Worked Examples

How to read these examples

Each example gives the machine, the I/O list, the logic (in ladder or IEC Structured Text), and the numbers worked out. The code is written in vendor-neutral IEC style; to use it, create the tags in your software, map the I/O, and translate the instructions (see the platform tables in How PLCs Work and Ladder Logic in Practice). All of them assume that stops are wired normally closed and that emergency stops are hardwired outside the PLC.

Example 1: a conveyor sorter

Boxes arrive on a belt. Tall boxes must be pushed off onto a side chute by a diverter cylinder 600 mm after the sensors; short boxes continue. The belt moves at 0.3 m/s.

Tag	Address (example)	Type	Meaning
Start_PB	I0.0	BOOL	Start button (normally open)
Stop_OK	I0.1	BOOL	Stop button, wired normally closed
EStop_OK	I0.2	BOOL	Emergency-stop status (hardwired circuit also cuts power)
PE1	I0.3	BOOL	Photoeye across the belt: any box
PE2	I0.4	BOOL	High photoeye: on only for tall boxes
Divert_Home / Divert_Out	I0.5 / I0.6	BOOL	Cylinder end sensors
Belt_Run	Q0.0	BOOL	Belt contactor
Divert	Q0.1	BOOL	Diverter valve
Track	(memory)	DWORD	Shift register: one bit per 100 ms tick of belt travel

(* Called every scan *)
(* 1. a 100 ms tick *)
Tick_T(IN := Belt_Run AND NOT Tick_T.Q, PT := T#100ms);
Tick := Tick_T.Q;                                   (* TRUE for one scan every 100 ms *)

(* 2. shift the tracking register once per tick (only while the belt runs) *)
IF Tick THEN
    Track := SHL(Track, 1);                          (* every box moves one stage *)
END_IF;

(* 3. record a new tall box at stage 0 *)
IF New_Tall THEN
    Track := Track OR 16#00000001;
END_IF;

(* 4. stage 20 is the diverter position: 20 ticks = 2.0 s after detection *)
At_Diverter := (Track AND 16#00100000) <> 0;
IF At_Diverter THEN
    Track := Track AND NOT 16#00100000;              (* consume it *)
END_IF;
Divert_Pulse(IN := At_Diverter, PT := T#500ms);      (* hold the valve 0.5 s *)
Divert := Divert_Pulse.Q;

(* 5. clear the register when the belt stops, so that old boxes are forgotten *)
IF NOT Belt_Run THEN Track := 0; END_IF;

• Make the tracking depend on the belt, not on time. If the belt slows or stops, boxes must not move on in the register; a belt encoder (a pulse every few mm) is better than a timer for accuracy.
• Verify the fire. Use the Divert_Out sensor: if the cylinder does not extend within 1 s, raise a fault.
• Test with the diverter disconnected first, then with a few boxes, and tune the delay by a few ticks until the pusher hits the box.

Example 2: tank level control

A tank is filled by a pump from a supply and emptied by the process. The pump must start when the level is low and stop when it is high, with alarms, protection against running dry, and two pumps that share the work.

Tag	Address (example)	Type	Meaning
LT101_Raw	IW64	INT	Level transmitter, 0–27 648 for 4–20 mA
Level_Pct	(memory)	REAL	Scaled level, 0–100 % of 3 m
LSLL_OK	I0.0	BOOL	Low-low float switch (true when the level is above it)
LSHH	I0.1	BOOL	High-high float switch (true when the level is at it)
Auto / Reset_PB	I0.2 / I0.3	BOOL	Auto mode switch, alarm reset
Pump1, Pump2	Q0.0, Q0.1	BOOL	Pump contactors (through interposing relays)
Lead	(memory)	INT	Which pump leads: 1 or 2
Alarm_High, Alarm_Low, Fault	(memory)	BOOL	Latched alarms

(* 1. scale the level: 4-20 mA = 0-27648 counts = 0-100 % *)
Level_Pct := INT_TO_REAL(LT101_Raw) / 27648.0 * 100.0;
Sensor_Fault := (LT101_Raw < -1382) OR (LT101_Raw > 29030);     (* wire break or overrange *)

(* 2. hysteresis: start below 40 %, stop above 80 % *)
IF Auto AND NOT Fault AND NOT Sensor_Fault THEN
    IF Level_Pct < 40.0 THEN Call_Pump := TRUE; END_IF;
    IF Level_Pct > 80.0 THEN Call_Pump := FALSE; END_IF;
ELSE
    Call_Pump := FALSE;
END_IF;

(* 3. alternate the lead pump every time the call drops out *)
Call_R(CLK := Call_Pump);
Call_F(CLK := Call_Pump);
IF Call_F.Q THEN Lead := 3 - Lead; END_IF;                 (* 1 <-> 2 *)

(* 4. protection: dry-run trip and high-high trip *)
IF NOT LSLL_OK THEN Fault := TRUE; END_IF;
IF LSHH         THEN Alarm_HH := TRUE; END_IF;
IF Reset_PB AND LSLL_OK AND NOT LSHH THEN Fault := FALSE; Alarm_HH := FALSE; END_IF;

(* 5. outputs *)
Pump1 := Call_Pump AND (Lead = 1) AND NOT Fault AND NOT LSHH;
Pump2 := Call_Pump AND (Lead = 2) AND NOT Fault AND NOT LSHH;
Alarm_Low  := Level_Pct < 30.0;
Alarm_High := Level_Pct > 85.0;

• Hysteresis keeps the pump from chattering around one level. The two-compare ladder version is on Timers, Counters and Data.
• Lag pump. Start the second pump if the level keeps rising above 90 % with the lead running, or if the lead pump trips.
• Run-time and starts. Add a run-hour counter for each pump and a start counter, for maintenance.
• The float switches protect against a failed transmitter: they trip independently and are wired normally closed.

Example 3: a batching sequence

A skid mixes a recipe: 100 L of water measured by a flow meter, 5 L of an ingredient, 120 s of mixing, then draining to the next stage. The flow meter gives 200 pulses per liter.

CASE State OF
  0:  (* idle *)
      XV1 := FALSE; XV2 := FALSE; M1 := FALSE; XV3 := FALSE;
      IF Start AND LSL_Empty AND NOT Fault THEN
          Pulses := 0; State := 10;                  (* clear the pulse count on entry *)
      END_IF;
  10: (* water: close early by the pre-act amount *)
      XV1 := TRUE;
      IF Pulses >= Water_Target - PreAct THEN XV1 := FALSE; Water_Done := TRUE; State := 20; Pulses := 0; END_IF;
      NoFlow(IN := XV1 AND (Pulses = 0), PT := T#5s);  IF NoFlow.Q THEN State := 90; END_IF;
  20: (* ingredient *)
      XV2 := TRUE;
      IF Pulses >= Ing_Target - PreAct2 THEN XV2 := FALSE; State := 30; END_IF;
  30: (* mix *)
      M1 := TRUE;  MixT(IN := TRUE, PT := T#120s);
      IF MixT.Q THEN M1 := FALSE; MixT(IN := FALSE); State := 40; END_IF;
  40: (* discharge until the tank is empty *)
      XV3 := TRUE;
      IF LSL_Empty THEN XV3 := FALSE; State := 0; Batches := Batches + 1; END_IF;
  90: (* fault: everything off *)
      XV1 := FALSE; XV2 := FALSE; M1 := FALSE; XV3 := FALSE;
      IF Reset AND NOT Fault_Cause THEN State := 0; END_IF;
END_CASE;

• Read the pulse counter from a high-speed input (flow at 100 L/min is 333 pulses per second, beyond a normal input). Use a counter in a periodic task.
• Weight-based batching works the same way with a load cell and the scaled weight in place of the pulse count.
• Store recipes in retentive data (an array or a structure per recipe), and have the HMI select the recipe number; the program reads targets from it.
• Log each batch (the actual volumes, times and the operator) for traceability.

Example 4: a bottle-packing line

Bottles pass a fill check, a reject pusher and a counting eye into cases of 24. The program counts good and rejected bottles, signals a full case, removes bottles that fail the fill check, and shows the line state.

(* good and rejected bottle counts *)
Fill_Fail_R(CLK := Fill_Check_Fail);                 (* edge: this bottle failed *)
IF Fill_Fail_R.Q THEN Reject_Track := Reject_Track OR 1; END_IF;
IF Tick THEN Reject_Track := SHL(Reject_Track, 1); END_IF;
Reject_Now := (Reject_Track AND 16#00001000) <> 0;       (* stage 12 = 1.2 s *)
IF Reject_Now THEN Reject_Track := Reject_Track AND NOT 16#00001000; Reject_Count := Reject_Count + 1; END_IF;
Reject_Pusher(IN := Reject_Now, PT := T#300ms);       (* push for 0.3 s *)

Count_R(CLK := Count_Eye);                            (* bottle reached the case *)
IF Count_R.Q THEN Good_Count := Good_Count + 1; Total_Good := Total_Good + 1; END_IF;
Case_Full := Good_Count >= 24;
IF Case_Full AND Case_Gone THEN Good_Count := 0; Cases := Cases + 1; END_IF;

(* reject rate shown on the HMI *)
IF (Total_Good + Reject_Count) > 0 THEN
    Reject_Pct := DINT_TO_REAL(Reject_Count) / DINT_TO_REAL(Total_Good + Reject_Count) * 100.0;
END_IF;

(* line state *)
Starved(IN := Line_Run AND NOT Bottle_Seen, PT := T#5s);       (* no bottles for 5 s *)
Blocked(IN := Case_Full AND NOT Case_Gone, PT := T#10s);       (* no case for 10 s *)
IF Blocked.Q THEN Line_State := 3; ELSIF Starved.Q THEN Line_State := 2; ELSIF Line_Run THEN Line_State := 1; ELSE Line_State := 0; END_IF;

• Totals in DINT, shift counts retentive or logged to a database, so that a power cut does not lose the day.
• Reject rate and states are cheap to add and very useful: they tell you whether the line is starved (upstream problem), blocked (downstream problem) or rejecting too many.
• Verify the pusher: a sensor on the reject bin confirms that the bottle left; a bad bottle that continues is a customer complaint.
• Counters need edges and filters. A bottle that wobbles in the beam gives two counts; filter the eye (a 10–20 ms on-delay) and check against a manual count.

Adapting an example to your platform

Step	What to do
1. Tags	Create tags with the same names and the right types (BOOL, DINT, REAL, TIME) in your software
2. I/O	Map each input and output to your hardware addresses, and check sinking or sourcing
3. Instructions	Replace the IEC names with your platform’s: timers, edge instructions, SHL/shift, MOD, INT_TO_REAL
4. Units	Check timer units (ms, 10 ms or 100 ms) and the analog count range of your card
5. Tasks	Run counters, tick timers and PID in a periodic task if your platform has one
6. Simulate	Test in a simulator, force the inputs, and walk through every state and fault before the real machine
7. Commission	Test with the load disconnected, then with it; back up the working program

Related

• PLC & Logic Control: the overview of this guide.
• Sequences and State Machines: the state-machine method behind the batch example.
• Timers, Counters and Data: scaling, counters and data types used here.
• PLC Troubleshooting, Safety and Documentation: commissioning and safety.
• Motors, Drives and PID Control: PID and drives for a variable-speed version of these machines.

FAQ

Can I use these programs on my PLC?

Use them as a pattern. The logic is vendor-neutral, but you must create the tags, map the I/O, and translate the instruction names and units for your platform. Test everything in a simulator and on the machine with the load disconnected before running it for real.

Why a shift register instead of a timer for tracking boxes?

A timer handles one item at a time. When boxes are closer together than the delay, several are in transit at once, and a shift register keeps one bit per position for all of them.

How do I handle the belt speed changing?

Drive the tracking from a belt encoder (shift once per fixed distance) instead of a time tick. Then the register follows the belt, whatever its speed.

Why close the valve early in batching?

The valve takes time to close and the liquid keeps moving, so closing at the target overshoots. Closing slightly early (the pre-act) lands on the target; adjust it from the measured error of several batches.

Do these examples include safety?

They include sensible interlocks and trips, but not a safety system. Emergency stops, guarding and any hazard need a risk assessment and a proper safety circuit; see PLC Troubleshooting, Safety and Documentation.

References

• IEC 61131-3 (data types, standard function blocks TON, TP, R_TRIG, F_TRIG, SHL).
• ISA-88 / IEC 61512 (batch control), ISA-TR88.00.02 (PackML).
• Flow meter and level transmitter datasheets for K-factors and ranges; valve closing times from the valve maker.
• Petruzella, F., Programmable Logic Controllers, McGraw-Hill; Bolton, W., Programmable Logic Controllers, Newnes.

Last reviewed: October 2026. Instruction names, addresses and data sizes differ by manufacturer: confirm against your controller’s manual, and keep safety functions independent of standard PLC logic.