A MODBUS extension for inexpensive distributed embedded systems

The MODBUS protocol on TIA/EIA-485 is a proven fieldbus that provides client-server data exchange between industrial devices with minimal hardware requirements. However, a few aspects of the protocol are not adequate to fulfill the needs of a contemporary distributed embedded system, especially for what concerns address space size, guaranteed bandwidth allocation, and controlled handover between multiple masters. This paper shows how MODBUS can be extended, in a backward-compatible way, to address these shortcomings. The extension is very simple and does not require any additional hardware, hence it is suitable for inexpensive, distributed embedded systems. The proposed extension was implemented with little development effort, starting from a Commercial Off-The-Shelf (COTS) protocol stack, for which the source code is readily available.