PLC-Course

Sprint 5 Cheat Sheet — Timers, Counters, Registers, Data

⏱️ Timer Family

Timer	Behavior	When done bit (Q) goes high
TON (on-delay)	Accumulator counts up while EN is true	When ACC ≥ PT (preset time)
TOF (off-delay)	Accumulator counts up while EN is false (after EN goes true→false)	While timing — drops when ACC ≥ PT
TP (pulse)	Fixed-duration pulse triggered by EN rising edge	For exactly PT seconds, regardless of EN
Retentive (RTO)	Holds ACC across EN drops, only resets on RES	When ACC ≥ PT (across multiple enables)

Timing diagram (TON)

EN    ────┐         ┌────────
          │         │
          └─────────┘
ACC   ────/┐         /┐──────
          /│         /│
PT  - - -/-│- - - -/-│- - -
        /  │       /  │
Q     ──────┐       ┌────────
            │       │
            └───────┘
              (high once ACC reaches PT)

🔢 Counter Family

Counter	Increment trigger	Reset
CTU	Rising edge of CU input	When RES is true
CTD	Rising edge of CD input	When LD (load preset) is true
CTUD	Up on CU↑, down on CD↑	RES or LD

Done bit (Q): high when ACC ≥ PV.

🚂 Shift Register (SHR / SHL)

Tracks bits moving through a sequence — perfect for parts on a conveyor.

Bit slot:    7  6  5  4  3  2  1  0
Initial:     0  0  0  0  0  0  0  0
After SHL:   0  0  0  0  0  0  0  ←IN
After SHL:   0  0  0  0  0  0  IN ←IN
...

Each shift, every bit moves one position; the new bit-in enters at one end, the old bit-out drops off the other.

Use case: photoelectric sensor at start of conveyor sets bit 0 when a part passes; each conveyor index pulse shifts the register; reject solenoid fires when the right slot reaches the inspection station.

🧠 Internal Relays (Memory Bits)

• Software-only flags — no physical wires
• Used for intermediate logic, status flags, one-shots
• Vendor names: M (Siemens), B3:0/0 (Allen-Bradley SLC), M (Mitsubishi)
• Rule: if you find yourself writing the same long Boolean expression in three rungs, store it in an internal relay and reference it.

🔁 Jumps, Calls, Subroutines

Instruction	Effect
JMP label	Skip to a label later in the program
LBL label	Marks the jump destination
JSR routine	Call a subroutine
RET	Return from subroutine

Hazard: if a jump skips past a coil, that coil keeps its previous state. This is the classic “stuck output after fault” bug. Always put outputs outside of jumps unless you really know what you’re doing.

📦 Data Handling Instructions

Instruction	Purpose
MOV src, dst	Copy value
ADD a, b, dst	Addition
SUB a, b, dst	Subtraction
MUL, DIV, MOD	Math
EQ, NE, GT, LT, GE, LE	Comparisons
LIM lo, val, hi	True if lo ≤ val ≤ hi
BCD_TO_INT, INT_TO_BCD	Conversions
SCL	Linear scaling (raw counts → engineering units)

Common scaling formula

For a 4–20 mA signal mapped to 0–100% in a 16-bit raw word (0–32767 counts for 4–20 mA on Siemens):

percent = (raw - raw_min) * (eng_max - eng_min) / (raw_max - raw_min) + eng_min

In ST:

Percent := REAL_TO_INT((Raw - 0) * 100 / 32767);

🏗️ Common Patterns You’ll Reuse

One-shot rising edge (rising-edge detector):

Pulse := Input AND NOT Last_Input;
Last_Input := Input;

Toggle (on/off button):

IF Button AND NOT Last_Button THEN
    Output := NOT Output;
END_IF;
Last_Button := Button;

Watchdog timer:

Heartbeat_Timer(IN := NOT Heartbeat_Timer.Q, PT := T#1s);
IF Heartbeat_Timer.Q THEN
    Heartbeat := NOT Heartbeat;
END_IF;

🐛 Watch out for

1. Timer never enables because EN is always true on power-up — use a one-shot.
2. Counter accumulator overflows because RES is never asserted — schedule resets.
3. Shift register skips a slot because shift signal pulses faster than scan time — use a synchronized index pulse.
4. MOV inside a jump skipped — the destination keeps stale data.

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