HMIs (Human Machine Interface) are used to monitor plant status and control plant. The HMI is usually mounted on the control panel local to the PLC, it usually has a direct connection to the PLC and is less likely to suffer with communication problems which can sometimes occur with networks.

Generally used instead of a SCADA, but also used when the SCADA is not in the vicinity of the PLC and the plant that it is controlling, the HMI gives the operator all the information that he/she requires about the process being controlled by the PLC. The operator can also control the operation of plant manually via the HMI, usually without affecting the automatic operation of the rest of the plant.

Setpoints can be adjusted via the HMI, so long as the operator has the required access. HMIs provide local, but limited control of the plant.

PLCs (Programmable logic Controllers)
HMIs (Human Machine Interfaces)
SCADAs (Supervisory Control And Data Aquasition)
Network Solutions
Programmed Languages