01
Read field inputs
Pressure, temperature, flow, level, position, and digital state from sensors and switches wired directly to its input modules.
PROGRAMMABLE LOGIC CONTROLLERS
The brainsat the asset.PLCs, demystified.
A Programmable Logic Controller is a hardened industrial computer that runs deterministic control loops at the edge — switching pumps, throttling valves, interlocking safety — and reporting its state up to SCADA. This page walks through what they are, how they think, and where they fit.
SCAN CYCLE
Read inputs → solve logic → write outputs. Repeats every few milliseconds.
LADDER LOGIC
Visual programming descended from relay diagrams — electricians can read it.
DETERMINISTIC
Same inputs, same outputs, in bounded time. Safety-critical guarantee.
01 / FUNDAMENTALS
What a PLC Actually Does
A Programmable Logic Controller is a single-purpose computer designed to survive a control cabinet — heat, vibration, voltage spikes — and execute a control program with millisecond-level reliability. It runs the same four-step cycle, forever.
Scan cycleRead inputs → execute logic → update outputs → housekeeping & comms. Repeat every 1–50 ms depending on the program. Deterministic by design.
02
Execute logic
Solve the control program — ladder, function block, or structured text — against current inputs. Same logic, same outcome, every cycle.
03
Drive outputs
Energize relays, modulate analog signals, command variable-frequency drives — close the loop on the physical process.
04
Report upstream
Publish state, counters, and faults to SCADA over DNP3, Modbus, EtherNet/IP, or OPC UA — the supervisory layer’s eyes.
02 / PROGRAMMING
Ladder Logic, Distilled
Ladder logic descends from physical relay wiring diagrams — two vertical power rails with horizontal “rungs” between them. Each rung is a logical condition. If power can flow from the left rail to the right rail through that rung’s contacts, the rung’s output coil energizes.
That visual metaphor is why ladder is the lingua franca of industrial control: electricians can read it without learning a programming language. The example on the right is a textbook seal-in motor starter — try the inputs.
Read it like this—| |— is a normally-open contact (closes when input is on). —|/|— is normally-closed (closes when input is off). —( )— is an output coil. Power flows left to right; if a coil gets power, its output turns on.
RUNG · MOTOR STARTER (M1)SCAN · 8 ms
Press START — the M1 coil energizes and seals itself in. Press STOP or trip the overload to drop it.
03 / INTEGRATION
How PLCs Hand Data To SCADA
The PLC owns the loop. SCADA owns the picture. Between them flows a steady two-way stream — telemetry rising every poll, operator commands descending on demand — over industrial protocols designed for noisy, lossy plants.
LEVEL 0FIELD
Sensors & Actuators
- Pressure transmitters
- Flow meters
- Limit switches
- Motor-operated valves
4–20 mA · 24 VDC · HART
LEVEL 1CONTROL
PLC
- Scan cycle: read → solve → write
- Ladder / FBD / Structured Text
- Local interlocks & safety
- Tag database
Scan 8 ms · IEC 61131-3
LEVEL 2SUPERVISORY
SCADA Server
- Polling engine
- Historian
- Alarm management
- HMI & dashboards
DNP3 · Modbus · OPC UA
04 / USE CASES
Where You’ll Actually Find PLCs
Three industries, same control philosophy. The hardware brands differ; the scan cycle does not. Pick a vertical to see what the PLC is doing on the ground.
PLCs sit at every wellhead, compressor station, and tank battery — controlling artificial-lift cycles, isolating leaks, and rolling telemetry up to the operator.
11,400+wellsite PLCs nationally
< 50 mssafety-trip response
24/7unattended operation
- Wellhead PLC
Plunger lift, gas lift cycle, ESP speed control
- Compressor station
Stage discharge interlocks, vibration trip logic
- Tank battery
Level transfer pumps, hatch detection, vapor recovery
- Pipeline block valve
Leak detection trip, ROW pressure containment
05 / DECISION
PLC or RTU? How to Choose.
They look similar from the outside — both are ruggedized controllers with I/O and comms. The difference is what they were optimized for. Here’s the practical breakdown.
DIMENSION
PLCIn-plant control
RTURemote telemetry
PRIMARY ROLE
Deterministic control of fast local loops.
Reliable telemetry from remote, comms-constrained sites.
TYPICAL ENVIRONMENT
In-plant control cabinets, MCC rooms, machine bases.
Pole-top, wellhead, lift station — outdoors, off-grid possible.
I/O DENSITY
High; hundreds to thousands of tags per chassis.
Modest; tens to a few hundred field points per unit.
PROGRAMMING
IEC 61131-3 — ladder, function block, structured text.
Vendor-specific config + lightweight scripting.
SCAN TIME
1–50 ms — fast enough for motor and process control.
100 ms – seconds — paced to comms and battery budget.
POWER PROFILE
AC mains; little tolerance for power loss.
Solar / battery friendly; ultra-low-power variants common.
COMMS
EtherNet/IP, Profinet, Modbus/TCP — high bandwidth.
DNP3, IEC 60870-5-101, cellular / radio / satellite.
WHEN TO CHOOSE
You need fast, deterministic control inside a plant.
You need rugged, low-power telemetry across geography.
Related Articles
RTU Primer
What's inside a Remote Terminal Unit and where you find them.
Modbus Explained
Function codes, data tables, and the float packing gotcha.
What is SCADA?
Architecture, layers, and the signal path from sensor to control room.
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