Mission 48 · PRO

Write your first state machine: PLC exercise

Use a numbered state to start, finish and reset a mixing job.

Run a mixing job with a CASE statement. State is a whole number (a ) that says which state the job is in: 0 Idle, 10 Running or 50 Complete. Start moves Idle to Running. MixDone moves Running to Complete. Reset moves Complete back to Idle. Change State at most once per , one pass through your code. Every state sets all four .

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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: Applied
Time: About 12 min
Reward: +250 XP
Rig: 3D process tank
Siemens editors: SCL
Rockwell editors: Structured Text
Beckhoff editors: Structured Text
Stage: 16. Write phases in text

Required behaviour

Write your first state machine: what your code must do

This is the first of three missions on the same mixing job. MixDone is the mixer’s done signal. It comes on after four seconds of mixing. The operating rules below list each state and what every output must be. The first move is written for you. There is no Stop or Hold yet. The next two missions add them.

0 · Idle: All outputs OFF. Start → 10. Other inputs do nothing.
10 · Running: PumpOn, ValveCmd and MixerOn ON. Done OFF. MixDone → 50. Other inputs do nothing.
50 · Complete: Pump, valve and mixer OFF. Done ON, and it stays ON after MixDone goes OFF. Reset → 0. It must not also start in that same scan. Other inputs do nothing.
Scan 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.

Tag table

Write your first state machine: inputs and outputs

Your program reads 3 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
`Reset`	`BOOL`	ReadsAn input your code needs	Reset pushbutton
`MixDone`	`BOOL`	ReadsAn input your code needs	Mixing finished
`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

Write your first state machine: what the tests check

Every state sets all four outputs: pump, valve, mixer and Done.
Each state has one way out: Start from Idle, MixDone from Running and Reset from Complete.
State changes at most once per scan, even with Start, MixDone and Reset all on together.

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 factory mixer is waiting, running or finished. The operator starts each batch, and resets the panel when it is done.

Without one clear state, a finished batch can start again by itself. A Done lamp that copies a sensor goes out as soon as the mixer stops.

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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