Usually, in a SCADA system there is need for gathering data from all the remote outstations to a central processing station. From there the operator can monitor and supervise all the stations that are under its control. At the Central Control Station is located the Master Terminal Unit – MTU. Subunits of that are:

• Modems that are responsible for the connection with the telecommunication network
• A communications processor that manages the telecom links
• One or more computers that allow human operators to monitor and control the automated system. The software that is used for that purpose belongs to the Human Machine Interface (HMI) category.

In several occasions it is possible that modems are connected directly to a workstation and the communication links are managed from the workstation with the use of specialized software (communication drivers). In that case there is no need for a separate communications processor.

In many occasions also happens that in place of the Central Control Station there is software that is responsible for the automated operation of the whole automated system. This piece of software can be running on a separate machine, which is usually called Master Control Unit and is attached to the communication processor or it’s an integrated system. If this software runs on a workstation and it’s part of the SCADA/HMI software, it is then called Soft Logic.