In contrast to PLCs, PCs have virtually unlimited memory, compared to traditional PLCs. In PC-based control systems, programming, MMI and data communication, access the same data through the same tag name. The end result is faster design cycles and less human error. In contrast with the PLCs, which normally implement an ON-OFF or PID modes of control the PC-based control systems provide very sophisticated analog control capabilities.
Concerning the programming languages, graphical flowchart programming lends themselves much better to the logical, sequential nature of communications interfaces required inside the control engine. PC-based control systems have pioneered the use of higher level programming. Some suppliers of RTUs have created simple graphical user interface in order to configure the RTU easily. Using very powerful language LabVIEW it is possible to speed up programming considerably as it is designed to take measurements, analyze data and present data to the user. LabVIEW make it easy to maintain good architecture in the applications because encapsulation and modularity are easy to implement through the use of sub.
Remote Terminal Units (RTUs), Programmable Automatic Controllers (PACs), differs from a PLC in that they are more suitable for local area control. Modern RTUs are capable of executing simple program autonomously without involving the host computers.
RTUs, PLCs, and PAC are increasingly beginning to overlap in responsibilities and many vendors sell RTUs with PLC. RTUs are always used in situations where communication is more difficult. But RTUs have poor programmability compared with PLC.