--- title: "What Is SCADA? Components and Architecture | Citia" description: "What is SCADA? It collects PLC and RTU data, shows it to operators and sends commands. See its components, architecture and examples from fuel terminals." url: https://citia-engineering.com/en/knowledge/scada language: en updated: 2026-11-13 type: article site: Citia Engineering Indonesia --- # What is SCADA? Components, architecture and industrial examples SCADA collects data from PLCs, RTUs and instruments, shows it to operators and sends supervisory commands. Components, architecture and real plant examples. | Field | Value | | --- | --- | | Topic | Control systems | | Author | Tim Engineering Citia | | Updated | 2026-11-13 | Key takeaways SCADA (Supervisory Control and Data Acquisition) is a system that collects data from PLCs, RTUs and field instruments, shows it to operators on HMI screens, stores history and alarms, and sends supervisory commands back to the equipment. SCADA watches and directs a process from above. The fast control logic still runs in the controllers in the field, which is why SCADA is called supervisory. This guide explains the components, the architecture from sensor to control room, how data and alarms move, and what SCADA looks like at fuel terminals and other Indonesian plants. - SCADA (Supervisory Control and Data Acquisition) collects data from PLCs, RTUs and field instruments, shows it to operators, stores history and alarms, and sends supervisory commands back to the equipment. - A SCADA system is a chain of field instruments, controllers, communication links, a SCADA server, HMI screens and a historian, and a fault in any link can look like a SCADA problem. - SCADA supervises while PLCs and RTUs control, so a well-designed plant keeps running under local control when the SCADA server stops. - Fuel terminals use SCADA for tanks, pumps, valves, loading and pipeline pressure, and water, power and factory utilities use the same architecture. - A SCADA command is complete only when field feedback confirms it, so every repair or change should be tested end to end. ## What SCADA does in a plant SCADA lets a small team watch and operate a large site from one control room. Instead of walking to every pump, tank and valve, the operator sees their state on a screen and acts from there. Technically, SCADA does four jobs. The first is acquiring data by polling controllers and gateways. Next, it presents that data as process graphics, trends and alarm lists. A historian records values and events so engineers can reconstruct what happened. Finally, SCADA sends supervisory commands, such as start, stop, open, close or a new setpoint, to the controller that owns the equipment. SCADA also changes how a site is run day to day. Shift reports, stock reconciliation and maintenance planning start to depend on its data, so an outage is felt in the office as well as in the control room. That dependency is the reason to design for data quality from the first day. The word supervisory matters. A PLC or RTU in the field runs the fast, protective logic on its own, scan after scan. SCADA sits above it. If the SCADA server stops, a well-designed plant keeps running under local control, and the operator loses the view rather than the process. NIST SP 800-82 Rev. 3, the US guide to operational technology (OT) security published in 2023, treats SCADA, distributed control systems (DCS) and programmable logic controllers (PLC) as core types of industrial control system. Most plants combine at least two of them. - Acquire: Polls PLCs, RTUs and gateways for values, states and diagnostics. - Display: Turns tags into process graphics, trends and alarm lists for operators. - Record: Stores time-stamped values, alarms and operator actions in a historian. - Command: Sends start, stop, open, close and setpoint changes to the controller. ## The components of a SCADA system A SCADA system is a chain of parts, and each part can fail on its own. Knowing the chain is the first step in design and in troubleshooting. Larger systems add redundant servers, an engineering workstation for configuration and an interface to business systems. In ISA-95, the model for linking plant control to enterprise software, PLCs and DCSs sit at level 2, SCADA at level 3 and ERP at level 4. The controller layer deserves its own explanation. How a PLC reads inputs, runs its program and writes outputs is covered in what a PLC is and how it works . - Field instruments : transmitters, switches, level gauges, flowmeters and actuators such as motor-operated valves (MOV). - Controllers : PLCs inside the plant and RTUs (remote terminal units) at remote or scattered sites. They read the instruments and run local logic. - Communication : serial links such as RS-485, Ethernet, fibre optic, radio or cellular, often with protocol gateways in between. - SCADA server : the software that polls devices, keeps the tag database and evaluates alarms. - HMI and operator stations : the screens where people read the process and issue commands. - Historian : the database of time-stamped values and events. - Field instruments: Wiring, calibration drift, damaged sensor - PLC / RTU: Power supply, I/O module, CPU battery - Communication: Converters, surge damage, address or baud-rate mismatch - SCADA server: Driver settings, licences, disk - HMI: Wrong tag mapping, stale display - Historian: Storage full, clocks out of sync ## SCADA architecture, from field to control room SCADA architecture describes how data climbs from a sensor to a screen and how a command travels back down. Drawn as layers, it reads from the field at the bottom to business systems at the top. At the field layer, instruments produce 4-20 mA signals, contact states or digital protocol data. The control layer turns them into values in PLC or RTU memory. A communication network carries those values to the SCADA server, which polls each device on a cycle and stores the result as tags. Operator stations and the historian read from the server. Some sites pass a filtered set of data upward to ERP or reporting systems. Commands follow the reverse path. An operator clicks open on a valve faceplate, the server writes a command bit to the controller, the controller checks its interlocks and drives the actuator, and the feedback comes back up the same chain. The diagram shows that path. Redundancy is added where a single failure would leave the operator blind. Typical choices are two SCADA servers in hot standby, a ring network between switches, and backed-up power for the control panels. Each choice adds cost and maintenance, so the design should start from what the operator must still see and control when one part fails. Power is the most common single point of failure. At the Donggala depot, Citia installed two UPS units and two automatic transfer switches so the safety and process control panels stay powered while supply moves between PLN and the generator set. See the Donggala UPS installation . - Instruments and actuators: Transmitters, gauges, flowmeters, MOVs and pump starters. - PLC / RTU: Reads signals, runs interlocks and local control. - Gateway and network: RS-485, Ethernet, fibre or radio, with protocol conversion. - SCADA server: Polls devices, keeps tags, evaluates alarms. - HMI / operator station: Graphics, trends, alarm list and commands. - Historian: Time-stamped values and events for analysis. - ERP and reports: Selected data for stock, maintenance and management. ## How SCADA reads data: polling, tags and alarms SCADA does not see the plant directly. It sees a list of tags, and each tag is one value read from one address in one device. In polling, the SCADA server or a gateway asks each device for a block of values on a fixed cycle, often through Modbus, an open protocol maintained by the Modbus Organization. The device answers, and the server updates the tags with a time stamp. If a device does not answer, a good system marks those tags as bad quality instead of showing the last value as if it were live. Alarms are rules on tags: a level above its high limit, a pump that did not start after a command, a lost link. Each alarm needs a priority and a defined operator response. The example on the right shows how one motor-operated valve can appear in a tag database. Time stamps decide whether history can be trusted. When the SCADA server, the historian and the controllers keep different clocks, an alarm can appear before the event that caused it. Synchronise clocks from one source and check them after every server change. How these tags and alarms are drawn matters as much as how they are read. The guide to HMI design that helps operators covers layout, colour and alarm presentation. ``` # Example only: tags for one motor-operated valve (MOV) TAG TYPE SOURCE MEANING MOV_A.OPENED BOOL gateway read valve fully open MOV_A.CLOSED BOOL gateway read valve fully closed MOV_A.POS REAL gateway read position, percent open MOV_A.REMOTE BOOL gateway read selector in Remote MOV_A.FAULT BOOL gateway read thermal, torque or general fault MOV_A.CMD_OPEN BOOL SCADA write open command MOV_A.CMD_STOP BOOL SCADA write stop command MOV_A.COMM_OK BOOL SCADA driver device answered last poll ``` Example tag list for one MOV. Names and fields vary by SCADA package. ## SCADA examples across industries The same SCADA idea appears wherever equipment is spread out or too numerous to watch by hand. What changes from one industry to another is the equipment, the distances and the communication links. A fuel terminal watches tanks, pumps, valves, truck loading and pipeline pressure. A water utility watches pumps and reservoirs across a service area. An electrical SCADA watches breakers, meters and generators. A factory often uses SCADA for utilities such as chillers, boilers and power distribution. The table compares them side by side. Fuel terminals add two requirements that many factories do not have. The first is stock data: tank levels and meter totals feed the terminal automation system and business reports, so their quality must be traceable from the gauge to the report. The second is safety equipment, such as emergency shutdown and tank overfill alarms. SCADA shows their state and records their events, but the protective function stays in dedicated hardware and logic. Water and power utilities face the opposite challenge: distance. Their RTUs sit at pump houses and substations far from the control room, so the communication link and its failure behaviour shape the design more than the software does. - Fuel terminal (TBBM): Tank level, pumps, MOVs, loading bays, pipeline pressure | Tank gauges, PLCs, MOV gateways, pressure transmitters, RTUs at jetties | Receipt, transfer and dispatch, alarms, shutdown status - Water and wastewater: Pumps, reservoirs, treatment steps, flows | PLCs at plants, RTUs at remote pump stations | Pump scheduling, levels, process quality alarms - Electrical distribution: Breakers, switchgear, power meters, generators | PLCs or RTUs reading meters over Modbus, protection relays | Switching status, trips, energy reports - Factory utilities: Chillers, compressors, boilers, panels | Machine and utility PLCs, Ethernet | Utility status, alarms, running hours - Pipelines: Pressure along the line, pumps, valves | RTU/PLC at stations, fibre or radio links | Leak and pressure alarms, emergency shutdown ## Field notes: SCADA at Indonesian terminals and plants Most SCADA work in Indonesian plants is not a blank-sheet design. It is adding, restoring or repairing links in a system that is already running. The examples below come from Citia project records. The shared lesson is that a SCADA fault rarely sits in the SCADA software alone. The PLC/SCADA assessment guide explains how to separate the layers before anything is replaced. When the fault is inside a controller, the safe PLC troubleshooting steps start with power, CPU status and I/O before any program change. - At the Jambi fuel terminal, Citia is restoring motor-operated valve communication. The path runs from a Flowserve Limitorque MOV over RS-485, through a surge protection device, to an Advantech ECU-1051 gateway and over Ethernet to InduSoft SCADA. See the MOV communication project . - At the Pertamina Bitung integrated terminal, failed VSD units and a pressure transmitter were replaced, fibre optic was pulled to the central control room, and pump data now appears on an AVEVA InTouch HMI. See the Bitung pump restoration . - At PT Bio Farma in Bandung, an electrical SCADA runs on ABB AC500 PLC masters that read power meters over Modbus RS-485 networks. Citia traced faults along the whole chain from SCADA server to meter during the ABB PLC troubleshooting . - At Mayora in Cianjur, chiller data and alarms stopped displaying because of a PLC-to-SCADA communication fault, and the repair had to be done without stopping production. See the chiller SCADA repair . - At Eltech in Bekasi, busbar power, humidity and temperature readings that could only be seen locally were brought into the central SCADA through a LAN-to-Ethernet converter, Modbus addressing and IP changes. See the Eltech HMI integration . ## SCADA, PLC and DCS: how they differ SCADA supervises, a PLC controls, and a DCS does both inside one integrated system. That is the short version, and it holds for most plants. A PLC is a rugged industrial computer that runs control logic for one machine or one process area. A DCS (distributed control system) combines controllers, operator stations, engineering tools and a shared database from one vendor, and is common in refineries and large continuous plants. SCADA is usually assembled from several vendors' parts and suits sites with many scattered or independent units, such as a terminal with separate pump, tank and loading areas, or a pipeline with remote stations. The boundaries blur in practice. A DCS such as ABB System 800xA, which Citia brought back to normal alarm and signal status at the Lomanis fuel terminal , includes its own HMI and alarm system. Many SCADA packages now offer redundancy and engineering tools that used to belong to DCS products. A side-by-side table of all four terms, including where the HMI fits, is in PLC vs DCS vs SCADA vs HMI . ## Keeping SCADA secure and recoverable A SCADA system that cannot be trusted or restored is a liability, even when every screen looks normal. Security and recovery belong in the design and in routine maintenance. In May 2025, the US Cybersecurity and Infrastructure Security Agency (CISA) and its partners published five primary mitigations for operational technology, listed alongside. They are short enough to check on any site. The same guidance asks for software backups and standby systems that are tested regularly, so that manual operation is safe during an incident. For a full programme, NIST SP 800-82 Rev. 3 and the ISA/IEC 62443 series give the structure most asset owners follow. Routine checks, alarm analysis and backup restore tests are part of Citia's Condition Monitoring and Maintenance service . Backups are where recovery most often fails. A PLC program or SCADA project file helps only if it matches what is running and loads with the software version on hand; the checklist is in PLC and SCADA backups you can actually restore . Systems whose hardware or operating system has left vendor support need a plan of their own. The options, from repair to phased migration, are compared in obsolete PLC and SCADA: repair or migrate . - Remove OT from the public internet: No HMI, PLC or SCADA server reachable directly from the internet. - Change default passwords: Use strong, unique passwords on every device and application. - Secure remote access: Private connection or VPN with phishing-resistant multi-factor login. - Segment IT and OT: Pass control data through a demilitarized zone, not a flat network. - Keep manual operation ready: Practise running the plant by hand and test backups and standby systems. ## Terms to know Test the command, not just the screen: After any SCADA, gateway or network change, send one real command from the operator station, confirm in the field that the equipment moved, and check that the feedback returned to the screen. A display that updates does not prove the field received the command. - SCADA: Supervisory Control and Data Acquisition: software and hardware that collect plant data, show it to operators and send supervisory commands. - RTU: Remote terminal unit: a field controller built for remote or scattered sites that reports data to SCADA over long-distance links. - PLC: Programmable logic controller: an industrial computer that reads inputs, runs a control program in repeated scans and drives outputs. - HMI: Human machine interface: the screen where an operator reads the process and gives commands. - Historian: A database that stores process values and events with time stamps for trends, reports and investigations. - Tag: One named value in the SCADA database, linked to one address in one device. - Polling: The SCADA server or gateway asking each device for its values on a regular cycle. - Alarm: A rule on a tag that tells the operator something abnormal needs a response. ## Frequently asked questions Q: What does SCADA stand for? A: SCADA stands for Supervisory Control and Data Acquisition. The name describes its two jobs: acquiring data from field controllers and instruments, and supervising the process by sending commands such as start, stop or a new setpoint to those controllers. Q: What are the main components of a SCADA system? A: The main components of SCADA are field instruments, PLCs or RTUs, a communication network, the SCADA server, HMI operator stations and a historian. Larger systems add redundant servers, an engineering workstation and an interface to business systems such as ERP. Q: What is the difference between SCADA and a PLC? A: A PLC controls equipment directly, while SCADA supervises many controllers from a central point. The PLC runs fast logic and interlocks in the field. SCADA collects data from many PLCs, shows it to operators, records history and alarms, and sends supervisory commands. Q: What is an example of SCADA at a fuel terminal? A: A fuel terminal SCADA typically shows tank levels, pump and VSD status, motor-operated valve positions and pipeline pressure in one control room. At the Jambi terminal, for example, MOV status and open, close and stop commands travel through an RS-485 gateway to InduSoft SCADA. Q: Which SCADA software is common in Indonesian plants? A: Common SCADA software in Indonesian plants includes AVEVA InTouch, InduSoft and Vijeo Citect, all of which appear in Citia project records. The right choice usually follows the installed base, the controllers already on site and the support available locally. Q: What is an RTU in SCADA? A: An RTU (remote terminal unit) is a field controller that collects data at a remote or scattered site and sends it to SCADA. RTUs are common at jetties, pipeline stations and pump houses, where the link to the control room is long and less reliable. Q: When should an old SCADA system be upgraded? A: An old SCADA system should be upgraded when spare parts, vendor support or security updates are no longer available, not simply because of its age. A healthy, supported system with tested backups can keep running; an unsupported one carries growing downtime and security risk. ## Sources and further reading 1. NIST SP 800-82 Rev. 3: Guide to Operational Technology (OT) Security, National Institute of Standards and Technology (NIST), 2023-09 (https://csrc.nist.gov/pubs/sp/800/82/r3/final) 2. ISA-95 Standard: Enterprise-Control System Integration, International Society of Automation (ISA) (https://www.isa.org/standards-and-publications/isa-standards/isa-95-standard) 3. Primary Mitigations to Reduce Cyber Threats to Operational Technology, Cybersecurity and Infrastructure Security Agency (CISA), 2025-05-06 (https://www.cisa.gov/resources-tools/resources/primary-mitigations-reduce-cyber-threats-operational-technology) 4. Modbus Organization, Modbus Organization (https://www.modbus.org/) 5. Motor-operated valve (MOV) communication, PT Citia Engineering Indonesia (https://citia-engineering.com/en/projects/motor-operated-valve-mov-communication) 6. PLC/SCADA assessment: start with symptoms, topology and backups, PT Citia Engineering Indonesia (https://citia-engineering.com/en/knowledge/reliability-plc-scada) 7. Cover photo by Nylki, CC0, Wikimedia Commons (https://commons.wikimedia.org/wiki/File:Control_room_of_BESSY_II,_2025-06-28.jpeg) Planning a SCADA change at your site?: Send siTia a short note on the equipment and what you see on the screen, and a Citia engineer will follow up. ## Contact - Company: PT Citia Engineering Indonesia - siTia - Virtual Assistance: [WhatsApp +62 813-2206-507](https://wa.me/628132206507). siTia answers questions about our products, projects and enquiries on WhatsApp. Replies are automated, and a person from the Citia team follows up. - Email: services@citiaengineering.com - Phone / WhatsApp (siTia): +62 813-2206-507 - Location: Bandung, Indonesia [Contact Citia](https://citia-engineering.com/en/contact.md) ## Structured data (JSON-LD) ```json { "@context": "https://schema.org", "@graph": [ { "@type": "Organization", "@id": "https://citia-engineering.com/#organization", "name": "Citia Engineering Indonesia", "legalName": "PT Citia Engineering Indonesia", "alternateName": "Citia", "url": "https://citia-engineering.com", "logo": { "@type": "ImageObject", "url": "https://citia-engineering.com/media/citia-logo.png", "width": 950, "height": 328 }, "description": "Citia is an engineering and system integration company in Bandung, Indonesia. 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