Using CitectSCADA > Communicating with I/O Devices > The Role of the I/O Device

The Role of the I/O Device

CitectSCADA is predominantly a supervisory system. It is the I/O Devices that directly monitor and control automation equipment. In most I/O Devices (such as PLCs) a program stored in the I/O Device controls the outputs. The logic (control strategy) of this stored program and the status of the inputs determine the value of each output.

The value of each input and output is stored in a separate memory register in the I/O Device. Each memory register is referenced by its address.

By reading and writing to memory registers in I/O Devices, CitectSCADA collects data from your plant or factory for monitoring and analysis, and provides high-level (supervisory) control of your equipment and processes.

You usually don't need to read (or write) to every register in the I/O Device: CitectSCADA lets you specify which inputs and outputs you want to monitor or control. After defining these register addresses, you can use them for system control, operator displays, trend analysis, data logging and alarm indication.

The choice of I/O Device is typically not open to the person engineering the communications system. In most cases, the I/O Device hardware is purchased in advance of the integration effort, or it is legacy equipment. However, if you have the luxury of influencing the choice of I/O Devices, communications capability is recommended to be one of the factors you consider.

Note: I/O devices such as programmable logic controllers (PLCs) usually have an internal program that controls the low-level processes within your plant. A PLC program continually scans the input registers of the PLC, and sets the output registers to values determined by the PLC program logic. While CitectSCADA can replace any PLC program, this is not recommended. PLCs are designed for high-speed response (typically 1 to 100 ms) and replacing this functionality with CitectSCADA could negatively affect your control system’s performance. Only use CitectSCADA to complement your PLC program (that is, for high level control and system monitoring).


Do not use CitectSCADA or other SCADA software as a replacement for PLC-based control programs. SCADA software is not designed for direct, high-speed system control.

Failure to follow these instructions can result in death, serious injury, or equipment damage.

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