OPC is stand for OLE for Process Control. OPC eases the user to access variables of PLC with DCOM. It will confined the variable of PLC as an object of OPC so you can show it visually with software of HMI/SCADA or other programming as the OPC is common standard. In sequence to get data from PLC, the OPC will function as a computer application, it will have to utilize protocols of communication. One of the protocols is Modbus.
Modbus is a protocol of communication between PLC even with PC using OPC. Modbus is a protocol communication applied from device to device. OPC is utilized from software to software for instance you may use Modbus between a computer (master) and a PLC (slave), or between remote-I/O subsystem and a PLC. If you use Modbus then you should prefer Modbus TCP than Modbus RTU via RS232 or RS485 for reasons of speed etc.
You may apply OPC among OPC server (hardware driver software) and OPC client (software of process visualization) on a Windows computer, or other client for example statistics, advanced control, etc. If the applications of server and client are in different computers they correspond across Ethernet. Each tool use different registers, and if the tool is programmable the registers will also be dissimilar for every application. Several tools apply integers while other utilizes floating point. You require referring to tool documentation to get which information is in what format and which register is used. Creating modifies often make many works.
For OPC you can surf the data without having to worry where it is placed and in what type of data. This creates it an entire lot simpler to use. OPC works on operating system of Windows so OPC base on the computers reliability. Suggestion: It is better to use Modbus for functions of closed loop control. Employ OPC for tie in with software, for instance with your software of PIMS and displays of graphics since PIMS and any other software may not support directly to Modbus. You may desire to utilize both OPC and Modbus: OPC for just signals displayed, and Modbus for the signals control.
You have to conscious how different OPC tags control systems charge. There is a high cost difference. Some charge only for the tags displayed total number at any one time for remote or local, while others accuse for all tags of OPC even though if the tags of OPC are not displayed.