PLC-Course

Sprint 3 Cheat Sheet — Ladder & FBD

🪜 Ladder Symbols

 ─┤ ├─    Normally Open contact   (passes power when bit = 1)
 ─┤/├─    Normally Closed contact (passes power when bit = 0)
 ─( )─    Output coil             (energizes when rung is true)
 ─(/)─    Negated output coil
 ─(L)─    Latch coil              (sets and holds)
 ─(U)─    Unlatch coil            (resets)

🔣 The 7 Logic Gates as Ladder

Gate	Ladder	Boolean
AND	Two NO contacts in series	Y = A · B
OR	Two NO contacts in parallel	Y = A + B
NOT	One NC contact	Y = ¬A
NAND	Two NC contacts in parallel	Y = ¬(A · B)
NOR	Two NC contacts in series	Y = ¬(A + B)
XOR	(A NO + B NC) parallel with (A NC + B NO)	Y = A ⊕ B
XNOR	(A NO + B NO) parallel with (A NC + B NC)	Y = ¬(A ⊕ B)

🔒 Latching — Two Patterns

Seal-in latch (most common):

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

• START energizes MOTOR
• The MOTOR contact in parallel “seals” it on
• STOP (NC) breaks the rung

SR latch (set/reset coils):

   START
 ─┤ ├────────( L MOTOR )

   STOP
 ─┤ ├────────( U MOTOR )

⚠️ STOP must be NO here, not NC — the unlatch coil only fires when the rung is true.

🔌 Power Flow Convention

• Left rail = power; right rail = neutral
• Power flows left to right through closed contacts
• Coil energizes when its rung evaluates true

This is a useful fiction. Internally the PLC just evaluates Boolean expressions — there is no electrical current in software.

🐛 Top 5 Ladder Bugs

1. Double-coiling — same output written from two rungs. Last rung wins.
2. Order-dependence — relying on rung A being evaluated before rung B for correctness.
3. Forgotten NC for STOP — using NO STOP means a broken wire prevents stopping (huge safety bug).
4. Latch trap — set-coil with no clear unlatch path. Output stuck on forever.
5. Race with one-shot — pulse generated by edge detection but consumed in same scan.

🧱 Function Block Diagram (FBD)

Same logic, drawn as wired blocks instead of rungs:

   A ───┐
        ├─[ AND ]─── Y
   B ───┘

When to prefer FBD over LD:

• Process control (PID, scaling, filtering)
• Reusable function blocks
• Analog signal flow (where ladder gets ugly)

When to prefer LD over FBD:

• Discrete interlocks
• Safety-critical relay-replacement logic
• Code that maintenance electricians will read

🧪 The Boolean → Ladder Translation Recipe

1. Write the spec as a sentence: “Motor runs when Start is pressed AND guard is closed AND no fault.”
2. Identify inputs, outputs, conditions.
3. Write the Boolean: MOTOR = START · GUARD · ¬FAULT
4. Convert each AND to series, each OR to parallel, each ¬ to NC contact.
5. Test mentally with all input combinations.

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