DCS Training: What a Distributed Control System Is and How to Learn One Free

DCS training teaches you how a distributed control system runs a process plant: controllers, I/O, operator stations and an engineering station sharing one database, with redundancy built in. You can learn the architecture and engineering workflow free on edwartens.com across five platforms, DeltaV, CENTUM VP, Experion PKS, System 800xA and PCS 7, without a licence.

This short lesson by ABB Control Systems introduces what a distributed control system is. It is the opening lesson of the free System 800xA course's first module, whose practice task asks you to write one paragraph on where a DCS fits in a cement plant against a PLC-plus-SCADA design. Watch it, then read on for the parts a four-minute overview cannot cover.

What a DCS actually is

A distributed control system spreads control across several controllers, each running part of the process, while the engineering and the operator view stay central. A typical system has:

• Process controllers, often in redundant pairs, running the control strategies.
• I/O, local or remote, frequently with redundant modules and hot-swap.
• A control network, usually redundant, linking controllers and stations.
• Operator stations with faceplates, trends and alarm lists.
• An engineering station holding the single configuration database.
• A historian recording process values and events.

The word "distributed" refers to where the control runs, not to the engineering. That is the important difference. In a DCS a tag is defined once. The controller strategy, the faceplate, the alarm and the history entry all come from that one definition.

DCS against PLC plus SCADA

The old line was simple: PLCs for discrete machines, DCS for continuous processes. It is blurred now. Modern PLCs run PID loops and batch sequences well, and several DCS platforms offer PLC-sized controllers. The practical differences that remain are about the whole system rather than the controller:

• One database. Change a range in a DCS and the faceplate and alarm limits follow. In a PLC plus SCADA design you usually change both projects.
• Redundancy as a default. Controllers, networks, power supplies and servers are commonly specified redundant, because an unplanned trip of a refinery unit is expensive.
• Online changes. Engineers expect to download changes to a running plant without stopping it, with the system managing the handover.
• Process libraries. Standard blocks for PID, motors and valves with modes, interlocks and faceplates, so every loop in the plant behaves the same way.

PLC plus SCADA is often the right answer for water networks, utilities and machine lines. DCS dominates in oil and gas, refining, chemicals, power generation and much of pharmaceuticals. The oil and gas automation guide looks at that industry in more detail.

What DCS engineers actually do

Configuration in a DCS is mostly about control strategies built from function blocks. Across the five platforms the free courses cover, the recurring tasks are the same:

1. Configure I/O and map field signals to tags, including bulk edits for hundreds of channels.
2. Build control modules: an analog input with scaling and alarms, a PID loop, a motor or valve block with interlocks.
3. Configure advanced control: cascade, split-range, override and feed-forward.
4. Write sequences in SFC or the platform's sequence language for batch steps and start-ups.
5. Build operator graphics and set alarm priorities, following an alarm philosophy such as ISA-18.2 (IEC 62682).
6. Maintain the running system: backups, controller replacement, redundant server synchronisation.

Batch plants add ISA-88, the standard model of procedures, unit procedures, operations and phases. Plant-wide integration follows the ISA-95 levels, with the DCS at level 2 below manufacturing operations systems.

The safety system is separate

A DCS controls the process. It does not provide the independent protection layer that stops a runaway reaction or an overpressure. That job belongs to the safety instrumented system, designed to IEC 61511 with each safety function given a SIL target. Our guide to safety PLCs, SIL and SIS explains the terms. Most plants keep the SIS on separate hardware from the DCS, even when both come from the same vendor. If you want to work on process plants, learn at least the vocabulary: SIF, SIL, LOPA, proof test, bypass. The free Safety Instrumented Systems and SIL to IEC 61511 course is advanced and best taken after one DCS course.

Free DCS training: the five courses

Every DCS course on edwartens.com is at intermediate level and assumes instrumentation and control basics. The software is licensed, so each course is written to be followed by watching and re-drawing. The vendors' own pages, such as Emerson's DeltaV and Honeywell's Experion PKS, describe the current product lines. The five courses:

• [Emerson DeltaV DCS](/free/emerson-deltav-dcs): controllers, CHARMs and electronic marshalling, Control Studio modules in FBD, SFC and ST, PID with cascade, override, split-range, feed-forward and MPC, and DeltaV Operate.
• [Yokogawa CENTUM VP DCS](/free/yokogawa-centum-vp-dcs): FCS, HIS and ENG, ST16 and LC64 logic, SEBOL sequences, MC-2E valve and pump logic, and a water-treatment project.
• [Honeywell Experion PKS DCS](/free/honeywell-experion-pks-dcs): C300 controllers and Series C I/O, control modules, forcing I/O safely, redundant server maintenance and controller recovery.
• [ABB System 800xA DCS](/free/abb-system-800xa-dcs): Control Builder with function blocks, SFC, ST and PID, the aspect object model and Plant Explorer.
• [Siemens SIMATIC PCS 7 DCS](/free/siemens-pcs7-dcs): AS and OS in a multi-project, CFC charts, motor and valve blocks, SFC sequences. It assumes STEP 7 or TIA Portal basics.

All five are listed together under free DCS courses, and the process control path groups the foundations.

A learning route that works

Start with signals. Industrial Instrumentation and Process Control is a beginner course and covers the transmitters, valves and 4-20 mA loops every DCS tag begins with. Then PID Control for PLC Engineers covers loops and tuning; the PID tuning post gives you a simulator exercise. A PLC course such as Siemens TIA Portal makes function blocks and sequences familiar, and it runs in a free trial with PLCSIM, which no DCS offers individuals.

Then choose one DCS. Pick the brand that appears most in adverts and plant sites near you. After the first, a second platform takes far less time, because the architecture is the same and only the names change.

How to practise without a DCS

• Re-draw each strategy you watch as a block diagram with tags, ranges and alarm limits.
• Write a loop sheet for one PID loop: transmitter, range, controller action, valve fail position.
• Take a P&ID of a simple process and list every control module it needs. How to read a P&ID explains the tags and symbols.
• Rebuild a DCS motor block in a PLC simulator: modes, interlocks, feedback timeout, fault reset.

The last exercise is the most useful. A PLC trial gives you a running controller, and a motor block with auto, manual and interlock handling is the same idea on every DCS.

Start the free DCS courses

Pick a platform from free DCS courses and create a free account so your progress is saved. Each course ends with one final assessment of 15 questions from the whole course, a 60% pass mark and three attempts, then a 24-hour wait and a fresh paper.

Learning is free in full. The optional EDWartens Certificate of Completion for an intermediate course such as these is a small one-off fee, and checkout shows your price. Anyone can check it at edwartens.com/verification. It is not an Emerson, Yokogawa, Honeywell, ABB or Siemens certification, and it is not a university or accredited qualification.

Take the free course

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DCS · Intermediate · Free

Emerson DeltaV DCS

Free

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Yokogawa CENTUM VP DCS

Free

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Honeywell Experion PKS DCS: Engineering and Maintenance

Free

Instrumentation · Beginner · Free

Industrial Instrumentation and Process Control