Reference

PLC glossary

Plain-English definitions of every term used in the course, with the lesson that teaches it.

ACC (accumulator)Lesson 6: The current value of a timer or counter: elapsed time or number of counts.
Alarm acknowledgeLesson 14: The operator telling the system "I have seen this alarm". Acknowledging usually silences the horn and turns a flashing light steady; the alarm itself stays until its cause is gone and it is reset.
Auxiliary contactLesson 5: An extra contact on a motor starter or contactor that changes state when the starter pulls in. Wired to a PLC input, it tells the program that the motor really is running.
Belt alignment switchLesson 3: A switch beside a conveyor belt that opens when the belt runs off track (mistracks). Usually wired normally closed so the conveyor can be stopped before the belt is damaged.
BitLesson 1: The smallest piece of data: a 1 or a 0. A BOOL tag is one bit.
BOOLLesson 1: A tag data type that holds a single bit: 1 (ON, true) or 0 (OFF, false).
Contact block: The part of a pushbutton or selector that holds its electrical contacts. The button operator and the contact block are separate parts, and contact blocks wear out.
COUNTER tagLesson 8: A tag of data type COUNTER. It holds PRE and ACC (counts) and the status bits CU, CD and DN, addressed as Name.ACC, Name.DN and so on.
CTD — Count DownLesson 13: A counter that subtracts one from its accumulator each time its rung goes from false to true. Usually shares a COUNTER tag with a CTU, so the accumulator is a running total of ins minus outs.
CTU — Count UpLesson 8: A counter that adds one to its accumulator each time its rung goes from false to true. DN turns on when ACC reaches PRE.
Double coilLesson 1: The same output tag written by OTE on more than one rung. Only the last rung scanned decides the final value, which makes the program confusing and is almost always a mistake.
EN, TT, DNLesson 6: Timer status bits. EN (enable) follows the rung. TT (timer timing) is on while timing. DN (done) turns on when ACC reaches PRE.
Fail-safe wiring: Wiring stop and fault devices normally closed, so a broken wire looks the same as the device operating. The machine stops instead of being unable to stop.
Fault resetLesson 10: The deliberate action, usually a RESET pushbutton, that clears latched faults. A good reset only clears a fault whose cause is gone, and never starts the machine by itself.
Field deviceLesson 1: Physical equipment wired to the PLC: pushbuttons, selector switches, sensors, motor starters, solenoid valves, pilot lights.
Flasher (timer oscillator)Lesson 14: Two timers that restart each other, producing a bit that turns on and off at a steady rate. Used to flash pilot lights and beacons.
Force: Overriding an I/O point from the programming software: a forced input reads the forced value whatever the field device does, and a forced output stays on or off whatever the logic says. Forces stay installed until removed, even in PROGRAM mode.
I/O status: Whether each input and output is on or off right now, shown by indicator LEDs on the I/O modules and by the tag values online. The first evidence to look at when a machine misbehaves.
IndexingLesson 9: Moving product forward one position at a time, stopping each piece at a station (for inspection, labelling or packing) before sending it on.
Input scanLesson 1: The first part of each scan. The PLC copies the state of every input terminal into its input tags. Those tags then stay fixed until the next input scan, even if the device changes mid-scan.
Input tagLesson 1: A tag connected to an input terminal. Written by the input scan, so your logic examines it but never writes it.
InterlockLesson 5: A condition that must be true for equipment to run, and that stops it if it goes false, for example "downstream conveyor running". Process interlocks in a standard PLC are not safety functions.
Intermittent fault: A fault that comes and goes. Hard to find because the machine often works perfectly while someone is watching.
Internal tag: A tag that is not wired to anything: a memory bit used inside the program, for example to remember a state or store a one-shot.
Jam detectionLesson 6: Finding a stuck product by timing how long a sensor stays blocked. A box normally passes a photoeye in well under a second; if the eye stays blocked much longer, the conveyor is stopped and a fault is shown.
JogLesson 1: Running a machine only while a pushbutton is held, used to inch a conveyor or position a part. Releasing the button stops it.
Lamp testLesson 3: A pushbutton that turns on every pilot light while it is held, so operators can spot a burned-out bulb before they rely on it.
Latched faultLesson 10: A fault bit set with OTL so it stays on after the condition that caused it goes away, until someone deliberately resets it. Operators can see what happened even if it was brief.
Momentary contactLesson 4: A pushbutton contact that changes state only while the button is pressed and springs back when released. Start, stop and jog buttons are momentary.
New vs acknowledged alarmLesson 14: A flashing light means an alarm nobody has acknowledged yet; a steady light means it has been seen but is still active. That difference lets an operator spot a new problem at a glance.
Non-retentiveLesson 1: Does not remember. An OTE is non-retentive: its bit follows the rung every scan.
Normally closed (N.C.)Lesson 2: A contact that is closed at rest and opens when operated. Stop buttons are usually N.C. so that a broken wire looks the same as a pressed Stop.
Normally open (N.O.)Lesson 2: A contact that is open at rest and closes when operated. A N.O. start button sends 1 to the PLC only while pressed.
Off-delay (run-on)Lesson 11: Keeping an output on for a set time after its condition goes away: a fan that cools a motor after it stops, or a conveyor that runs on to clear product.
Online: Connected to a running controller with the programming software, so you can watch rungs and tag values change live.
ONS — One ShotLesson 9: An input instruction that passes power for exactly one scan when its rung goes from false to true. It needs its own storage bit to remember the previous rung state.
Open circuit: A break in a circuit, such as a broken wire, a loose terminal or a burnt-out coil, so no current can flow.
OTE — Output EnergizeLesson 1: An output instruction that writes the rung condition to its bit every scan: 1 when the rung is true, 0 when it is false. Non-retentive.; See also: Non-retentive
OTL — Output LatchLesson 10: Sets its bit to 1 when the rung is true. A false rung leaves the bit unchanged. Retentive.
OTU — Output UnlatchLesson 10: Clears its bit to 0 when the rung is true. A false rung leaves the bit unchanged.
Output scanLesson 1: The last part of each scan. The PLC copies its output tags to the output terminals, turning field devices on or off.
Output tagLesson 1: A tag connected to an output terminal. Written by your logic and sent to the field device by the output scan.
Overload relay: A device on a motor starter that trips when the motor draws too much current for too long, to protect the motor. A trip is a symptom: something made the motor work too hard.
Parallel branch (OR)Lesson 3: Two or more paths side by side on a rung. Power passes if at least one path is true.
PermissiveLesson 5: A condition that must be true to START equipment but does not stop it once running.
PhotoeyeLesson 1: A sensor that detects an object by whether it breaks or reflects a beam of light. Commonly used to detect boxes on a conveyor.
Pilot lightLesson 1: An indicator lamp on an operator station that shows a machine status, such as running, faulted or part present.
PLC (Programmable Logic Controller)Lesson 1: An industrial computer that reads field devices such as pushbuttons and sensors, runs a control program, and switches outputs such as motor starters, valves and lights. Built to run continuously in harsh environments.; See also: Scan cycle
PRE (preset)Lesson 6: The target value of a timer or counter, for example 5000 ms.
Prescan: A pass the controller makes when it enters RUN, before the first real scan, that puts instructions in a known starting state. For example, OTE bits are cleared.
PROGRAM modeLesson 1: The controller does not execute logic and all outputs are off. Used for editing and downloading.
Program scanLesson 1: The part of each scan where the PLC solves the ladder logic: rungs from top to bottom, instructions from left to right.
RailsLesson 1: The two vertical lines on each side of a ladder diagram. Power flows from the left rail toward the right rail.
READY lightLesson 5: A pilot light that shows every start condition is met: the machine is stopped and pressing START would start it.
RES — ResetLesson 8: An output instruction that clears a timer or counter accumulator and its status bits when its rung is true.
RetentiveLesson 10: Keeps its value when the conditions that set it go away, and through a PROGRAM/RUN change. Latched bits are retentive.
Root cause: The underlying reason for a failure, as opposed to its symptom. Resetting a tripped overload treats the symptom; freeing the seized roller that tripped it fixes the root cause.
RTO — Retentive Timer OnLesson 12: A timer that accumulates time while its rung is true and keeps its accumulated value when the rung goes false, through stops and mode changes. Only a RES clears it.
RUN modeLesson 1: The controller executes the scan cycle and controls outputs.
Run-hoursLesson 12: The total time a machine has actually run. Maintenance is often scheduled on run-hours rather than calendar days.
RungLesson 1: One line of ladder logic between the left and right rails. Input instructions on the left decide whether power reaches the output instructions on the right.
Rung condition (continuity)Lesson 1: Whether a path of true instructions connects the left rail to an output. A rung with continuity is "true". The simulator highlights it in green.
Safety-rated control: Emergency stops, guard interlocks and similar functions that protect people are built with hardwired safety circuits or certified safety controllers. Ordinary PLC logic, like the logic in this course, is not a safety function.
Scan cycleLesson 1: The loop a PLC repeats continuously: read all inputs (input scan), solve the program top to bottom (program scan), then write all outputs (output scan). One pass usually takes a few milliseconds.; See also: Input scan, Program scan, Output scan
Seal-in (holding circuit)Lesson 4: A contact of an output placed in parallel with its start condition, so the output keeps itself on after the start button is released, until a stop condition breaks the rung.
Selector switchLesson 3: A rotary switch with two or more fixed positions, such as RUN / MAINT. It stays where it is put (maintained), unlike a momentary pushbutton. Each position closes its own contact.
Sequence (step logic)Lesson 7: A machine operation broken into steps that happen in order, such as idle, warning, running. Each step is a bit; a transition condition (a button, a timer done bit) moves the machine to the next step.
Series logic (AND)Lesson 3: Instructions placed one after another on a rung. Power passes only if every one of them is true.
Start-up warningLesson 7: A horn or beacon that sounds for a few seconds before a conveyor or machine starts, so anyone nearby can step clear. Many plants require one on long or remotely started conveyors.
Storage bitLesson 9: The internal BOOL an ONS uses to remember whether its rung was true on the previous scan. Each ONS needs its own storage bit, and nothing else should write it.
TagLesson 1: A named location in PLC memory, such as Conveyor_Motor or Jog_PB. Instructions read and write tags by name.; See also: BOOL
Three-wire controlLesson 4: The classic hardwired motor circuit: a normally-closed STOP in series with a normally-open START, with an auxiliary contact of the motor starter wired across START to hold the coil in. After a power loss the motor stays off until START is pressed again.
TIMER tagLesson 6: A tag of data type TIMER. It holds PRE and ACC (in milliseconds) and the status bits EN, TT and DN, addressed as Name.PRE, Name.ACC, Name.DN and so on. Each TON needs its own TIMER tag.
TOF — Timer Off DelayLesson 11: A timer whose done bit turns on as soon as its rung is true, and stays on for the preset time after the rung goes false. Used to keep something running for a while after it is told to stop.
TON — Timer On DelayLesson 6: A timer that accumulates time while its rung is true and sets its done bit when the accumulated time reaches the preset. A false rung resets it.
Transition (edge)Lesson 8: The moment a condition changes: false to true (rising edge) or true to false (falling edge).
Troubleshooting method: Find out what the machine should do, see what it does instead, then follow the signal from the operator to the PLC input, through the logic, to the output and the field device, until the place where it stops matching what it should be.
Up/down counterLesson 13: A CTU and a CTD on the same COUNTER tag: one counts things in, the other counts them out, so ACC is how many are in between.
Work orderLesson 15: A written request for work, with requirements. In this course, each challenge is a work order and the automated tests check each of its requirements.
XIC — Examine If ClosedLesson 1: An input instruction that is true when its bit is 1. It asks "is this bit ON?" It does not care how the field device is wired.; See also: XIO — Examine If Open
XIO — Examine If OpenLesson 2: An input instruction that is true when its bit is 0. It asks "is this bit OFF?"; See also: XIC — Examine If Closed
Zone trackingLesson 13: Keeping count of how many products are in a section of a conveyor, using a sensor where they enter and another where they leave.