PLC, DCS, SCADA, HMI – What are the differences?

You’ve landed on this blogpost, so you probably know what the definitions are, but over the past decade the functionality of different control systems has been merging. Therefore it is important to explain how we interpretent the terms in this blogpost:

• PLC – Programmable Logic Controller
  A PLC can handle the binary input and output of the logic statement which are stored in its’s memory. In fact, a PLC is a small DCS. The status of the PLC and process can be graphically visualized in a HMI or SCADA. A operator can also give commands to the PLC and view the output in a HMI or SCADA.
• DCS – Distributed Control System
  A DCS is a automated control system that is distributed throughout a machine to provide instructions to different parts of the machine. Each section / part of a machine has its own computer that controls and manages the operation.
• SCADA – Supervisory Control And Data Acquisition
  A SCADA system is a application or system utilized for real-time collection of data from one or more remote locations. The collected information is utilized to monitor and manage ongoing processes. SCADA is quite a lot like a HMI, but SCADA is used for displaying large scale process data and is used in control rooms, far away from the machine itself.
• HMI – Human Machine Interfaces
  A HMI is just like a SCADA a monitoring device which displays the status of the machine. The main difference is that SCADA is a remote monitoring system, where as HMI is local to machine. A HMI for example, would be placed on the control panle near a part of a machine, where as SCADA would be set up in a control room, far away from the machine itself.

Okay, so now we understand that HMI and SCADA are morelikely just interfaces. However, the question ‘What is the difference between a DCS and a PLC?’ still remains. The answer isn’t simple as a PLC/HMI combination can do a lot of the same things a DCS does.

PLC vs DCS

When using a PLC, 99% of the times you will need a HMI, which displays a graphical representation of a process or machine so operators can monitor and control the process. It is possible to use a PLC without a HMI, but when a process or machine will be regularly monitored and controled by operators, a HMI is a must. A DCS typically covers an entire proces and is capable of covering an entire plant. This means a DCS is a lot more expensive.

When deciding to choose for a PLC setup or a DCS, you shouldn’t just look to the dollar signs. The question is: do you want to pay now or pay later. That sounds a bit sour, but you should realize that if your setup needs a DCS (or in the future), but you choose for a PLC-setup, you are going to pay more down the road in engineering time.

1. Industrial control SCADA &HMI Solution-- with source codes!

2. Visualization & HMI Solution-- with source codes!