In many modern industrial control systems there may be multiple DCS's, PLC's and RTU's running control logic from multiple locations. A SCADA system is used to connect all of these processors together and provide a central interface for operating and monitoring the whole system.
A SCADA system may also be used on a smaller scale and provide the HMI for a particular site or sites.
The SCADA system is comprised of software running on PC based machines which generally are servers that collect tag data from any attached controllers and save that data into a database.
This data is then used in graphic screens and trends to provide the interface by which the operators can see and control multiple processes simultaneously.
These graphic screens will be displayed on HMI clients. Most SCADA systems will have an architecture that incorporates thick, thin and zero clients. Clients are computers that are accessing services or programs on other servers over a network rather than running a program or service locally on themselves.
Chances are that if you are working with SCADA systems you will need to understand the differences in the clients on the system.
A Thin Client depends heavily on a remote server to fulfill its traditional computational roles. A thin client has a lightweight OS, and does not typically maintain local storage capabilities.
Thick ("Fat") Clients
A Thick client is one that maintains the capability for both local storage and processing. A system that can operate as a “stand-alone” node.
Zero (“Ultra-Thin”) Clients
A Zero Client is one that relies wholly on a remote server for computing needs. A Zero Client does not have a full OS; instead, the kernel merely initializes the network, begins the networking protocol, and handles display of the server's output.