Topic
Fundamentals
23 guides across the knowledge base.
PLC Architecture
How a programmable logic controller is physically organized: the chassis, backplane, processor, I/O modules, remote racks, and what those choices mean for wiring, expansion, and failure.
8 min readUpdated Sep 5, 2026
The PLC Processor
What the processor module does on every scan, how its memory is organized, what the mode switch and status lights mean, and how it faults and recovers.
8 min readUpdated Sep 5, 2026
PLC I/O Systems
Discrete and analog input and output modules, sinking and sourcing, relay and transistor outputs, wiring conventions, and the module diagnostics that find field faults from the panel.
9 min readUpdated Sep 5, 2026
Controller Memory and Addressing
How a controller organizes its data: data tables and tags, the input and output images, the elementary data types and what they hold, and the naming discipline that keeps a program readable years later.
8 min readUpdated Sep 5, 2026
Tasks and Execution Priority
How a controller schedules its work: the continuous task, periodic tasks at a fixed rate, event tasks triggered by something happening, how priority preempts, and where each kind of logic belongs.
7 min readUpdated Sep 5, 2026
The Watchdog Timer
What the scan watchdog protects against, why it faults the controller instead of pausing it, what makes a scan run long, and why raising the limit is the wrong first response.
6 min readUpdated Sep 5, 2026
Retentive Memory
What survives a power cycle in a controller and what does not, how retention is backed up by battery, capacitor, or nonvolatile storage, what should be retentive, and how retained data gets lost.
8 min readUpdated Sep 5, 2026
IEC 61131-3
The standard behind PLC programming languages: the five languages, the common elements of data types, variables, and program organization units, what conformance actually means, and how the standard shows up in real platforms.
9 min readUpdated Sep 5, 2026
Program Faults
Faults the program causes: an index out of range, division by zero, an invalid indirect address, a missing routine, a type mismatch, or task overlap. Reading the fault record, what a fault handler should do, clearing safely, and logic that does not fault.
8 min readUpdated Sep 5, 2026
Watchdog Faults
A task that did not finish in time: what the watchdog protects, why the scan grew, and how to fix it. Measuring task times, finding the loop or instruction that took the time, communications load, and when raising the watchdog is legitimate.
7 min readUpdated Sep 5, 2026
Raw Counts
What an analog input module actually hands the program: the count range, where 4 mA and 20 mA fall in it, how under-range and over-range show up, and why the module manual is the only source that counts.
7 min readUpdated Sep 5, 2026
SCADA Architecture
The parts of a SCADA system and how data moves between them: I/O servers, the tag database, clients, the historian, the alarm server, and the topologies from a single panel PC to a redundant, distributed plant.
9 min readUpdated Sep 5, 2026
Historians
What a process historian does that a database does not, how data gets into it, how compression and retrieval work, how to size and keep it, and the settings that quietly throw data away.
8 min readUpdated Sep 5, 2026
Differential Pressure Flow Measurement
Orifice plates, venturis, and flow nozzles: the square-root relationship, why turndown is limited, where the square root is taken, impulse line rules, and when a DP element is still the right choice against a mag meter.
10 min readUpdated Sep 5, 2026
No Control Power
The control power path from the disconnect to the last relay, and how to work along it when the panel is dead: transformer and fuses, control breaker, emergency stop and safety relay, DC supply and distribution, what trips them, and design that makes it easy.
10 min readUpdated Sep 5, 2026
No Control Power in Panel
The panel is dead: no pilot lights, no controller, no HMI. The order of checks from the disconnect through the control transformer and its fuses, the control breaker, the emergency stop chain, and the DC supply, with the readings that locate the break.
8 min readUpdated Sep 5, 2026
Fuse Blows Repeatedly
A control fuse that opens again after replacement has a cause the replacement did not touch: a shorted coil, a ground fault in a wet field device, an overloaded circuit, the wrong fuse type for inrush, or an intermittent short. How the timing identifies it.
8 min readUpdated Sep 5, 2026
Power Supply Failure
A 24 volt DC supply whose output is gone, low, cycling, or noisy: an overload that has put it into current limit, a supply killed by heat and age, input voltage out of range, or surge damage. One test tells an overloaded supply from a dead one.
8 min readUpdated Sep 5, 2026
Troubleshooting Checklist
A general checklist for any control system problem, usable at 2 a.m.: define the problem, ask what changed, make the process safe, gather the evidence before touching anything, divide the chain, change one thing at a time, restore, and record.
8 min readUpdated Sep 5, 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
The Purdue Model
The reference architecture most segmentation designs still start from: what each level contains, where the DMZ goes, and how it maps to a water utility.
8 min readUpdated Aug 20, 2026
What Is SCADA?
Supervisory control and data acquisition: the layer that collects data from controllers across a system, presents it to operators, records it, and lets them intervene.
9 min readUpdated Aug 4, 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
Other topics
Direct contact
Have a controls question?
Reach Eric Sullivan directly about anything on this site, a controls or automation topic, or one of his personal projects.