PLC Articles
Longer-form technical writing, published as it is finished and revised as field experience corrects it. Every entry carries its published and updated dates.
123 entries
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
Control Power Supplies
Sizing the 24 VDC supplies that run a controller and its field devices, separating loads that should not share a supply, and keeping the controller alive through a power event.
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
Structured Text
The text language of IEC 61131-3: syntax, control structures, how it fits alongside ladder, the conventions that keep it readable for the next person, and the mistakes that make structured text a liability in a plant that troubleshoots online.
10 min readUpdated Sep 5, 2026
Function Block Diagram
The graphical data-flow language of IEC 61131-3: how blocks, pins, and wires work, execution order, where FBD beats ladder and structured text, building reusable equipment blocks, and the habits that keep a diagram readable.
9 min readUpdated Sep 5, 2026
State Machines in PLC Programs
Writing sequences as explicit states instead of tangled interlock chains: what a state is, how transitions are written, the pattern in structured text and in ladder, publishing the state to the HMI, and the failure handling every state machine needs.
11 min readUpdated Sep 5, 2026
PLC Will Not Run
A controller that stays in program mode, faults on every attempt to run, or will not power up: the mode switch, the major fault log, the power supply, the memory and battery, the firmware and program mismatch, and the I/O configuration errors that stop a processor, in the order that finds them.
9 min readUpdated Sep 5, 2026
I/O Not Updating
Inputs that do not change and outputs that do not respond while the controller runs: the module and rack status, the inhibited and faulted connections, the input filter and the scan, the remote adapter link, the forced points, and the program that stopped writing the tag.
9 min readUpdated Sep 5, 2026
Permissives
The conditions that must be true before a device may start: how permissives differ from interlocks and trips, the list a pump or feeder usually needs, coding them so the HMI can say why a start was refused, and the rules for time delays and bypasses.
9 min readUpdated Sep 5, 2026
Sequencers
Step-based control for operations that always run in the same order: backwash, pump and valve startup, batch makeup, membrane cleans. What every step needs, three ways to implement one, handling faults and power loss mid-sequence, and what the HMI shows.
10 min readUpdated Sep 5, 2026
Program Organization
Structuring a controller program so that someone else can maintain it: a layout that follows the plant, one routine per job, standard device blocks, I/O mapped in one place, a fixed execution order, naming that means something, and comments that say why.
10 min readUpdated Sep 5, 2026
Control Strategies
Choosing how a loop is controlled before choosing how to program it: on-off with deadband, staging, PID, ratio and feedforward, cascade, split range, and override, with the water and wastewater applications each fits and the questions that pick between them.
11 min readUpdated Sep 5, 2026
Alarms in the Controller
Generating alarm conditions in the controller rather than in the graphics: why the PLC should own the alarm state, a standard alarm block with setpoints, deadband, and delays, equipment alarms, and packing alarm bits for SCADA.
9 min readUpdated Sep 5, 2026
Sequential Function Chart
The IEC 61131-3 language for sequences: steps, transitions, actions and their qualifiers, alternative and simultaneous branches, where it fits in water and wastewater, and the rules that keep a chart from sticking in a step.
9 min readUpdated Sep 5, 2026
Analog Signal Problems
The controller side of a bad analog value: raw counts against the module range, channel diagnostics, scaling that does not match the transmitter, wiring by device type, noise and filtering, resolution, and a symptom table that points at the check.
8 min readUpdated Sep 5, 2026
Communication Failures
What to read when a controller stops talking to a device, another controller, or SCADA: port and connection status, message error codes, timeouts and retries, addressing, limits, and the order of checks that separates controller, cable, device, and network.
8 min readUpdated Sep 5, 2026
Network Problems
The controller as a network device: addressing, duplicate addresses, link speed and duplex, connection limits, packet rate and storms, multicast for remote I/O, ring protocols, and time synchronization, with the diagnostics that show which is wrong.
8 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
Modicon M340
Field notes on the Schneider Electric Modicon M340, a rack-based mid-range controller common in water plants: rack and CPU layout, X80 I/O, memory card behavior, Ethernet and serial communications, programming in Control Expert, and what to check on site.
8 min readUpdated Sep 5, 2026
Modicon M580
Field notes on the Schneider Electric Modicon M580, the Ethernet-backplane controller at the top of the range: remote I/O drops on a ring, hot standby CPUs, built-in security features, the networks it defines, Quantum migration, and what to check on site.
9 min readUpdated Sep 5, 2026
Control Expert
Field notes on Control Expert, the Schneider Electric engineering software for the Modicon M340 and M580: project structure, the five languages and derived function blocks, versions and files, online changes, upload information, and security settings.
8 min readUpdated Sep 5, 2026
Unity Pro
Field notes on Unity Pro, the earlier name of Control Expert and the software behind most installed Modicon M340, Quantum, and Premium applications: versions, moving projects to Control Expert, keeping an old version alive, and the Concept and PL7 originals.
7 min readUpdated Sep 5, 2026
ControlLogix
Field notes on the Rockwell Automation ControlLogix, the chassis-based controller of larger water plants: chassis and modules, tag-based memory and tasks, EtherNet/IP and older networks, controller redundancy, version matching, and what to check on site.
9 min readUpdated Sep 5, 2026
CompactLogix
Field notes on the Rockwell Automation CompactLogix, the smaller Logix controller behind most lift stations and small plants: families and I/O, the two Ethernet ports, what it shares with ControlLogix and what it lacks, and what to check on site.
8 min readUpdated Sep 5, 2026
Studio 5000
Field notes on Studio 5000, the Rockwell Automation software for ControlLogix and CompactLogix: the version-to-firmware rule, side-by-side installs, project files, add-on instructions, online editing, documentation in the controller, source protection.
8 min readUpdated Sep 5, 2026
Siemens
Field notes on Siemens controllers in water and wastewater: the S7-1200 and S7-1500 with TIA Portal, the older S7-300 and S7-400, PROFINET and PROFIBUS, the block model and memory cards, protection levels, and why it arrives inside packaged equipment.
8 min readUpdated Sep 5, 2026
AutomationDirect
Field notes on AutomationDirect controllers in water and wastewater: the Productivity, Do-more and BRX, and CLICK families with free software, the legacy DirectLOGIC line, Modbus and EtherNet/IP support, what they suit, and what to check on site.
8 min readUpdated Sep 5, 2026
Legacy Systems
Keeping older controllers running and planning a migration: the families still found in water plants, the risks that grow each year, software to keep alive, batteries and memory, tested spares, the security problem, and a migration the plant can survive.
9 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
Filtering Analog Inputs
How to damp a noisy analog input in the controller without hiding a real process change: the first-order filter, the moving average, the median, where each belongs, and what never to filter.
8 min readUpdated Sep 5, 2026
Deadband and Hysteresis
Why a single setpoint chatters, how a deadband turns it into a start point and a stop point, how wide to make it, and where deadbands belong in alarms, level control, and controller outputs.
7 min readUpdated Sep 5, 2026
Signal Validation
Catching a failed analog input in logic before it runs a pump on a dead transmitter: range checks, module status bits, frozen-value detection, rate checks, and what the program does with a bad value.
8 min readUpdated Sep 5, 2026
Engineering Units
Choosing and documenting the units a tag carries: where the unit is decided, how it flows from transmitter range to controller scaling to screen and historian, the conventions used in water and wastewater, and the mismatches that put a wrong number on screen.
7 min readUpdated Sep 5, 2026
EtherNet/IP
How EtherNet/IP moves control data: CIP over standard Ethernet, implicit I/O connections on a requested packet interval, explicit messaging, producer and consumer tags, and what the network has to provide.
9 min readUpdated Sep 5, 2026
Protocol Gateways
Protocol converters between a controller and devices that do not speak its language: what a gateway does, the common pairings in water and wastewater, the mapping table that is the whole configuration, the failure modes a gateway adds, and when to use a native interface instead.
9 min readUpdated Sep 5, 2026
Remote I/O
Extending controller I/O to another panel or another site over a network: the adapter and the rack, the update rate and what it costs, what the outputs do when the link fails, the difference between remote I/O and a remote controller, and the wiring, addressing, and diagnostics that make it dependable.
9 min readUpdated Sep 5, 2026
Alarm Suppression
Suppressing alarms by design: state-based logic that removes alarms that mean nothing in the current state, parent-child logic that stops a flood at its source, out-of-service suppression for maintenance, the rules that keep it safe, and how to test it.
9 min readUpdated Sep 5, 2026
Frozen Values
Why a SCADA value stops changing while looking normal: the five layers where it can freeze, from a transmitter holding its output to a client that lost its server, how to find the layer by comparing each, and the heartbeat that makes a freeze announce itself.
9 min readUpdated Sep 5, 2026
Time Synchronization
Why every clock in a control system must agree and what happens when they do not: out-of-order events, historian gaps at daylight saving, authentication failures, wrong totals. The time source and hierarchy, how each device is synchronized, and the checks.
9 min readUpdated Sep 5, 2026
Rockwell FactoryTalk
Field notes on the Rockwell Automation FactoryTalk family: View Site Edition and Machine Edition, the directory, Linx with direct controller tag access, device-based Alarms and Events, the PI-based Historian, ViewPoint and Optix, redundancy, and site checks.
8 min readUpdated Sep 5, 2026
Digital Signals
Discrete signals between instruments and controllers: dry and wetted contacts, sinking and sourcing transistor outputs, two-wire sensor leakage, NAMUR sensors with wire-break detection, contact ratings at low current, and wiring conventions.
8 min readUpdated Sep 5, 2026
Frequency Signals
Frequency as a process signal: the instruments that produce it, how frequency differs from pulse counting, the K-factor and the conversion to flow, frequency inputs and period measurement at low rates, converters to 4-20 mA, noise, and how to verify one.
7 min readUpdated Sep 5, 2026
Pulse Signals
Pulse outputs for totalizing: flowmeters, energy meters, rain gauges, and feeders, output types, scaling in volume per pulse, counter inputs, pulse width and rate, debouncing reeds, totalizer retention and rollover, and verifying against the register.
7 min readUpdated Sep 5, 2026
Hand-Off-Auto Selectors
What each position of an HOA selector does, how it is wired into the starter circuit, which protections must survive Hand, what the controller should see, and the mistakes that let a pump run with nobody watching.
7 min readUpdated Sep 5, 2026
Panel Power Supplies
The 24 V DC supply behind everything in a modern panel: sizing with real inrush and duty, single versus redundant with diode modules, how a switch-mode supply behaves under a short, distribution and fusing per branch, monitoring, and the failure modes that take out a whole panel at once.
9 min readUpdated Sep 5, 2026
Failed Power Supplies
Diagnosing the 24 volt supply in a control panel: dead, low, noisy, or cycling, a failed supply against an overloaded one, loads that grew, heat and capacitor aging, redundant supplies, sag on the input, and what to specify so the next failure is an alarm.
8 min readUpdated Sep 5, 2026
Panel PLC Faults
Working a controller fault from the panel side: reading the indicators before opening the software, faults caused by the panel rather than the program, power and grounding, I/O module and wiring faults, heat, loose connections, and the order of checks.
8 min readUpdated Sep 5, 2026
PLC Power
Powering the controller section of a panel: the control transformer and 24 volt supplies, separating controller power from field I/O power, sizing with margin, the UPS and what rides through, protection and monitoring, the DC ground, and restart sequencing.
8 min readUpdated Sep 5, 2026
Panel I/O
Arranging the I/O in a controller panel: module types for the field devices, sinking, sourcing, and isolated inputs, fused and relay outputs, the path from field terminal to module, spare capacity, and addressing that follows the I/O list.
8 min readUpdated Sep 5, 2026
Relays in the PLC Panel
The relays a controller panel needs and why: interposing relays between outputs and loads, input relays for high-voltage and remote signals, isolation and safety relays, monitoring relays, coil and contact selection, and keeping logic out of relays.
7 min readUpdated Sep 5, 2026
Isolation
Isolating the controller from the field: why grounds differ and what that does to signals, signal isolators and isolated channels, isolation transformers and their limits, relays as isolation, fiber as the ultimate isolator, and when isolation is worth it.
8 min readUpdated Sep 5, 2026
Panel Surge Protection
Surge protection of a controller panel as a system: a single entrance and the ground bar it lands on, staged protection on the power, protectors on every loop and data line that leaves the building, the radio feed, layout, and the checklist after a strike.
8 min readUpdated Sep 5, 2026
Panel Networking
The network inside a controller panel: which networks exist and how they are kept apart, where the switch sits, copper patching and fiber entry, the radio or cellular router as a panel component, cable management, shield grounding, and labeling.
7 min readUpdated Sep 5, 2026
Filtration
Controlling gravity and pressure filters: effluent rate and level control, head loss and turbidity on every filter, the backwash triggers and sequence, filter-to-waste and ripening, the turbidity rules that filter monitoring answers to, and the operator view.
11 min readUpdated Sep 5, 2026
Controller Hardening
What can be locked down on a PLC or RTU: keyswitch and mode, credentials, unused services, access lists, firmware, and the engineering workstation that is the real target.
12 min readUpdated Sep 5, 2026
PLC Program Backups
What a complete controller backup contains, how often to take one, where to keep it, why the upload from the running controller is not the master, and the verification that turns a folder of files into a recovery plan.
9 min readUpdated Sep 5, 2026
What to Back Up in a Control System
What a utility must be able to restore after a failure or attack: controller programs and firmware, SCADA and historian, HMI panels, drive and instrument parameters, network configurations, licenses, installers, and documentation, and how often each changes.
10 min readUpdated Sep 5, 2026
Controller Mode Switch and Keyswitch
The physical switch on the front of a controller: what RUN, REMOTE, and PROGRAM mean, why most controllers are left in REMOTE and why that matters, a policy for a utility, how mode is monitored and alarmed, and what to do on platforms with no keyswitch.
8 min readUpdated Sep 5, 2026
Controller Program Integrity
Knowing that the program in the controller is the one that was approved: checksums and change counters, the scheduled compare against the trusted backup, signed and protected projects, what an unexplained difference means, and making the check routine.
8 min readUpdated Sep 5, 2026
Shared Account Problems
Why the shared operator login, the vendor account everyone knows, and the single PLC password are the weakest point in most utilities: no accountability, no offboarding, no detection. Containing what cannot be avoided and migrating the rest.
8 min readUpdated Sep 5, 2026
Password Policy for OT
A password policy that fits a control system rather than an office: length over complexity, rotation on events, screening against breached lists, multi-factor authentication, no lockouts that take control from an operator, and unique vaulted device passwords.
8 min readUpdated Sep 5, 2026
Credential Storage
Where the passwords and keys of a control system are kept: a vault with individual access and a log, break-glass copies in a safe, platform credential stores for service accounts, and the places they must not be, from spreadsheets to project files.
8 min readUpdated Sep 5, 2026
Backup Testing
Why backups that have never been restored fail when needed, and how to find out: spot restores, full restores of a server to an isolated machine, controller program restores to a spare, archive reads, timing each one, and the findings and record that follow.
8 min readUpdated Sep 5, 2026
Offline Copies
The backup copy that survives ransomware: why anything reachable over the network will be encrypted too, what offline and immutable mean, options from rotated drives to immutable storage, what goes in the offline set, and the restore test that uses it.
8 min readUpdated Sep 5, 2026
Configuration Baselines
A known-good record of how every part of the control system is configured, captured at acceptance and kept current through change management: what to baseline, how to capture and store it, how to compare on a schedule, and what an unexplained difference means.
8 min readUpdated Sep 5, 2026
Program Change Detection
Knowing when a controller program changed and whether it was supposed to: change counters and signatures the SCADA can alarm on, scheduled compares, audit logs, mode and network monitoring, what each catches, and what to do when one fires.
9 min readUpdated Sep 5, 2026
Recovery and Restoration
Bringing a control system back after a compromise without reintroducing it: controllers verified against baselines first, servers rebuilt from clean media and project exports, every credential changed, hardening, staged reconnection, and end-to-end tests.
9 min readUpdated Sep 5, 2026
Building an OT Asset Inventory
The list of everything in the control system with a network interface or a program, and how to build one that is right: what counts, the fields to record, the site walk, the switch tables and passive listening that find the rest, and keeping it current.
9 min readUpdated Sep 5, 2026
Passive Discovery
Finding out what is on a control network by listening rather than probing: why active scans are risky around older controllers, how a mirror port feeds a sensor, what the traffic reveals, the blind spots, and how the sensor becomes continuous monitoring.
8 min readUpdated Sep 5, 2026
Documenting Firmware Versions
Why the firmware version of every controller, module, drive, switch, radio, and instrument belongs in the inventory with the date it was read: advisories are issued by version. Where to read each one, the several versions one controller has, and the cadence.
8 min readUpdated Sep 5, 2026
Advisories and Alerts
Where a utility learns its equipment has a known vulnerability and what to do with the news: the government and vendor advisory feeds, the sector center, the exploited catalog, subscribing by product, a weekly triage on exposure and consequence, and records.
8 min readUpdated Sep 5, 2026
Risk-Based Patching
Deciding what to patch first when everything cannot be patched at once: priority from exposure, exploitation, and consequence rather than scores, tiers with timelines, different rules for servers, network gear, and controllers, and the written accept decision.
8 min readUpdated Sep 5, 2026
Compensating Controls
What to do about a vulnerability that cannot be patched now or ever: restricting who can reach it, disabling the service, blocking dangerous protocol functions, allowlisting, monitoring, physical and procedural controls, matched to the attack path.
8 min readUpdated Sep 5, 2026
Session Logging
Recording every remote session into the control system: who connected, from where, when, to what, and what they did, from the VPN log through jump host session recording to the SCADA audit trail and controller change log, stored off the systems and reviewed.
8 min readUpdated Sep 5, 2026
Deep Packet Inspection
What a firewall can do when it understands the control protocols: allowing reads and blocking writes, permitting programming only from one workstation in a window, rejecting malformed messages, and alerting on function codes that should never appear.
8 min readUpdated Sep 5, 2026
Firmware Management
Updating the firmware of controllers, modules, drives, HMIs, switches, and radios without breaking the plant: why firmware lags and why it still matters, tracking versions, deciding what to update and when, and the update as a planned outage with a rollback.
9 min readUpdated Sep 5, 2026
Disabling Unused Services
Every service a device runs is a way in, and most ship with more running than the plant uses: web servers, file transfer, shells, discovery, management protocols, unused ports. Finding what is listening, deciding what the plant needs, and turning off the rest.
8 min readUpdated Sep 5, 2026
How to Create a PID Loop in a PLC
Set up a PID instruction from scratch: scale the PV and CV, pick the action, set the execution rate, configure limits and anti-windup, tune conservatively, and test the manual and auto transitions before it controls anything.
10 min readUpdated Sep 5, 2026
How to Add an Alarm
From a request to a working alarm: define it against the philosophy, build the condition in the PLC with deadband and delays, configure the SCADA alarm with priority and message, test it end to end, and record it in the master alarm database.
9 min readUpdated Sep 5, 2026
How to Configure a Modbus Connection
Set up a Modbus RTU or TCP link between a controller and a device: get the register map, match the physical and protocol settings, build the reads and writes, handle data types and the addressing offset, and verify with a protocol tool before trusting a value.
10 min readUpdated Sep 5, 2026
How to Build a Sequencer
Build a step sequencer in a controller that will not stick, restart into the wrong step, or leave equipment half way: define the steps from the narrative, choose a step register, and test every path before the plant depends on it.
9 min readUpdated Sep 5, 2026
How to Diagnose Read and Write Communications
Diagnose a controller that cannot read from or write to a device: capture the exact request and response, classify the failure as no link, no answer, or a rejected exchange, read the error or exception code, compare with a test tool, and fix the port.
8 min readUpdated Sep 5, 2026
How to Configure Remote I/O
Add a remote I/O rack to a controller over Ethernet: plan the network and addresses, set the adapter, and prove every point and the failure behavior before the rack goes into service.
8 min readUpdated Sep 5, 2026
Processor Faulted
A PLC with its fault light on and no logic running. How to read what it is telling you, the causes in order of likelihood, and how to restart it without repeating the fault.
8 min readUpdated Sep 5, 2026
Outputs Not Energizing
The program says the output is on and the equipment is not running. Where the signal gets lost between the output image and the field device, and how to find the point in a few measurements.
8 min readUpdated Sep 5, 2026
Inputs Not Reading
A switch closes in the field and the program never sees it. How the wetting voltage, the common, sinking and sourcing, and the wiring each lose the signal, and how to find which one did.
8 min readUpdated Sep 5, 2026
Program Will Not Download
The software refuses to download, or the download starts and fails. The version, path, mode, memory, and protection problems behind it, and the order to check them in.
7 min readUpdated Sep 5, 2026
Logic Not Executing As Expected
The rung looks right and the plant does something else. The scan-order, duplicate-output, unscheduled-routine, and data-type traps behind it, and how to find which one you are in.
8 min readUpdated Sep 5, 2026
Retentive Data Lost
The plant came back from a power cycle with zero setpoints, zero totals, and the wrong pump as lead. Which backup failed, which restore overwrote what, and how to get the data back and keep it.
7 min readUpdated Sep 5, 2026
Floating Reference Between Panels
Signals between two panels that misbehave because the panels do not share a DC reference: inputs that never turn on or half turn on, strange voltages to ground, and noise that comes and goes. How DC commons should be bonded and how to cross a panel boundary.
9 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
Relay Chatter
A relay or contactor that buzzes, cycles, or will not stay pulled in: low coil voltage from an overloaded supply or a sag, an AC coil with a dirty armature or broken shading ring, a solid-state output leaking, or an input bouncing at its threshold.
8 min readUpdated Sep 5, 2026
Loss of Control Voltage
Control voltage that drops out for a moment and comes back, resetting controllers and dropping contactors: utility sags when a large load starts, a transfer switch operation, a loose neutral, or an overloaded control transformer. Catching it with a recorder.
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
Generator Transfer Problems
The generator started and something still went wrong: no transfer, controls reset during the gap, drives faulted, pumps ran backward, or the retransfer caused a second outage. The sequence and its timers, neutral and rotation, and the monthly test.
9 min readUpdated Sep 5, 2026
Loop Powers Up but Reads Zero
The transmitter is lit and the controller shows zero, so loop current is not reaching the input: a loop wired for power but not signal, an active and passive mismatch, a missing jumper, an unconfigured channel, a blown fuse, or a transmitter sending nothing.
7 min readUpdated Sep 5, 2026
Read Works but Write Fails
Polling a device returns good data and writing a setpoint does nothing or returns an exception: a write-protected device, a read-only register, the wrong function code, a wrong address or type, logic that overwrites the value, a security rule, or a local mode.
8 min readUpdated Sep 5, 2026
Wrong Register Data
The poll succeeds and the numbers are nonsense: an offset by one, the wrong table, swapped words in a 32-bit value, a float read as an integer, signed read as unsigned, a scale factor not applied, a stale gateway value, or the wrong unit answering.
8 min readUpdated Sep 5, 2026
I/O Lists
The document that ties the field to the controller: what every I/O list column means, how tags, terminals, and module channels are assigned, spare capacity rules, how the list drives the drawings and the program, and how to keep it true after commissioning.
9 min readUpdated Sep 5, 2026
IEC Standards for Controls
The International Electrotechnical Commission standards a controls practitioner meets: IEC 61131 for programmable controllers and their languages, IEC 62443 for security, IEC 61511 for safety instrumented systems, IEC 60529 ingress protection ratings, the telemetry protocols, and how IEC and North American standards fit together.
8 min readUpdated Sep 5, 2026
Startup Checklist
The checklist for bringing a control system and its process into operation after commissioning or an outage: verification of power, I/O, communications, setpoints, forces, and backups; staged energization; manual first, then automatic; and the watch.
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
Where the Logic Should Live: Controller, SCADA, or Relay
Every control decision has a home, and putting it in the wrong one is the root of most plants that are hard to operate. A rule for deciding what belongs in the controller, what belongs in SCADA, and what belongs in a relay.
9 min readUpdated Sep 5, 2026
Measure Before You Replace
Parts-swapping is the most expensive way to troubleshoot and the most common. A method for controls faults that starts with a measurement, splits the problem in half at every step, and ends with a cause instead of a coincidence.
8 min readUpdated Sep 5, 2026
The Backup That Would Not Restore
A controller failed on a Friday, the spare was on the shelf, and the saved program would not go in. What a PLC backup has to contain to actually put a system back, why the program file alone is not enough, and how a small utility keeps backups that work.
9 min readUpdated Sep 5, 2026
The Vendor Laptop Is on Your Network Now
Every service call ends with a laptop plugged into the control network, and most utilities have no rule about it. What can go wrong when a vendor connects, and how to have the conversation without losing the vendor.
9 min readUpdated Sep 5, 2026
The Intermittent Fault Is a Data Problem
A fault that happens once a week at random cannot be found with a meter, because it is never happening while the meter is connected. How to turn an intermittent problem into a recorded one, and how the patterns in the record point at the cause.
9 min readUpdated Sep 5, 2026
How to Program Lead/Lag Pump Control
Build duplex pump control with alternation, failure detection, minimum run and off timers, and an operator override that survives a restart.
10 min readUpdated Aug 24, 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
How to Scale a 4-20 mA Input in a PLC
Turn raw analog counts into engineering units, verify at three points, and add the validity checks that stop a failed transmitter from producing a believable number.
7 min readUpdated Aug 12, 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
Modbus RTU
Serial Modbus over RS-485: function codes, register addressing, the off-by-one that catches everyone, and the physical layer details that decide whether a bus is stable.
10 min readUpdated Jul 21, 2026
Analog Scaling: Raw Counts to Engineering Units
The arithmetic that turns an analog card reading into feet, psi, or gallons per minute — plus the range mismatches that silently corrupt it.
8 min readUpdated Jul 16, 2026
Interlocks and Permissives
A permissive lets you start. An interlock stops you. Confusing the two produces equipment that either will not run or will not protect itself.
7 min readUpdated Jun 27, 2026
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.
Content on CurryControls.com is general technical reference information provided for educational and reference purposes only. It is not engineering advice for a specific installation. All information must be independently verified against current codes, standards, manufacturer documentation, project requirements, and qualified professional judgment before it is relied upon or applied.