DLDevLadder📒 My Notebook
Roadmap › 5. Automation & Industry 4.0 › Step 53
PLC

Industrial Automation — PLC, SCADA, Modbus and OPC UA

Automation pyramid and Purdue model, sensors and actuators, signals (digital, 4–20 mA), PLC hardware and scan cycle, IEC 61131-3 ladder logic and structured text, timers and counters, PID control, HMI/SCADA/DCS, historians, fieldbuses and industrial Ethernet, Modbus RTU/TCP, OPC UA, connecting plant data to .NET, safety and OT security (IEC 62443), with interview questions.
Quick revision for interview day. Print it or save it as a PDF.

Levels (ISA-95 / Purdue)

0 field devices · 1 PLC/DCS control · 2 SCADA/HMI · 3 MES/historian · 4 ERP. DMZ between 3 and 4.

PLC basics

Scan: inputs → program → outputs → housekeeping. Languages: LD, FBD, ST, SFC (IL deprecated). Timers TON/TOF/TP; counters CTU/CTD; edges R_TRIG/F_TRIG.

Signals

Digital 24 V (PNP/NPN) · 4–20 mA (live zero, broken-wire detection) · 0–10 V · scaling (raw−rMin)/(rMax−rMin)×(eMax−eMin)+eMin.

Modbus

Coils FC01/05/15 · discrete inputs FC02 · input registers FC04 · holding registers FC03/06/16 · TCP port 502 · two registers per float — check word order · no security.

OPC UA

Nodes + information model · read/write/subscribe/methods/history/alarms · certificates + encryption · opc.tcp://host:4840 · PubSub over MQTT · companion specs.

Safety & security

E-stop NC and on safety PLC/hard-wired · SIL/PL · IEC 62443 zones & conduits · no internet to PLCs · audited writes only.