Modbus protocol.
This protocol defines a message structure that devices (PLC,..) will recognize and use, regardless of the type of networks over which they communicate. It describes the process a device uses to request access to another device, how it will respond to requests from the other devices, and how errors will be detected and reported.
Modbus /TCPIP.
The MODBUS messaging service provides a Client/Server communication between devices
connected on an Ethernet TCP/IP network.
check out the following link:
http://www.modbus.orgRegards,
Sergejs