The libmodbus project is a real success on many platforms and to conquer the world, I've written a version dedicated to Arduino devices. My goals are:
- to talk with a standard Arduino UNO or Duemilanove via the integrated USB serial line (8N1)
- to have the lowest footprint (binary sketch size and SRAM)
- to be robust
OK, you're right, other projects exist already:
- Master/slave by jpmzometa and others is great but the reading in loop doesn't handle serial latency and the code contains too many arrays IMHO
- ModbusMaster is a master and I want a slave.
Arduino-Modbus, the code is, eh, ugly!
- modbusmq by Mario Di Bacco is based on libmodbus but it aims Modbus TCP.
So I've extracted/adapted only the slave, RTU, reading/writing registers of libmodbus. To reduce the footprint even further, I've replaced the fast CRC code based on precomputed arrays (512 bytes) by a slow one (not so slow!). The result is clean library for Arduino able to handle exceptions if an error occurs.
The binary sketch size is 2,450 bytes and around 1,830 bytes of memory still free when running on 2K SRAM model.