scope for both Modbus RTU and Modbus ASCII implementations within the code base. Additionally, this library has been implemented with a keen awareness of portability for different platforms and as such includes distinct APIs for database and target platform integration.
This embedded Modbus slave C library can be obtained from GIT at https://bitbucket.org/rob_au/modbus. Similar to the IEC 61131-3 virtual machine project, this library is licensed under CC BY-NC 4.0.