Structured Text: Zero to Advanced

Write a PROGRAM that runs, and learn the read–execute–write scan that every line of Structured Text lives inside.

Declare BOOL, INT, DINT, REAL, TIME and STRING variables, give them initial values and choose the type that fits the signal.

Arithmetic, precedence, integer division, MOD and type conversion — and the rounding mistakes that hide in them.

AND, OR, NOT and XOR as machine permissives: seal-in circuits, fail-safe stops and interlocks you can read aloud.

Choose between paths with IF, order ELSIF branches by priority, and avoid the missing-ELSE trap that latches outputs.

Compare at the right boundary, add hysteresis and dead bands, and use LIMIT, MIN, MAX and SEL instead of long IF chains.

Select one behaviour from many with CASE: label lists, ranges, ELSE, and mode selectors that stay readable.

Detect rising and falling edges, set and reset latches with clear priority, and decide what the program must remember.

Use TON for delays, debouncing and confirmation times, and read its IN, PT, Q and ET pins scan by scan.

Keep things running after a signal drops with TOF, fire fixed pulses with TP, and build a flasher from two timers.

Count parts and events with CTU, CTD and CTUD, reset them deliberately, and count edges instead of scans.

Turn a machine sequence into named states and transitions with CASE, timers and clear entry conditions.

Store many readings in an ARRAY and process them with FOR: totals, averages, minimum and maximum, all inside one scan.

Loop until a condition with WHILE and REPEAT, leave early with EXIT, and never use a loop to wait for the machine.

Group related values into a STRUCT, name states with an ENUM, and build arrays of structures for real equipment.

Build messages with STRING functions, compute with TIME values, and keep text out of the control decisions.

Write FUNCTIONs that turn inputs into one result with no memory, and use them for scaling, limits and checks.

Write FUNCTION_BLOCKs with inputs, outputs and memory, create instances, and reuse one motor block for many motors.

Scale 4–20 mA raw counts to engineering units, filter noise, detect broken wires and raise delayed alarms.

Ramp setpoints, close a loop with proportional and PI control, and limit the output so the actuator survives.

Name, comment and organise ST so it reads like the machine, and carry it across CODESYS, TwinCAT, TIA and Studio 5000.

Detect faults, latch alarms until acknowledged, record the first-out cause and drive every output to a safe state.

Test ST with plant cases before the machine exists, watch values online, and debug from a hypothesis instead of guessing.

Build a complete filling-line controller in ST — modes, sequence, timers, counts, alarms and tests — then plan what to learn next.

Pack and unpack status words, test and set single bits, shift and rotate with SHL, SHR, ROL and ROR, and decode a drive status word.

Share data with VAR_GLOBAL and VAR_EXTERNAL, split a program across tasks with different cycle times, and keep shared data consistent.

Read POINTER TO, REF_TO and ADR in vendor code, know when indirection helps, and recognise the faults it causes when it goes wrong.

Work with DATE, TOD and DT, the real-time clock and time stamps, and meet WSTRING, LREAL, subranges, unions and aliases.

Design command and status words, heartbeats and handshakes so an HMI, SCADA system or another PLC can talk to your program safely.

Store recipes in arrays of structures, validate a download, apply it at a safe moment and fall back cleanly when values are wrong.

Power, home and move a servo axis with PLCopen MC blocks, handle Busy, Done and Error, and sequence moves from a state machine.

Track parts along a conveyor with shift registers and ring buffers, log events in a FIFO, and keep data matched to the product.