Basic instructions

Contacts and coils

They are the basis of every Ladder program:

Symbol	Name	What it does
┤ ├	NO contact (open)	Passes power when the bit is 1
┤/├	NC contact (closed)	Passes power when the bit is 0
( )	Coil	Writes the rung result to the bit (1 if power reaches it)
(/)	Negated coil	Writes the opposite of the rung result

A contact is not a push button: it is a question about a bit’s value. An NC contact of I0.0 asks “is I0.0 equal to 0?”, no matter whether the field device is an NO or NC button.

Set and reset

• Set (S): when power reaches it, it sets the bit to 1. When it does not, it does nothing: the bit keeps its value.
• Reset (R): when power reaches it, it sets the bit to 0.

Together they form a memory: an alarm that stays active until someone acknowledges it, or a machine that remembers which stage it is in.

Edges

A rising edge (P) passes power for only one scan cycle, right when the signal goes from 0 to 1. A falling edge (N) passes power for one cycle when it goes from 1 to 0.

They are used so that an action happens only once per press, even if the button is held down: adding 1 to a counter, changing mode, logging an event.

Seal-in and interlocks

A seal-in (or latch) keeps an output on after the button that turned it on is released: a contact of the output itself is placed in parallel with the start button.

  ┤ START ├──┬──┤ STOP ├──( MOTOR )
  ┤ MOTOR ├──┘

An interlock is different: it prevents two incompatible actions from happening at the same time. For example, on a reversing motor, the forward contactor can only turn on if the reverse contactor is off, and vice versa:

  ┤ FORWARD ├──┤/ REVERSE_K ├──( FORWARD_K )
  ┤ REVERSE ├──┤/ FORWARD_K ├──( REVERSE_K )

Timers

All IEC 61131-3 timers have an input IN, a preset time PT, an output Q and the elapsed time ET:

Type	Name	Behaviour of Q
TON	On-delay	Turns on when IN has been 1 for PT. Turns off as soon as IN drops.
TOF	Off-delay	Turns on with IN; when IN drops, stays at 1 for PT.
TP	Pulse	When IN rises, gives a pulse of exactly PT.

Typical examples:

• TON: confirm a sensor has been active for 2 s before reacting (filtering bounce), or wait for a pump to start.
• TOF: keep a fan running for a while after an oven is switched off.
• TP: open a solenoid valve for a fixed time each time a command arrives.

Counters

Type	Name	Behaviour
CTU	Up counter	Adds 1 on each rising edge of CU. Q = 1 when CV ≥ PV.
CTD	Down counter	Subtracts 1 on each edge of CD. Q = 1 when CV ≤ 0. LD loads PV.
CTUD	Up/down	Adds with CU, subtracts with CD. QU = CV ≥ PV and QD = CV ≤ 0.

The R (reset) input sets the count to 0. Counters count edges, not how long the signal stays on: a box that stops in front of the sensor counts only once.

Comparisons

They compare two numbers and pass power if the comparison is true: equal (==), not equal (<>), greater (>), greater or equal (>=), less (<) and less or equal (<=).

They are used for level or temperature alarms, to act at certain moments of a timer (as in the traffic light example) or to compare a counter with a variable value.

Maths and MOVE

• ADD, SUB, MUL, DIV: addition, subtraction, multiplication and division.
• MOVE: copies a value to another address. For example, loading a recipe.
• Scaling: converts the number an analog module delivers (for example, 0 to 27,648) into engineering units (0 to 100 %, or 0 to 10 bar).

Practise

The traffic light example uses a timer and comparisons on its elapsed time. And in the challenges section you can test set/reset, timers and counters with automatic grading.

Try it in the simulatorOpen a preloaded example and experiment yourself.

Summary

• Contacts ask about a bit; coils write it.
• Set/reset form memories; the last instruction executed wins.
• Edges last a single scan cycle.
• A seal-in keeps an output on; an interlock prevents incompatible actions and is reinforced in the wiring.
• TON, TOF and TP handle time; CTU, CTD and CTUD count edges.
• Comparisons, maths and MOVE work with numbers.