Modbus
Modbus is an open protocol that determines how a device knows it's address, recognizes a message addressed to it, determines the kind of action to be taken, and the extraction of any data or other information contained in the message.
Modbus messages may be transported over many different networks and mediums.
Modbus serial networks may be RTU mode (8-bit bytes that contain two 4-bit hexadecimal characters, CRC error checking) or ASCII mode (8-bit byte of two ASCII characters, LRC error checking). Up to 255 devices may be addressed.
Modbus Plus is a proprietal high speed local area network. A standard Modbus message packet is contained within the Modbus Plus message, this allows the easy application of bridge devices to go between Modbus and Modbus Plus networks. Up to 64 devices may be addressed.
Modbus TCP is an open implementation of Modbus messages on standard TCP/IP networks. Again, bridge devices may be utilized to provide standard Modbus devices with Ethernet connectivity.