MODBUS is an application layer messaging protocol that provides client/server communications between devices connected on different types of buses or networks. It is commonly used as "MODBUS over Serial Line" in RS422/485 networks, but can be implemented using TCP over Ethernet as well ("MODBUS TCP").
Two different serial transmission modes for MODBUS are defined: "RTU mode" for 8 bit binary transmissions, and "ASCII mode". "RTU mode" is the default mode that must be implemented by all devices.
See http://www.modbus.org for a complete specification of the MODBUS protocol.