Mission 57 · PRO

Hold a job, then resume it: state machine PLC exercise

Add a Held state to the mixing job, and decide which request wins.

Add Hold, Resume and Stop to the mixing job. State is still one whole number (a ) in one CASE statement, with two new states: 30 Held and 90 Stopped. Hold pauses the job with every OFF. Resume carries on, but only once Hold is released. Stop wins over everything, and then only Reset lets the job start again. Change State at most once per .

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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 process tank this mission runs on

Level: Advanced
Time: About 18 min
Reward: +400 XP
Rig: 3D process tank
Siemens editors: SCL
Rockwell editors: Structured Text
Beckhoff editors: Structured Text
Stage: 16. Write phases in text

Required behaviour

Hold a job, then resume it: what your code must do

This is the second of three missions on the same mixing job. The operating rules below list each state and what every output must be. When two requests arrive in the same scan, the order of your IF and ELSIF lines decides which one wins: Stop first, then Hold, then finishing.

0 · Idle: All outputs OFF. Start → 10, but only if Stop and Hold are released. Other inputs do nothing.
10 · Running: PumpOn, ValveCmd and MixerOn ON. Done OFF. Stop → 90. If not, Hold → 30. If not, MixDone → 50.
30 · Held: All outputs OFF. Stop → 90. If not, Resume AND NOT Hold → 10. Start, MixDone and Reset cannot restart it.
50 · Complete / 90 · Stopped: Pump, valve and mixer OFF. Done is ON only in 50. Reset → 0, but only with Stop released. It must not also start in that same scan.
Scan and reset rules: State starts at 0. Change State at most once per scan. On the scan where State changes, the outputs may match the old state or the new one, as a full set. From the next scan, they must match the current state. Reset does nothing in Running or Held.

Tag table

Hold a job, then resume it: inputs and outputs

Your program reads 6 inputs and sets 4 outputs. 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 pushbutton
`Stop`	`BOOL`	ReadsAn input your code needs	Stop pushbutton
`Reset`	`BOOL`	ReadsAn input your code needs	Reset pushbutton
`MixDone`	`BOOL`	ReadsAn input your code needs	Mixing finished
`Hold`	`BOOL`	ReadsAn input your code needs	Hold pushbutton
`Resume`	`BOOL`	ReadsAn input your code needs	Resume pushbutton
`PumpOn`	`BOOL`	WritesAn output your code sets	Pump command
`ValveCmd`	`BOOL`	WritesAn output your code sets	Inlet valve command
`MixerOn`	`BOOL`	WritesAn output your code sets	Mixer command
`Done`	`BOOL`	WritesAn output your code sets	Job complete

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

Hold a job, then resume it: what the tests check

Every state sets all four outputs: pump, valve, mixer and Done.
Held waits for Resume, and only once Hold is released.
Stop works from Running and from Held, and beats Hold, Resume, MixDone and Reset. Reset only works from Complete or Stopped.

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

An operator pauses a mixing batch to check the tank, then carries on. If something is wrong, Stop ends the batch.

A job that restarts when Hold is let go can catch the operator out. Stop must win, even when another button is pressed in the same moment.

Where you'd find thisMaking one batch of drink

At a drinks factory, valves let measured amounts of water and syrup into a mixing tank. An electric mixer blends them into one batch. An outlet valve then sends the drink to a holding tank. Filling machines bottle it from there.

Ingredient tanksHold the water and syrup
Mixing tankFill, mix and measure one batchYou are here
Holding tankKeeps the finished drink ready
Bottle fillerPuts the drink into bottles

On a real site: places, parts and safety

Drinks factory — blending water and syrup
Paint factory — mixing ingredients into a consistent colour
Water-treatment works — preparing a measured chemical mixture
Brewery — heating water for washing equipment

A full tank does not close its own inlet. Water keeps arriving until the valve closes, so an open inlet can flood the room.

A heater that is not covered by liquid can overheat. Real tanks have separate safety devices as well as the PLC program.

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

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