Glossary
Every word you meet in the academy, in plain English, with where you'd see it on a real site and what Siemens and Rockwell call it.
- 4–20 mAalso: 4-20 mA, 4–20mA, current loop
- A sensor sends a changing electrical current to represent a measurement. Usually 4 milliamps means the range's bottom and 20 its top.
- On site A tank instrument sends a small current through its cable to report the water level.
- Siemens Configure the analogue input's current range and scale its reading to engineering units.Rockwell Configure the analogue module's current range and scaling to match the transmitter.
- Taught in: From milliamps to a number you can trust
- Accumulatoralso: ACC, CV, accumulated value
- The current total inside a timer or counter. It shows time passed or events counted, rather than the target.
- On site A counter's current total reads two after two separate bottles arrive.
- Siemens ET for elapsed timer time; CV for the counter value.Rockwell ACC for a timer or counter's accumulated value.
- Taught in: Make time explicit
- Acknowledgealso: Ack, acknowledgement, acknowledgment
- Tell the system that the operator has noticed an alarm. This can silence a horn without fixing or clearing the problem.
- On site An operator silences the high-level horn while investigating why the tank is filling.
- Taught in: Acknowledge the horn, retain the alarm
- Actuatoralso: final control device
- The part that makes equipment move or change when commanded. It turns a control request into a physical action.
- On site An air-powered actuator turns a tank valve open or closed.
- Alarmalso: warning alarm
- A warning that something needs attention. The programme decides when it appears, whether it stays and how it clears.
- On site A mixing tank warns the operator when a valve fails to open in time.
- Taught in: Separate request from evidence
- Analoguealso: analog, analogue signal, analog signal
- A measurement that can vary across a range, rather than only ON or OFF. Temperature and tank level are examples.
- On site A level instrument reports how full a tank is throughout its filling cycle.
- Taught in: Follow the signal
- ANDalso: and operator
- Both conditions must be true. Start AND Permit allows a request only when permission is also present.
- On site A wash pump needs both an operator request and enough water in its supply tank.
- Siemens AND in SCL, series contacts in LAD and & in FBD.Rockwell AND in ST, series instructions in Ladder and BAND in FBD.
- Taught in: Build permission into the logic
- AOIalso: Add-On Instruction, add on instruction
- A reusable instruction an engineer defines for a repeated job. Each use needs separate storage when its machines must act independently.
- On site A factory uses one tested pump instruction for several independent pumps.
- Siemens A Siemens FB provides a comparable reuse pattern, with different interfaces and lifecycle rules.Rockwell A Logix Add-On Instruction definition is called with a backing instance tag.
- Taught in: One definition, separate instances
- Arrayalso: one-dimensional array
- A collection of numbered values kept under one name. Each position holds the same kind of value.
- On site A level filter keeps the last few measurements in a numbered collection.
- Siemens ARRAY[0..7] OF REAL declares eight values in the supported SCL interface.Rockwell REAL[8] declares eight values, indexed 0 through 7, in Logix tags.
- Taught in: Four values, one bounded loop
- Assignmentalso: :=, assign
- Writing a value into a named place. In text code, the name on the left receives the value calculated on the right.
- On site PumpOn := Start makes the pump command follow the current Start request.
- Siemens SCL uses :=; graphical MOVE boxes copy values.Rockwell ST uses :=; Ladder MOVE copies values (formerly MOV).
- Taught in: Think in scans
- At speedalso: at-speed, at reference, AtReference
- A drive's report that the motor has reached the speed it was asked for. While the drive ramps, this signal is off.
- On site A pump's ready lamp waits for its drive to report at speed.
- Siemens SINAMICS drives report it in status word ZSW1.Rockwell PowerFlex add-on profiles report it as AtReference.
- Taught in: Ask a drive, then check
- BOOLalso: Boolean, Boolean value
- A value with only two choices: TRUE or FALSE. It cannot hold a measured amount such as 42 litres.
- On site A float switch gives an ON or OFF reading when water reaches it.
- Siemens BOOL in block interfaces and PLC tags.Rockwell BOOL in controller/program tags.
- Taught in: Know what a tag represents
- CASEalso: CASE statement
- Choose one section of code using a value, such as a state number. Only the matching section runs in that CASE call.
- On site State 10 runs the batch; State 30 keeps the equipment stopped while held.
- Siemens CASE ... OF ... END_CASE in SCL.Rockwell CASE ... OF ... END_CASE in ST.
- Taught in: Write one phase as a state machine
- Comparisonalso: compare
- A test that compares two values and gives TRUE or FALSE. Level > 90 asks whether the level exceeds 90.
- On site The controller compares the measured tank level with its filling limit.
- Siemens SCL comparison operators; CMP boxes in graphical languages.Rockwell ST comparison operators; v38 Ladder uses LT/GT/EQ/NE/LE/GE.
- Taught in: Build permission into the logic
- Counteralso: event counter
- An instruction that remembers how many events occurred. Counting an arrival once is different from adding one on every scan.
- On site A drinks line stops after the third bottle passes the sensor.
- Siemens CTU; SCL calls the declared IEC_COUNTER instance's CTU operation.Rockwell CTU in Ladder; CTUD with an FBD_COUNTER backing tag in ST.
- Taught in: Count events, not scans
- Deadbandalso: alarm deadband, alarm hysteresis
- A gap between the point where an alarm comes on and the point where it goes off. It stops a rippling reading from flashing the alarm.
- On site A high-level alarm comes on at 85 percent and clears only below 80 percent.
- Siemens WinCC analogue alarms can add hysteresis; you can also write it in the programme.Rockwell The ALMA instruction and FactoryTalk alarms have a deadband setting.
- Taught in: Give alarms a deadband
- DINTalso: double integer
- A whole-number type with a larger range than INT. It is useful for totals and numbered programme states.
- On site A filling line keeps its accumulated bottle count in a DINT.
- Siemens 32-bit signed DINT.Rockwell 32-bit signed DINT, also used for classic counter PRE/ACC.
- Taught in: Know what a tag represents
- Dutyalso: duty pump, lead pump
- The equipment chosen to do the normal job first. Another unit may be available as a spare.
- On site One water pump normally supplies the upper floors of a block of flats.
- Taught in: One request, one eligible pump
- Elapsed timealso: ET, timer elapsed
- How much of a wait has passed so far. It is different from the target duration.
- On site The operator checks how long the valve has been commanded open without confirming movement.
- Siemens ET on the IEC timer instance.Rockwell ACC on the timer backing tag.
- Taught in: Make time explicit
- Engineering unitsalso: EU, engineering unit
- The units people use for a measurement, such as percent, bar or degrees. Scaling turns raw input counts into them.
- On site The tank screen shows the level in percent, not in input counts.
- Siemens SCALE_X sets them with its MIN and MAX.Rockwell SCP calls them Scaled Min and Max; SCL calls them InEUMin and InEUMax.
- Taught in: From milliamps to a number you can trust
- Falling edgealso: negative edge, TRUE-to-FALSE
- The moment a signal changes from TRUE to FALSE. A signal that was already FALSE has not just fallen.
- On site A bottle clearing the sensor creates one falling edge.
- Siemens F_TRIG in SCL; N contact or N_TRIG in LAD.Rockwell OSF in Ladder; this academy's ST exercise compares the current and previous input explicitly.
- Taught in: Count events, not scans
- FALSEalso: false value
- A value meaning no, false or inactive. FALSE on Start means no start request is present.
- On site A valve-open command becomes FALSE when the tank has enough water.
- Taught in: Know what a tag represents
- Faultalso: fault condition
- A sign that something is not working as expected. Its effect on commands must be defined in the programme.
- On site A failed duty pump makes the water system choose the spare pump.
- Taught in: Build permission into the logic
- Feedbackalso: actual-state feedback
- A reading that reports what equipment actually did. A command to open a valve and proof that it opened are different things.
- On site A valve's open switch confirms movement after the controller asks it to open.
- Taught in: Separate request from evidence
- Flow meteralso: flowmeter, flow transmitter
- An instrument that measures liquid or gas passing through a pipe. Its reading can show whether a pump command actually produced flow.
- On site A water-pump meter reports how many litres pass each second.
- Taught in: Separate request from evidence
- FORalso: FOR loop
- Repeat code while a counting value moves from a chosen start to an end. Both ends are included in these languages.
- On site FOR from zero to three visits all four stored readings.
- Siemens FOR ... TO ... BY ... DO ... END_FOR in SCL.Rockwell FOR ... TO ... BY ... DO ... END_FOR in ST.
- Taught in: Four values, one bounded loop
- Function blockalso: FB
- A reusable piece of programme with its own remembered values. Give each separate machine its own copy of that memory.
- On site Two pumps use the same control design but keep separate start totals.
- Siemens An FB definition is called with its own instance data.Rockwell An AOI provides reusable custom control logic; its instance tag holds data.
- Taught in: One definition, separate instances
- Function block diagramalso: FBD
- A diagram language that connects the results of boxes to other boxes. Each connection carries a value used in a calculation or decision.
- On site An engineer follows a tank measurement through scaling and level comparison boxes.
- Siemens FBD networks use Siemens instruction boxes and parameter names.Rockwell FBD routines use Logix blocks, backing tags and their execution rules.
- Taught in: Follow the signal
- GRAPHalso: Siemens GRAPH
- Siemens' language for describing a job as steps, conditions between steps and actions. It makes the order of a batch visible.
- On site An engineer reads the Fill, Mix and Drain steps of a tank's batch programme.
- Siemens GRAPH in TIA Portal.Rockwell The comparable sequence language is SFC; its execution settings differ.
- Taught in: Describe a process in steps
- HMIalso: Human-Machine Interface, operator panel
- A screen where an operator sees readings and makes requests. It shows the programme's information but does not replace the controller.
- On site A tank panel shows level and lets the operator request a batch start.
- Siemens WinCC is a Siemens HMI software family.Rockwell PanelView terminals and FactoryTalk View are Rockwell HMI products.
- Hysteresisalso: level band, two-threshold control
- Using different points to start and stop an action. Between those points, keep the previous decision instead of switching repeatedly.
- On site A tank pump starts below 30 percent and keeps filling until 70 percent.
- Taught in: Control a band, not a point
- Indexalso: array index
- The whole number selecting one position in an array. You may use only positions inside the declared range.
- On site Readings[0] selects the first stored measurement in a zero-based collection.
- Taught in: Four values, one bounded loop
- Inputalso: input signal
- Information arriving at the controller, such as a button request or a measured water level.
- On site A bottle sensor tells the controller when a bottle is under the nozzle.
- Taught in: Know what a tag represents
- Instancealso: instance data, backing tag
- The separate storage used by one timer or reusable block. Sharing the design does not mean sharing every machine's remembered values.
- On site The duty and spare pumps have separate memory even though they use the same control block.
- Siemens Static timer instances; FB instances have instance data blocks or multi-instance storage.Rockwell Instruction backing tags and separate AOI instance tags.
- Taught in: Make time explicit
- INTalso: integer
- A whole-number type that can hold positive or negative values. It cannot keep a fractional reading such as 41.5.
- On site A small batch counter records three bottles as a whole number.
- Siemens 16-bit signed INT; the supported IEC_COUNTER uses INT values.Rockwell 16-bit signed INT; Logix counter ACC is DINT.
- Taught in: Know what a tag represents
- Interlockalso: run inhibit
- A condition that prevents or stops an action when another condition is unsuitable. Its exact rules belong in the programme.
- On site A low-water switch stops a pump before it runs dry.
- Taught in: Build permission into the logic
- Ladderalso: LAD, LD, ladder logic, RLL
- A diagram language using conditions and output instructions along paths. Read which conditions allow each path to reach its output.
- On site An engineer checks both the Start path and the holding path before accepting a pump circuit.
- Siemens LAD networks with contacts, coils and instruction boxes.Rockwell Ladder rungs using instructions such as XIC, XIO and OTE.
- Taught in: Read a rung
- Latchalso: latched command
- A remembered ON value that stays on after the original request ends. Another instruction must deliberately turn it off.
- On site A high-level alarm stays displayed until the water falls and the operator resets it.
- Siemens S/R coils or explicit SCL assignments can store and clear a BOOL.Rockwell OTL/OTU instructions or explicit ST assignments can store and clear a BOOL.
- Taught in: Read a rung
- Level transmitteralso: level instrument
- An instrument that reports how much liquid is in a vessel. Unlike a simple level switch, it can report a changing measurement.
- On site A mixing-tank instrument sends its level reading to the controller.
- Taught in: From milliamps to a number you can trust
- Limit switchalso: position switch, open limit, closed limit
- A switch that reports when equipment reaches a particular position. It gives an ON or OFF answer rather than a complete travel measurement.
- On site A valve's open limit switch confirms that it reached its open position.
- Taught in: Separate request from evidence
- Loopalso: iteration
- A section of code that repeats. A clear stopping rule keeps it from repeating forever.
- On site The programme adds four meter readings by visiting each stored value once.
- Siemens FOR loops are supported in this academy's SCL subset.Rockwell FOR loops are supported in this academy's ST subset.
- Taught in: Four values, one bounded loop
- Memberalso: structure member
- One named value inside a group of values. A dot separates the group's name from the value's name.
- On site Status.Level is the level value inside the Status group.
- Siemens Local SCL example: #Status.Level.Rockwell ST example: Status.Level.
- Taught in: Data is not executable logic
- Motor starteralso: starter, contactor starter
- Equipment that switches motor power on and off. It usually includes protection against excessive motor current.
- On site The PLC sends a small run command to a starter, which switches the pump motor's power.
- Nalso: N action, non-stored action
- A step action active only while its step is active. It stops being active when the sequence leaves that step.
- On site The mixer command is active only during the Mix step.
- Siemens N action qualifier in GRAPH.Rockwell N action qualifier in SFC.
- Taught in: Make every command’s lifetime clear
- NOTalso: not operator
- Reverses true and false. NOT Stop is true while Stop is inactive.
- On site A pump may run only while its fault signal is NOT active.
- Siemens NOT in SCL; an inverted contact tests a false operand in LAD.Rockwell NOT in ST; XIO tests a false operand in Ladder.
- Taught in: Build permission into the logic
- Off-delayalso: off delay, run-on delay
- A wait before switching off after a request ends. A new request during the wait keeps the output on.
- On site A flushing pump keeps washing a pipe briefly after the wash request ends.
- Siemens TOF with IN/PT and Q/ET; SCL uses a TOF_TIME instance here.Rockwell TOF in Ladder; TOFR with TimerEnable/PRE and DN/ACC in ST.
- Taught in: Make time explicit
- On-delayalso: on delay, start delay
- A wait before switching on. The timing request must remain true for the whole wait; removing it starts the wait over.
- On site A wash pump ignores a brief button tap but starts after a held request.
- Siemens TON with IN/PT and Q/ET; SCL uses a TON_TIME instance here.Rockwell TON in Ladder; TONR with TimerEnable/PRE and DN/ACC in ST.
- Taught in: Make time explicit
- One-shotalso: one shot, edge pulse
- A TRUE pulse lasting one scan when a signal changes. It turns one physical event into one countable request.
- On site One bottle under a sensor produces one arrival pulse, rather than a count on every scan.
- Siemens R_TRIG/F_TRIG or P/N edge instructions, depending on the language.Rockwell ONS/OSR/OSF in Ladder; OSRI or explicit input-history logic in the supported ST exercises.
- Taught in: Count events, not scans
- ORalso: or operator
- At least one condition must be true. Either condition, or both together, can make the result true.
- On site Either Stop or a low-water condition can remove the pump command.
- Siemens OR in SCL, parallel paths in LAD and >=1 in FBD.Rockwell OR in ST, parallel branches in Ladder and BOR in FBD.
- Taught in: Build permission into the logic
- Outputalso: output command
- A command sent by the controller to equipment. An ON command does not prove that the equipment moved.
- On site A pump command asks its motor starter to supply power to the motor.
- Taught in: Know what a tag represents
- Permissivealso: permit, permission
- A condition that must be satisfied before equipment may run. In these exercises it must also remain satisfied while running.
- On site A brewery pump needs enough water in the supply tank before it can run.
- Siemens A project condition, often represented by a BOOL such as Permit.Rockwell A project condition, often represented by a BOOL such as Permit.
- Taught in: Build permission into the logic
- PLCalso: Programmable Logic Controller
- An industrial computer that reads inputs, follows your programme and sends commands to equipment.
- On site A brewery PLC controls the valves that add water to a mixing tank.
- Siemens S7-1500 controllers in this academy; programmed using TIA Portal.Rockwell CompactLogix/ControlLogix controllers in this academy; programmed using Studio 5000 Logix Designer.
- Taught in: Think in scans
- Presetalso: PT, PRE, PV
- The target given to a timer or counter. It says how long to wait or how many events to count.
- On site A packing counter has a preset of three bottles before it stops the belt.
- Siemens PT is the timer duration; PV is the counter target.Rockwell PRE is the timer or counter preset; timer values here are milliseconds.
- Taught in: Make time explicit
- Ralso: R action, reset action
- An action that clears a command kept on by an S action. Leaving the original step alone does not clear it.
- On site The finishing step clears the stored recirculation pump command.
- Siemens R action qualifier in GRAPH.Rockwell R action qualifier in SFC.
- Taught in: Make every command’s lifetime clear
- Rampalso: ramp time, acceleration ramp
- The steady rate at which a drive changes speed. It protects the motor, the pump and the pipes from sudden changes.
- On site A pump drive takes four seconds to go from stopped to full speed.
- Taught in: Ask a drive, then check
- Raw valuealso: raw counts, input counts
- The number an input card gives the PLC before scaling. It means nothing to an operator until it is converted.
- On site A level transmitter's 12 mA arrives as 13824 counts on an S7-1500 input.
- Siemens S7-1500 inputs set to 4–20 mA use 0 to 27648 counts for the normal range.Rockwell Logix modules can report raw counts, or scale them to mA or engineering units in the module.
- Taught in: From milliamps to a number you can trust
- REALalso: real number, floating-point value
- A number type that can keep fractional measurements. It has limited precision, so some calculations are approximate.
- On site A tank reading of 41.5 percent needs a type that keeps the half percent.
- Siemens REAL in the block interface.Rockwell REAL in controller/program tags.
- Taught in: Know what a tag represents
- Resetalso: reset request
- A request to return a value or piece of logic to a chosen starting condition. What it clears depends on the programme.
- On site An operator clears the bottle total before counting the next group.
- Siemens Counter R input; chart reset and output clearing follow the chart's defined policy.Rockwell Ladder RES or the ST counter Reset member; chart output clearing is a separate programme policy.
- Taught in: Count events, not scans
- Retentive timeralso: retained timer
- A timer that keeps elapsed time when its request stops. A separate reset clears the stored time.
- On site A maintenance total can add several separate periods of machine use.
- Siemens Retentive timing is a reading topic here, rather than a supported practical timer exercise.Rockwell RTO/RTOR are retentive; TONR in these ST lessons is non-retentive despite its name.
- Taught in: Make time explicit
- Rising edgealso: positive edge, FALSE-to-TRUE
- The moment a signal changes from FALSE to TRUE. A signal held TRUE is not another rising edge.
- On site A bottle arriving beneath the sensor creates one rising edge.
- Siemens R_TRIG in SCL; P contact or P_TRIG in LAD.Rockwell ONS/OSR in Ladder; OSRI or explicit previous-input logic in ST.
- Taught in: Count events, not scans
- Rungalso: network
- One row of ladder logic containing conditions and an action. Branches can provide alternative paths through that row.
- On site One row checks the pump's permission before writing its run command.
- Siemens LAD groups are called networks.Rockwell Ladder rows are called rungs.
- Taught in: Read a rung
- Salso: S action, stored action
- A step action that keeps a command on after its step ends. A separate reset action must clear that command.
- On site A recirculation pump stays commanded on through a later waiting step.
- Siemens S action qualifier in GRAPH.Rockwell S action qualifier in SFC.
- Taught in: Make every command’s lifetime clear
- SCADAalso: Supervisory Control and Data Acquisition
- A site-wide system for watching equipment, readings and alarms. It can record history and let operators make control requests.
- On site A water-treatment control room watches several pump stations and their alarms on one system.
- Siemens WinCC systems can provide supervisory monitoring.Rockwell FactoryTalk View SE systems can provide supervisory monitoring.
- Scalingalso: scale conversion
- Turning a sensor's raw number into a useful measurement. The conversion must match the sensor's chosen range.
- On site A tank display converts input counts into a level from zero to 100 percent.
- Siemens NORM_X then SCALE_X in these exercises.Rockwell SCL with a SCALE backing tag in these exercises; this SCL instruction is not Siemens' programming language.
- Taught in: Follow the signal
- Scanalso: scan cycle
- One pass through reading inputs, running instructions and updating outputs. This academy normally repeats it every 20 milliseconds.
- On site While one bottle covers a sensor, the controller can read it many times.
- Siemens A cyclic organisation block, such as OB1, repeatedly runs the programme.Rockwell A task schedules routines; these lessons use a periodic task with buffered inputs.
- Taught in: Think in scans
- SCLalso: Structured Control Language, Siemens SCL
- Siemens' text language for writing controller instructions. Local names can start with #, and := writes a value.
- On site An engineer writes #PumpOn := #Start to make a pump follow a request.
- Siemens SCL routines in TIA Portal.Rockwell The comparable text language is Structured Text; Logix's SCL instruction instead performs analogue scaling.
- Taught in: Think in scans
- Seal-inalso: seal in, holding circuit, self-hold
- A way to remember a brief Start request. The pump stays commanded on until Stop or another stopping condition breaks that memory.
- On site A caretaker presses Start once and the water pump keeps running after the button is released.
- Siemens LAD parallel holding contact or SCL logic using the previous output.Rockwell Ladder parallel holding branch or ST logic using the previous output.
- Taught in: Read a rung
- Sensoralso: sensing device
- A device that detects or measures something. Its reading tells the controller about the real equipment.
- On site A light sensor detects a bottle beneath the filling nozzle.
- Taught in: Know what a tag represents
- Sequencealso: sequencing
- A programme that carries out a job in ordered stages. Each stage has commands and a rule for moving on.
- On site A mixing tank fills, mixes its contents and then drains the finished batch.
- Siemens GRAPH is Siemens' step-and-transition language.Rockwell SFC is Logix's step-and-transition language.
- Taught in: Describe a process in steps
- Sequential function chartalso: SFC
- A diagram that shows steps of a job and the conditions for changing steps. Actions say what each step does.
- On site A filling sequence waits for a bottle, fills it and then moves it onward.
- Siemens Siemens GRAPH provides a related sequence language, with its own rules.Rockwell SFC routines in Studio 5000 Logix Designer.
- Taught in: Describe a process in steps
- Setpointalso: target value
- The value you want a process to reach or stay near. It is different from the value currently measured.
- On site A wash-water heater aims for a chosen temperature rather than heating forever.
- Taught in: Build permission into the logic
- Solenoid valvealso: solenoid-operated valve
- A valve operated using an electrical coil. The coil can move the valve itself or control air that moves a larger valve.
- On site A small electrically operated valve supplies air to the ingredient valve's actuator.
- Speed referencealso: speed setpoint, speed command, frequency command
- The speed the PLC asks a drive to run at. The drive ramps towards it and reports when it gets there.
- On site A pump controller sends its drive 60 percent to slow the flow.
- Siemens On a SINAMICS drive, 16384 means 100 percent of the reference speed.Rockwell PowerFlex add-on profiles call it FreqCommand; check its units in the drive manual.
- Taught in: Ask a drive, then check
- Standbyalso: standby pump, spare pump
- Equipment kept ready to take over when the usual unit cannot run. It still needs permission and a usable supply.
- On site The spare water pump takes over after the usual pump fails.
- Taught in: One request, one eligible pump
- State machinealso: state-machine
- A programme that remembers which part of a job it is in. It defines commands and permitted changes for each part.
- On site A batch can be Running, Held or Complete, with different commands in each state.
- Siemens Often written with CASE and a stored state value in SCL.Rockwell Often written with CASE and a state tag in ST.
- Taught in: Write one phase as a state machine
- Stepalso: sequence step
- One part of a sequence that is currently active. It says what to do while waiting for the next change.
- On site The Fill step runs the inlet pump and valve until the tank reaches its target level.
- Siemens A GRAPH step with assigned actions.Rockwell An SFC step with assigned actions.
- Taught in: Describe a process in steps
- Structured Textalso: ST, Rockwell ST
- A text language for writing controller instructions as statements. IF chooses conditions, and := writes a value.
- On site An engineer writes a temperature decision as a short text routine.
- Siemens Siemens uses SCL as its text programming language.Rockwell Structured Text routines in Studio 5000 Logix Designer.
- Taught in: Think in scans
- Tagalso: variable
- A name for a value in the programme. Start and PumpOn can name a button request and a pump command.
- On site A tank level reading is stored under a name the engineer can watch.
- Siemens Local SCL references use #; global data can use named data-block members.Rockwell Tags can be controller-scoped or program-scoped; ST references do not use #.
- Taught in: Know what a tag represents
- Timeralso: timing instruction
- An instruction that measures time against a chosen wait. Its stored values let it follow that wait across many scans.
- On site A valve alarm waits briefly so a healthy valve has time to open.
- Siemens A declared IEC timer instance, such as TON_TIME.Rockwell TIMER in classic Ladder; FBD_TIMER for the supported ST/FBD timing instructions.
- Taught in: Make time explicit
- Transitionalso: sequence transition
- A condition that allows a sequence to leave one step and enter another. It must become true before the change happens.
- On site Reaching the filling target lets the batch move from Fill to Mix.
- Siemens Transition condition between GRAPH steps.Rockwell Transition condition between SFC steps.
- Taught in: Describe a process in steps
- Tripalso: protective trip
- A stop caused by a protective device or detected problem. Removing the original request is not necessarily the same as clearing it.
- On site A motor's overload protection trips when it draws too much current.
- Taught in: Trips, resets and restarts
- TRUEalso: true value
- A value meaning yes, true or active; FALSE means no or inactive. TRUE on Stop means a stop was requested.
- On site The inlet valve receives TRUE when the programme asks it to open.
- Taught in: Know what a tag represents
- UDTalso: user-defined data type, structure
- A named group of different values that belong together. Declaring the group stores information; it does not run equipment by itself.
- On site One pump status group holds its running state, start total and measured level.
- Siemens A named PLC data type with STRUCT members.Rockwell A User-Defined Data Type used as a tag's type.
- Taught in: Data is not executable logic
- VFDalso: Variable Frequency Drive, variable-speed drive, VSD, inverter drive
- An electronic drive that changes a motor's speed. The controller can request a speed rather than only ON or OFF.
- On site A water pump slows down when the building needs less water.
- Siemens A drive exchanges commands and status with the PLC; SINAMICS is a Siemens drive family.Rockwell A drive exchanges commands and status with the controller; PowerFlex is a Rockwell drive family.
- Taught in: Ask a drive, then check
- Wire breakalso: broken wire, open circuit
- A cut or loose cable that stops a sensor's signal reaching the PLC. A 4–20 mA input then reads close to 0 mA.
- On site A damaged transmitter cable makes the level reading drop far below empty.
- Siemens S7-1500 analogue cards can report it as a diagnostic, with the value 32767.Rockwell Logix analogue modules flag it with channel fault status bits.
- Taught in: When the signal dies