Controls
PLC Systems
Architecture, programming, analog control, communications, and troubleshooting for programmable logic controllers.
Written guides in this section
Full references, procedures, and diagnostic guides published under PLC Systems.
4-20 mA Current Loops
Why the industry standardized on current instead of voltage, how a two-wire loop is powered, what the live zero buys you, and how to read a loop with a meter.
12 min readUpdated Aug 29, 2026
What Is a PLC?
A programmable logic controller is a ruggedized industrial computer that reads field inputs, solves a control program, and drives outputs on a repeating, deterministic cycle.
8 min readUpdated Aug 22, 2026
PID Control for Pumps and Valves
What each term actually does, how to tune a loop on a real pump station, and the handful of mistakes that cause most of the oscillation people blame on tuning.
11 min readUpdated Aug 19, 2026
Ladder Logic Fundamentals
Rungs, contacts, coils, and the relay conventions the language inherited — plus the patterns you will meet in almost every industrial program.
10 min readUpdated Aug 11, 2026
The PLC Scan Cycle
Read inputs, solve logic, write outputs, housekeeping. Understanding scan order explains most PLC behavior that looks wrong at first glance.
9 min readUpdated Jul 30, 2026
Modbus TCP
Modbus over Ethernet: what the MBAP header changes, why the CRC disappears, how the unit identifier works, and what does not get easier just because it is on Ethernet.
8 min readUpdated Jul 28, 2026
Topics in this area
PLC Fundamentals
How a programmable logic controller is built and how it executes.
10 topics
Programming
The five IEC languages and the structures that make a program maintainable.
12 topics
Analog Control
Getting a field measurement into a controller and acting on it correctly.
8 topics
Communications
The protocols that move data between controllers, devices, and SCADA.
8 topics
Troubleshooting
Symptom-first diagnosis for a controller that is not behaving.
7 topics
Platforms
Platform-specific notes. Vendor names are used for identification only.
12 topics
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