Since the PLC is connected to field sensors and it is controlling field devices all of the information about the process being controlled and monitored is available within the PLC. A SCADA/HMI (Supervisory Control & Data Acquisition/Human-Machine Interface) allows us to extract that information and display it on a Personal Computer screen. We can extract and display only the data which interests us and in a manner which we can design. These HMIs can also be configured to permit operator control of the process from the screen.

While there are many SCADA/HMI packages available on the market there are three main players - Citect, Intellution FIX and Wonderware InTouch.
These applications can be connected to almost any PLC you care to mention. Although certain manufacturers, such as Allen Bradley, sell HMI software specifically for their PLCs.

A less expensive way of displaying process data extracted from the PLC is to use small displays. These are less flexible but may be right for the job.