1. DCS, PLC and SCADA are different systems, but all of them are automation systems. DCS is a system suitable for operations requiring many analog inputs and outputs with multiple controllers, while PLC is for operations where all its functions can be contained within one controller and for tasks that need operational speed. DCS may contain several PLCs in one unit. In the case of DCS, the operator will be monitoring the I/O (input/output) through a display, and the operator operates the central controllers. In DCS, there are also distributed controllers distributed across all other subcomponents of a system.
In PLC, the alarm system is smaller than that of DCS. In PLC, the alarm system is managed in different places, including the HMI, but in DCS, the alarm system is handled by the operator centrally. For SCADA, the alarm system will sound for the whole process event if it needs emergency intervention. SCADA does not sound an alarm based solely on a point changing state in contrast with DCS.
The most important task for SCADA is data gathering from the system and event processing, whereas in a DCS and PLC, the task is to have immediate control of the subunits in the processes of a system.
2. PLC stands for Programmable Logic Controller. DCS stands for Distributed Control System. SCADA stands for Supervisory Control and Data Acquisition. PLC is a microprocessor-based device that is used in controlling the input variables of a process. A DCS is a process control system that controls multiple controllers, and actuators via some human-machine interfaces. A SCADA is a process control system that acquires and stores data from field devices, and sends them to central human-machine interfaces.
3. In digital I/O points, the inputs and outputs are in the form of binary digits or bits 1 and 0. Digital I/Os are for cases where the system can only be in two states, for example ON or OFF, START or STOP. In analog I/O points, the inputs and outputs are in the form of a continuum of numerical values, hence they are used for cases where the system can be in multiple states, for example temperature level, and flow rate.
4. The PLC consists of an input module that takes in inputs from sensors, an output module that produces outputs to be sent to controllers of an operation, and a central processing unit (CPU) which acts as a mediator between the input module and output module, and process the inputs to generate the outputs, with instructions taken from a human-machine interface (HMI), for example a computer (Shekhawat, 2014). The CPU and the HMI interacts both ways, i.e. the CPU sends messages to the HMI, and the HMI sends instructions to the CPU. The PLC has its own power supply providing power to the input module, CPU, and the output module.
5. In SCADA, the data acquisition (DAQ) system gathers data from the field devices which take their inputs from the field signals of the processes. The operator will obtain the acquired data from the DAQ. In SCADA, the acquired data is saved in a data storage for future uses, for example in displaying graphs. From the acquired data, the operator will decide on any procedures to take in cases of abnormal operations. Data storage is needed because it can store the last good values of the system in case no new/current information from the system is given to the operators by other means, including the DCS.
6. The DCS contains a central processing unit (CPU) that takes in instructions from a human-machine interface (HMI) via a data bus (Shekhawat, 2014). The CPU processes instructions from the HMI to operate multiple input/output (I/O) modules. The input signals from the field are received by the multiple I/O modules in the DCS. As the DCS has multiple I/O modules, the DCS actually contains more than one PLC per unit.
7. The DCS System Database manages the security of the information such that only authorized users are allowed to access them (Umar, 1988). The DCS System Database also manages the consistency of the data, such that the data reflects the state of the system. In addition, it also manages the currency of the data, such that the database contains the most recent information on the system. Further, it also manages the concurrency of the data, such that the data is simultaneously accessible to multiple users.
8. DCS alarm is a message displayed for the operator, which is coupled with an audible alarm that is produced when a pre-defined condition is detected. DCS alarm management consists of a system of tools and procedures that enables the efficient operations of processes by improving the effectiveness of the alarm systems (Honeywell, 2011). A typical alarm management is capable of displaying an alert to the operator showing what has changed, describing it, and provide guidelines on what to do to handle the change. Without a good alarm management system, the alarms can be overwhelming to the point that the operator is distracted by it, when the operator should have been optimizing the efficiency processes in the plant.
9. Both Foundation Fieldbus (FF) and Hart are networks to better measure and control the continuous physical processes (Liptak, 2002). This can only be done through analog inputs, and hence is fitted to DCS rather than a PLC. This is because PLCs only take digital inputs, and is more concern with manufacturing automation but DCS is more concern with process automation. FF and Hart focuses on information transfer network to better assist in proper operation of a distributed control system.
10. AMS stands for Asset Management Solutions. AMS provides the operator with an interface that mediates between the FF or Hart-enabled devices and a human-machine interface like a computer (Broadley-James, 2010).
11. DDC, an abbreviation for Direct Digital Control, is a controller managing both analog and digital I/Os under one controller card. In contrast, DCS has multiple controllers managing the analog and digital I/Os. Hence, a DDC is cheaper than a DCS. DDC is used mainly in heating, ventilating, and air-conditioning.
12. OPC stands for OLE (Object Linking and Embedding) for Process Control. It is a protocol that allows different industrial hardware devices, and software to communicate with one another (Cogent, 2010). Typically these different hardware devices contain proprietary softwares, hence they are not designed to be able to interface/link and communicate with another device from a different company. The OPC solves the interfacing/linking problem of these proprietary devices.
Broadley-James. (2010). “What is AMS?”. Retrieved from
Cogent. (2010). “What is OPC?”. Retrieved from
Honeywell. (2011). “Alarm Management: What, Why, Who and How?”. Retrieved from
Liptak, B.G. (2002). “Process Software and Digital Networks”. 3rd ed. Danvers, MA: CRC
Shekhawat, J. S. (2014). “PLC & DCS”. Retrieved from
Umar, A. (1988). “Distributed Database Management Systems: Issues and Approaches”.
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