Mission 87 · PRO

Log what explains a stop: PLC SCADA training exercise

Store a value when it changes enough, never go silent, then read the trend to find what failed.

Give the factory's history what it needs to explain a stop. Write the store for two : the air pressure, kept when it moves 0.2 bar or after 10 s, and the line's state, kept on every change or after 10 s. Then choose what to log and how, and read a recorded stop to find its cause.

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Every Coaching lesson, from zero
Every mission and lesson, as you level up
A practical test for every title, with a certificate
Rewind any test, one scan at a time
Worked solutions when you're stuck
The code converter, as much as you like

The 3D model factory this mission runs on

Level: Advanced
Time: About 45 min
Reward: +500 XP
Rig: 3D model factory
Siemens editors: Ladder (LAD), SCL
Rockwell editors: Ladder, Structured Text
Beckhoff editors: Structured Text
Stage: 45. SCADA 3: log what explains a stop

Required behaviour

Log what explains a stop: what your code must do

The plant control is written and working. A historian can't keep every scan of every tag, so each tag is stored on change, with a slowest rate so a flat line is known to be alive. Write that store in the PLC, as a remote station does, so only what matters is sent. Air pressure: add 0.02 to AirAge every scan. When AirPressure is 0.2 bar or more above or below AirStored, or AirAge reaches 10.0 s, store it: copy AirPressure to AirStored, add 1 to AirSamples and set AirAge to 0. The line: add 0.02 to LineAge every scan. When LineRun differs from LineStored, or LineAge reaches 10.0 s, store it: copy LineRun to LineStored, add 1 to LineSamples and set LineAge to 0. The Operator screen tab draws what your PLC stores over the real signal. There, choose which tags the history keeps, how often and with what deadband. Then read the stop recorded last night and say what failed.

Air: Store on a 0.2 bar change either way, or after 10 s.
Line: Store on any change of LineRun, or after 10 s.
A store: Copy the value, add 1 to the count, set the age to 0.

Tag table

Log what explains a stop: inputs and outputs

Your program reads 17 inputs and sets 18 outputs. 2 more inputs drive the simulation or show readings, for you to test with. The names are the same in every maker’s editor.

Tag	Data type	Your code	What it means on the machine
`Start`	`BOOL`	ReadsAn input your code needs	Start the factory
`Stop`	`BOOL`	ReadsAn input your code needs	Stop the factory where it is
`Reset`	`BOOL`	ReadsAn input your code needs	Reset after an emergency stop or a light-curtain trip
`EStop`	`BOOL`	ReadsAn input your code needs	Emergency stop pressed
`AirPressure`	`REAL`	ReadsAn input your code needs	Air receiver pressure, bar
`SteamPressure`	`REAL`	ReadsAn input your code needs	Boiler steam pressure, bar
`VesselLevel`	`REAL`	ReadsAn input your code needs	Mixing vessel level, %
`VesselTemp`	`REAL`	ReadsAn input your code needs	Batch temperature, °C
`SugarAdded`	`REAL`	ReadsAn input your code needs	Sugar the weigh feeder has added to this batch, kg
`BufferLevel`	`REAL`	ReadsAn input your code needs	Filler buffer tank level, %
`BadBottle`	`BOOL`	ReadsAn input your code needs	Camera: a bottle with no label is passing
`GateEye`	`BOOL`	ReadsAn input your code needs	A bottle is in front of the reject gate
`AccumFull`	`BOOL`	ReadsAn input your code needs	Three packs are waiting for the robot
`CurtainClear`	`BOOL`	ReadsAn input your code needs	Light curtain beams unbroken
`PalletFull`	`BOOL`	ReadsAn input your code needs	A full pallet is waiting or on its way to the wrapper
`PalletOnWrapper`	`BOOL`	ReadsAn input your code needs	A pallet is on the wrapper turntable
`WrapDone`	`BOOL`	ReadsAn input your code needs	The pallet on the wrapper is wrapped
`PersonInCell`	`BOOL`	Not neededFor testing or watching; your code can leave it out	Try it: someone walks into the robot cell
`CompressorFault`	`BOOL`	Not neededFor testing or watching; your code can leave it out	Try it: the compressor trips
`Compressor`	`BOOL`	WritesAn output your code sets	Run the air compressor
`BoilerOn`	`BOOL`	WritesAn output your code sets	Switch the boiler on
`WaterValve`	`BOOL`	WritesAn output your code sets	Water into the mixing vessel
`Feeder`	`BOOL`	WritesAn output your code sets	Run the sugar weigh feeder
`Agitator`	`BOOL`	WritesAn output your code sets	Run the agitator
`SteamValve`	`BOOL`	WritesAn output your code sets	Steam to the vessel jacket
`TransferPump`	`BOOL`	WritesAn output your code sets	Pump the batch to the buffer tank
`LineRun`	`BOOL`	WritesAn output your code sets	Run the bottling line
`RejectGate`	`BOOL`	WritesAn output your code sets	Fire the reject gate
`RobotEnable`	`BOOL`	WritesAn output your code sets	Let the robot palletise
`PalletOut`	`BOOL`	WritesAn output your code sets	Move the full pallet to the wrapper
`Wrap`	`BOOL`	WritesAn output your code sets	Run the stretch wrapper
`AGVCall`	`BOOL`	WritesAn output your code sets	Call the AGV to collect the wrapped pallet
`GreenLamp`	`BOOL`	WritesAn output your code sets	Stack light: producing
`AmberLamp`	`BOOL`	WritesAn output your code sets	Stack light: starting or waiting
`RedLamp`	`BOOL`	WritesAn output your code sets	Stack light: emergency stop or light-curtain trip
`AirStored`	`REAL`	WritesAn output your code sets	History: the air pressure last stored, bar
`LineStored`	`BOOL`	WritesAn output your code sets	History: the line state last stored

In Siemens SCL, write a block’s own tags with # in front, as TIA Portal does: #Start. Rockwell and TwinCAT use the name on its own: Start.

Automatic tests

Log what explains a stop: what the tests check

AirStored follows AirPressure to within 0.2 bar.
A steady pressure is still stored every 10 s, so a flat line is known to be live.
LineStored takes every change of LineRun on the it happens, and a 10 s heartbeat.
AirSamples and LineSamples count one for each store: tens a minute, not thousands.
The history keeps the measurement, the command and the state that explain a stop, on change, with a just above the 's noise.
The recorded stop is read correctly: the compressor was called and the pressure kept falling.

Try it Use the buttons below the model and watch the rig respond.
Test solution Every check runs on your code in one press.
Rewind PRO (needs Pro) If one fails, step back through it one scan at a time to see why.

Stuck on this mission? Ask the members in the Locker Room.

Why it matters

Where you’d see this on site

A plant historian keeps years of readings from thousands of tags. After every unexplained stop, the first thing an engineer opens is the trend of the minutes before it.

A trip that happens at night with nobody watching leaves only what was logged. If the command and the pressure were not both kept, at a rate fast enough to see which moved first, the cause is a guess.

Where you'd find thisOne controller for a whole factory: when each machine may run, and what keeps it safe

A small drinks factory makes an orange drink. Sugar and water are batched and pasteurised, then pumped to a filler. Bottles are filled, capped, labelled and inspected, packed in sixes, stacked on pallets by a robot, wrapped and carried to the dock by a driverless forklift. Every machine has its own controls; the plant controller decides when each may run, hands work from one to the next, and stops everything safely.

BatchMix and pasteurise
FillFill, cap, label
InspectReject bad bottles
PackRobot stacks packsYou are here
ShipWrap, AGV to dock

On a real site: places, parts and safety

Soft drinks and juice bottling halls
Sauces, dairy and liquid food plants
Household and personal-care liquids

An emergency stop is wired through a safety relay that cuts the power on its own. The program must then hold everything off and wait for a deliberate Reset and Start, so nothing moves by surprise.

Robot cells are fenced. Light curtains and interlocked doors stop the robot when someone goes in, and a restart needs a reset from outside the cell, in the spirit of ISO 10218 and ISO 13849.

The sizes, timings and cutaways are teaching choices, not a drawing of any one real installation.

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