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// IEC 61131-2 — 24V DC digital signal thresholds
// Type 1 inputs (most common industrial standard)
//
//  Voltage range    │  Guaranteed state
//  ─────────────────┼─────────────────────────────────
//  −3V  to  +5V     │  Logic 0 (OFF)  — guaranteed
//   +5V to +15V     │  UNDEFINED      — avoid this zone
//  +15V to +30V     │  Logic 1 (ON)   — guaranteed
//
// Real sensor output voltages (good design):
//   Sensor OFF:  0V to 1V  (well within logic 0 zone)
//   Sensor ON:   22V to 24V  (well within logic 1 zone)
//
// Common problems that push into undefined zone:
//   Long cable + high sensor current → voltage drop
//   Shared 0V rail with heavy loads → ground lift
//   Damaged sensor output transistor → partial conduction
//   Missing termination on shielded cable
//
// Test procedure:
//   Measure signal with multimeter (DC) at PLC input terminal
//   Expected OFF: < 5V.  Expected ON: > 15V
//   Anything between 5–15V: INVESTIGATE before commissioning

// Signal type wiring examples — 24V DC system
//
// ── PNP (Sourcing) sensor → PLC input ──────────────────────
// +24V ──────────── Sensor Vcc
//                   Sensor OUT ──────── PLC Input +
// 0V  ──────────── Sensor GND ──────── PLC Input − (Common)
//
// When sensor detects: OUT → +24V → PLC sees HIGH
// When sensor not det: OUT → floating/0V → PLC sees LOW
// Current path: +24V → Sensor → PLC input impedance → 0V

// ── NPN (Sinking) sensor → PLC input ───────────────────────
// +24V ──────────── Sensor Vcc ──── Pull-up resistor ──────┐
//                   Sensor OUT ──────────────────────── PLC Input
// 0V  ──────────── Sensor GND ──── PLC Input − (Common)
//
// When sensor detects: OUT pulls to 0V → PLC sees LOW? No:
// NPN sinking means current flows INTO sensor OUT
// PLC input sees +24V when sensor OFF, 0V when sensor ON
// → Input logic is INVERTED for NPN with pull-up wiring

// ── DRY CONTACT (potential-free) → PLC input ───────────────
// +24V ──── PLC Input + supply ──── Contact A
//                                    Contact B ──── PLC Input
// 0V  ──── PLC Input − (Common)
//
// When contact closes: current flows through contact → PLC HIGH
// Galvanically isolated from any external voltage source
// Used for: E-Stop, safety relay contacts, manual switches

// ── PUSH-PULL sensor → PLC input ────────────────────────────
// Works with EITHER PNP or NPN wiring
// ON state: output drives to +24V (PNP transistor active)
// OFF state: output drives to 0V (NPN transistor active)
// No pull-up or pull-down needed — actively drives both states

// Input/output current matching calculation
//
// Step 1: Check sensor vs PLC input
//   Sensor max output current: 200 mA (typical PNP sensor)
//   PLC input current draw:    6 mA at 24V  (from datasheet)
//   → OK: 6 mA << 200 mA
//
// Step 2: Check PLC output vs load
//   PLC output max current:    0.5 A continuous
//   Solenoid valve coil:       P=4.8W, I=4.8/24=0.2A steady
//   Solenoid inrush (pick-up): I_inrush = 5 × I_steady = 1.0A
//                              Duration: ~20 ms
//   → 1.0A > 0.5A: OVERLOAD at inrush!
//   → Solution: Use output rated ≥ 1A, or add external relay
//   → OR: Use solenoid valve with electronic inrush limiter
//          (modern ASCO/Festo valves: inrush = 1.2× steady)
//
// Step 3: Group fuse calculation
//   8 outputs per group, all solenoids 0.2A steady
//   Simultaneous on: 8 × 0.2A = 1.6A
//   Add 25% margin: 1.6 × 1.25 = 2.0A
//   Select: 2A fast-blow fuse (or 2.5A slow-blow for inrush)
//
// Step 4: PSU total current
//   All I/O current + PLC CPU + HMI + field devices
//   Total: 12A estimated → use 15A / 360W PSU @ 24V
//   Add UPS buffer capacitor: 15A × 20ms = 0.3 Ah

// Digital input channel — internal signal path
//
// Field terminal (+) ─┬── [TVS diode to 0V]  ← clamps spikes
//                     │
//                     └── [R_input ~3.3kΩ]   ← current limiter
//                              │               I = 24V/3.3kΩ = 7.3mA
//                              ▼
//                         [C_filter ~1µF]     ← RC debounce
//                              │               τ = 3.3kΩ × 1µF = 3.3ms
//                              ▼
//                      [Optocoupler LED]       ← 5mA min to switch
//                      [Optocoupler photo-transistor]
//                              │               3kV isolation barrier
//                              ▼
//                       [3.3V logic level]    ← PLC CPU side
//                              │
//                       [Input register]      ← Read each PLC scan
// Field terminal (−) ─── [0V common]
//
// Key parameters from datasheet:
//   Input voltage range: −3V to +30V (continuous)
//   Min. current for logic 1: 4 mA
//   Max. current (continuous): 15 mA
//   Default filter time: 3.2 ms
//   Filter time programmable: 0.1–20 ms
//   Isolation voltage: 500V (field to backplane)
//   Input impedance at 24V: ~3.3 kΩ (= 7.3 mA → above 4mA min)

// Freewheeling diode — why it is MANDATORY for inductive DC loads
//
// When a transistor output switches OFF a coil:
//   L × (dI/dt) = V_spike
//   Coil inductance L = 0.1H, I = 200mA, turn-off time = 10µs
//   V_spike = 0.1 × (0.2 / 0.00001) = 2000V !!!
//
// This 2000V spike appears across the output transistor:
//   → Exceeds transistor V_DS rating (typically 50–60V)
//   → Transistor avalanche breakdown → eventual failure
//   → Also radiates EMI affecting nearby electronics
//
// Freewheeling diode solution:
//   Diode placed: anode to 0V, cathode to +24V
//   (i.e. reverse-biased during normal operation)
//   When output turns OFF: inductor drives current through diode
//   Coil energy dissipated safely: V_clamp = 0.7V (diode forward V)
//   Spike: 0.7V instead of 2000V
//
// Practical notes:
//   Most modern PLC output cards include internal freewheeling diodes
//   External is still recommended for long cable runs (cable inductance)
//   For faster coil release: Zener diode in series with freewheeling
//   (e.g. 27V Zener): V_clamp = 27V, faster energy dissipation
//   → Relay/valve opens in ~3ms vs ~50ms with simple diode
//
// Protection components per output channel (external):
//   Inductive DC load:     1N4007 freewheeling diode across load
//   Inductive AC load:     RC snubber (47Ω + 47nF) across load
//   Lamp load (inrush):    Series NTC thermistor or current fuse

// IO-Link device data model (IEC 61131-9)
// Single 3-wire cable: L+ (24V), L− (0V), C/Q (data)
// Communication: 24V UART, 4800/38400/230400 baud

// Process Data (cyclic, every master cycle ~2ms):
//   Bit 0:    Switch state (ON/OFF) — same as digital I/O
//   Bit 1–7:  Extended status flags (output overload etc.)

// Device Parameters (acyclic, on demand):
//   Index 1:   Vendor name      (e.g. 'SICK')
//   Index 2:   Device name      (e.g. 'IME08-1B5NSZW2S')
//   Index 5:   Serial number    ('AB12345678')
//   Index 16:  Switching distance (mm) — read/write!
//   Index 17:  Hysteresis       (mm)
//   Index 18:  Output logic     (NO/NC)

// Diagnostic Data (acyclic, on event):
//   Temperature          (°C, internal)
//   Operating hours      (h)
//   Short-circuit count  (events)
//   Signal quality       (0–100%)

// PLC code — read IO-Link device temperature:
VAR
  iolink_master : IO_LINK_MASTER_REF;
  sensor_temp   : REAL;
  read_done     : BOOL;
END_VAR

iolink_master.ReadParameter(
  port    := 3,           // Port 3 on master
  index   := 32,          // Temperature object index
  subindex:= 0,
  data    => sensor_temp,
  done    => read_done
);
// sensor_temp populated after read_done = TRUE

// Edge detection — all methods (IEC 61131-3)

// ── METHOD 1: R_TRIG / F_TRIG function blocks ─────────────
VAR
  start_button   : BOOL;    // Direct input
  rtrig_start    : R_TRIG;  // Rising edge detector
  ftrig_start    : F_TRIG;  // Falling edge detector
  start_pressed  : BOOL;    // Single-scan pulse on press
  start_released : BOOL;    // Single-scan pulse on release
END_VAR

rtrig_start(CLK := start_button);
ftrig_start(CLK := start_button);
start_pressed  := rtrig_start.Q;  // TRUE for exactly 1 scan
start_released := ftrig_start.Q;

// ── METHOD 2: Manual edge detection (no function blocks) ──
VAR
  input_now  : BOOL;
  input_prev : BOOL := FALSE;
  rising_now : BOOL;
  fall_now   : BOOL;
END_VAR

rising_now := input_now AND NOT input_prev;  // 0→1 transition
fall_now   := NOT input_now AND input_prev;  // 1→0 transition
input_prev := input_now;  // Store for next scan — MUST be last!

// ── METHOD 3: Pulse-stretching for very short inputs ──────
// Sensor pulse may be shorter than PLC scan time (e.g. 0.5ms pulse)
// Use hardware input filter OFF + software latch:
VAR
  short_pulse   : BOOL;   // Hardware HSC captures, OR input filter=0
  pulse_latch   : BOOL;
  pulse_timer   : TON;
END_VAR

IF short_pulse THEN pulse_latch := TRUE; END_IF;
pulse_timer(IN := pulse_latch, PT := T#50ms);
IF pulse_timer.Q THEN pulse_latch := FALSE; END_IF;
// pulse_latch now stretches short input to 50ms — visible to slow scan

// Structured interlock pattern — machine axis enable
// All conditions must be TRUE for axis to enable

VAR
  // Raw input signals
  estop_ok        : BOOL;   // E-Stop chain healthy (TRUE = no estop)
  guard_closed    : BOOL;   // Safety guard confirmed closed
  home_done       : BOOL;   // Axis has completed homing
  pneum_ok        : BOOL;   // Pneumatic pressure OK (> 5 bar)
  no_fault        : BOOL;   // Drive has no active fault
  mode_auto       : BOOL;   // Machine in AUTO mode
  upstream_ready  : BOOL;   // Upstream station has released

  // Interlock result
  axis_interlock  : BOOL;   // Combined interlock — all must be TRUE
  axis_enable     : BOOL;   // Final output to drive STO

  // Diagnostic — which interlock is preventing enable?
  interlock_word  : WORD;   // Bit-mapped diagnostic for HMI
END_VAR

// Evaluate all interlocks ONCE per scan
interlock_word.0 := estop_ok;
interlock_word.1 := guard_closed;
interlock_word.2 := home_done;
interlock_word.3 := pneum_ok;
interlock_word.4 := no_fault;
interlock_word.5 := mode_auto;
interlock_word.6 := upstream_ready;

// Single combined result
axis_interlock := estop_ok AND guard_closed AND home_done
               AND pneum_ok AND no_fault
               AND mode_auto AND upstream_ready;

// Output: only enable if all interlocks pass
axis_enable := axis_interlock;

// HMI shows interlock_word as bitfield:
// Bit 0 OFF → operator sees 'E-Stop active'
// Bit 1 OFF → 'Guard open'
// Bit 3 OFF → 'Low pneumatic pressure' etc.
// One word carries all diagnostic info — efficient and clear

// Production counter with reset, preset, and HMI output
VAR
  part_sensor     : BOOL;   // Photoelectric sensor — part detected
  reset_button    : BOOL;   // Operator reset
  rtrig_part      : R_TRIG;
  rtrig_reset     : R_TRIG;
  counter_CTU     : CTU;
  batch_size      : INT := 500;
  batch_complete  : BOOL;
  total_produced  : DINT := 0;  // Persistent across power cycles
END_VAR

rtrig_part(CLK := part_sensor);
rtrig_reset(CLK := reset_button);

counter_CTU(
  CU := rtrig_part.Q,     // Count on each part detection
  R  := rtrig_reset.Q,    // Reset on button press
  PV := batch_size,
  Q  => batch_complete,   // TRUE when batch_size reached
  CV => batch_count       // Current count (0 to PV)
);

// Accumulate total (persist across resets)
IF rtrig_part.Q THEN total_produced := total_produced + 1; END_IF;

// Full machine cycle state machine — RDFD pattern
// (simplified to show DI/DO interaction)
CASE machine_state OF
  0: // IDLE — waiting for start
    IF start_button AND interlock_ok THEN
      machine_state := 1;
    END_IF;
  1: // EXTEND cylinder
    cylinder_extend := TRUE;
    IF cylinder_fwd_sensor THEN  // End of stroke confirmed
      dwell_timer(IN:=TRUE, PT:=T#200ms);
      IF dwell_timer.Q THEN
        dwell_timer(IN:=FALSE);
        machine_state := 2;
      END_IF;
    END_IF;
  2: // RETRACT
    cylinder_extend := FALSE;
    cylinder_retract := TRUE;
    IF cylinder_back_sensor THEN
      cylinder_retract := FALSE;
      machine_state := 0;
    END_IF;
END_CASE;

// Inductive sensor — correction factor calculation
// Sensor: Balluff BES 516-300-S272 Sn=5mm (mild steel)
//
// Target material correction factors:
//   Mild steel (Fe360):     1.00  (reference)
//   Stainless steel 304:    0.70  → Actual Sd = 5 × 0.70 = 3.5mm
//   Stainless steel 316L:   0.65  → Sd = 3.25mm
//   Cast iron:              1.10  → Sd = 5.5mm
//   Aluminium:              0.45  → Sd = 2.25mm
//   Brass:                  0.40  → Sd = 2.0mm
//   Copper:                 0.35  → Sd = 1.75mm
//
// CRITICAL: Always verify actual switching distance on
//           the real machine with the real target material!
//           Spec sheet gives starting point, not final value.
//
// Mounting correction:
//   Flush (shielded, embeddable):   100% Sn, can embed in metal
//   Non-flush (unshielded):         100% Sn but needs 3Sn clearance
//                                   around sensor body
//
// Temperature effect:
//   Standard range: −25°C to +70°C  (Sn varies ±10%)
//   Extended range: −40°C to +85°C  (Sn varies ±15%)
//   At −25°C: switching distance may increase by 10%
//   At +70°C: switching distance may decrease by 10%
//   For tight applications: measure at actual operating temperature
//
// Output types available:
//   NO (normally open):  outputs HIGH when target present
//   NC (normally closed): outputs LOW when target present
//   NO+NC (dual output): both signals simultaneously
//   IO-Link:  digital + analog + diagnostics on one wire

// Photoelectric sensor selection guide
//
// APPLICATION: Detect transparent bottles on conveyor
// Problem: diffuse mode cannot detect transparent objects
//          (light passes through, no reflection back)
// Solution: Through-beam or retro-reflective with polarised filter
//
// APPLICATION: Detect shiny metal lids
// Problem: diffuse may false-trigger on ambient light
//          retro-reflective: lid surface may reflect back
// Solution: Background suppression diffuse sensor (laser triangulation)
//           or colour mark sensor if mark is present
//
// Excess gain concept:
//   Excess gain = actual signal level / minimum detection level
//   Excess gain = 1.0: sensor just barely switches — UNRELIABLE
//   Excess gain = 5.0: sensor has 5× signal margin — GOOD
//   Excess gain > 10: robust in dirty/foggy environments
//   Rule: always mount sensor to achieve excess gain > 2.0
//         after accounting for worst-case contamination
//
// Response time:
//   Mechanical conveyor at 1.5 m/s, part width 10mm:
//   Part passes sensor in: 10mm / 1500mm/s = 6.7 ms
//   Sensor response time must be << 6.7ms
//   Standard sensor: 1–3 ms response → OK
//   High-speed sensor: < 0.1 ms → for small parts at high speed
//
// Beam diameter must be < part size:
//   Min part size to reliably detect: 1.5 × beam_diameter
//   Laser spot: 1–3 mm → detects parts > 1.5–4.5 mm
//   LED spot: 5–20 mm → minimum part size 7.5–30 mm

// Limit switch selection — precision positioning
// Application: CNC axis end-of-travel limits

// Key parameters for precision limit switches:
//   Repeatability: ±0.01mm (Omron D4C series)
//   vs standard limit switch: ±0.5mm
//   Operating force: 0.5–2N (low for delicate mechanisms)
//   Pre-travel: 0.5–2mm (distance from free to operate)
//   Over-travel: 5–10mm (mechanical stops beyond operate point)
//
// Reed switch inside pneumatic cylinder:
//   Cylinder bore: 32mm, stroke: 100mm, piston has magnet
//   Mount reed switch in T-slot on cylinder body
//   Sensing range: 3–5mm through aluminium wall
//   Response time: 1–3ms (contact bounce 2–5ms — use input filter)
//   Max speed: 2m/s typical (piston must slow near end of stroke)
//   Temperature: −20°C to +70°C standard
//
//   Common problem: reed switch misalignment
//   Symptom: sensor activates mid-stroke (magnet field misaligned)
//   Fix: slide switch 5–10mm toward piston travel direction
//
// Hall-effect digital switch (solid-state reed replacement):
//   No contact bounce → no input filter needed
//   Faster response: < 0.1ms
//   Vibration resistant: survives 30g shock vs 10g for reed
//   IO-Link version: reports magnetic field strength
//   If field_strength < threshold → 'magnet wear' alarm
//   Service life: > 10^9 operations (effectively unlimited)

// Dual-channel safety input — wiring and monitoring
// Hardware: Pilz PNOZ m B0 safety relay
// Input: E-Stop button with 2× NC contacts
//
// Channel 1: +24V → button NC1 → terminal S11 of relay
// Channel 2: +24V → button NC2 → terminal S12 of relay
// Common:    terminal S14 → 0V
//
// Safety relay internal monitoring:
//   Continuously measures voltage on S11 and S12
//   Both HIGH (button released, NC contacts closed): OK
//   Both LOW simultaneously: E-Stop pressed → SAFE STATE
//   One HIGH, one LOW: DISCREPANCY
//     → If discrepancy < 500ms: temporary (acceptable)
//     → If discrepancy > 500ms: FAULT → latched fault state
//     → Requires manual reset after fault cleared
//
// Cross-circuit detection (short between channels):
//   Relay sends diagnostic test pulse alternately on ch1/ch2
//   If ch2 responds when ch1 is pulsed: short circuit detected
//   → Relay locks out immediately
//
// Safety relay outputs (2× NO contacts):
//   Contact K1 and K2 in series → drive contactor coil
//   Even if K1 welds shut: K2 still opens safely
//   Relay monitors K1/K2 feedback to detect welded contacts
//   If welded contact detected: relay will not re-enable
//   → Forces maintenance before machine can restart

// IEC 62061 PFH calculation (simplified):
// PFH = λ_D × DC where λ_D = dangerous failure rate, DC = diagnostic coverage
// For dual-channel (Cat.3): PFH ≈ λ_D² × T_proof / 2
// Typical result: PFH = 10^-8 /h → achieves SIL 2

// Safety PLC F-DI module — channel configuration (TIA Portal / Siemens)
// Module: SM 326 F-DI 8×24VDC (6ES7326-1BK02-0AB0)

// Channel configuration (per pair of channels):
// Channel mode: 2-channel equivalent evaluation
//   Channel 0 + Channel 1 are one safety input (E-Stop button)
//   Module monitors both channels and their agreement
// Discrepancy time: 100 ms
//   Channels must agree within 100ms → otherwise safety fault
// Supply voltage: 24V DC
// Input filter: 3ms (adjustable 0.1–20ms)
//
// Safety integrity:
//   SIL 3 / PL e capable (single module, dual channel)
//   PFH: 1.0 × 10^-9 /h
//   MTTFd: 474 years
//   Proof test interval: 20 years
//
// PROFIsafe communication:
//   F-Address: must be unique across all F-peripherals
//   Watchdog time: 150ms (if no valid safety telegram in 150ms → fault)
//   CRC: 4-byte checksum on every safety telegram
//
// Safety program (SCL / F-FBD):
// Access safety data ONLY through F-approved blocks
VAR
  E_stop_F    : BOOL;   // Read from F-DI via ESTOP1 block
  Muting_en   : BOOL;   // From standard program (muting request)
  Safety_OK   : BOOL;   // Safety output to F-DO
END_VAR

// ESTOP1: standard emergency stop evaluation block (PLCopen Safety)
ESTOP1_instance(
  E_STOP   := NOT F_DI_channel_0,  // NC contact → invert
  RESET    := reset_button_safe,
  T_MUTING := T#500ms,
  Q1       => Safety_OK,
  Q2       =>  (* feedback monitoring output *)
);

// Muting sequence for safety light curtain
// Application: product enters machine through curtain on conveyor
// Muting sensors: S_MUT_1 and S_MUT_2, spaced 200mm apart on conveyor
// Sequence: S_MUT_1 must trigger BEFORE S_MUT_2 (confirms product direction)
// Time window: S_MUT_2 must trigger within 5s of S_MUT_1

VAR
  lc_beam_broken  : BOOL;   // Light curtain OSSD output (NC logic)
  s_mut_1         : BOOL;   // Upstream muting sensor
  s_mut_2         : BOOL;   // Downstream muting sensor
  rtrig_mut1      : R_TRIG;
  rtrig_mut2      : R_TRIG;
  muting_window   : TON;    // 5-second muting window timer
  muting_active   : BOOL;   // Muting state
  safety_curtain  : BOOL;   // Final safety output (to F-CPU)
END_VAR

rtrig_mut1(CLK := s_mut_1);
rtrig_mut2(CLK := s_mut_2);

// Muting logic — strict sequence required
IF rtrig_mut1.Q AND NOT muting_active THEN
  muting_window(IN:=FALSE);  // Reset timer
  muting_window(IN:=TRUE, PT:=T#5s);
END_IF;

// S_MUT_2 must follow S_MUT_1 within window
IF rtrig_mut2.Q AND muting_window.IN AND NOT muting_window.Q THEN
  muting_active := TRUE;
END_IF;

// Deactivate muting when both sensors clear
IF NOT s_mut_1 AND NOT s_mut_2 THEN
  muting_active := FALSE;
  muting_window(IN:=FALSE);
END_IF;

// Timeout: no second sensor → cancel muting
IF muting_window.Q AND NOT muting_active THEN
  muting_window(IN:=FALSE);
END_IF;

// Safety output: safe unless beam broken AND muting not active
safety_curtain := NOT lc_beam_broken OR muting_active;
// This runs in SAFETY PLC — standard PLC cannot modify safety_curtain

// Digital I/O fault diagnosis — systematic approach
//
// ── INPUT NOT READING CORRECTLY ──────────────────────────────
//
// Step 1: Check PLC status LEDs
//   Input LED ON but PLC reads FALSE:
//     → Wrong input address in program (check wiring diagram)
//     → Input card failed (rare)
//   Input LED OFF but device is activated:
//     → No signal reaching PLC terminal → go to Step 2
//
// Step 2: Measure voltage at PLC input terminal (+ vs COM)
//   Sensor claimed to be ON:
//     Measure > 15V → signal OK → input card fault
//     Measure 5–15V → signal in undefined zone → sensor or cable fault
//     Measure < 5V → no signal → go to Step 3
//
// Step 3: Measure voltage at sensor output pin
//   Sensor ON condition:
//     > 20V → cable fault between sensor and PLC (check connector)
//     < 1V  → sensor output stage failed or not powered
//     No supply voltage: check Vcc and 0V at sensor connector
//
// Step 4: Check sensor supply
//   Measure Vcc at sensor: should be 18–30V DC
//   < 18V: cable resistance drop (long cable + high current)
//          or PSU overloaded (measure at PSU terminals)
//   0V: fuse blown, broken wire, wrong pin connection
//
// ── OUTPUT NOT ACTIVATING LOAD ──────────────────────────────
//
// Step 1: Is PLC commanding the output?
//   Force output ON in PLC → output LED lights?
//   NO → PLC output card fault or output disabled (safety?)
//   YES → output card OK, problem is in wiring/load → Step 2
//
// Step 2: Measure voltage at output terminal (+ vs COM)
//   Output commanded ON:
//     > 20V: signal OK but load not working → check load
//     < 15V: voltage drop → check fuse, check load resistance
//     0V:    broken wire or blown output transistor
//
// Step 3: Check load
//   Solenoid valve: measure coil resistance (Ω)
//     Expected: P=4.8W → R = V²/P = 576/4.8 = 120Ω
//     Measured 0Ω: coil short → replace valve
//     Measured ∞Ω: coil open → replace valve
//   Lamp: measure filament resistance
//   Relay coil: measure coil resistance

// EMC noise diagnosis and prevention
//
// SYMPTOM: Input randomly toggles with no physical change
//
// Diagnosis sequence:
// 1. Check: does it correlate with any machine output?
//    (motor start, solenoid fire, welding?) → Conducted/inductive EMI
// 2. Check: does it correlate with day/time? → Mains supply quality issue
// 3. Measure: voltage at PLC input terminal during false trigger
//    > 5V transient? → Noise couple into cable
//    < 5V:           → Ground reference problem (ground loop)
//
// FIX 1: Increase input filter time
//   Change filter from 3ms to 10ms
//   Cost: slightly slower input response (acceptable for most sensors)
//   Eliminates: short transients < 10ms
//
// FIX 2: Cable shielding and routing
//   Signal cables: run in separate cable tray from power cables
//   Minimum separation: 100mm (200mm near VFD output cables)
//   Shield: connect at PLC end only (single-point ground)
//   Cross at 90°: when signal and power cables must cross
//
// FIX 3: Ground loop elimination
//   Problem: sensor and PLC have different local earth potentials
//   e.g. sensor ground = 0.8V above PLC ground → 0.8V common mode
//   Fix: ensure all 0V connections return to a single earth point
//   Use star topology: ALL 0V wires back to same PSU 0V terminal
//   Never: daisy-chain 0V through sensor connectors (adds resistance)
//
// FIX 4: Ferrite cores on cable
//   Snap-on ferrite on cable: ~50Ω impedance at 25MHz
//   Attenuates HF conducted noise
//   Use: 3–5 turns through core for maximum impedance at target frequency
//   Zero cost to system operation (no effect on DC/50Hz signals)
//
// FIX 5: Optical isolation (ultimate solution)
//   If all else fails: use an isolating signal converter
//   Input: 24V DC from field device
//   Output: 24V DC to PLC, galvanically isolated
//   Common mode rejection: > 1000V
//   Cost: €15–50 per channel — use only where necessary

// Digital I/O predictive diagnostics
// Monitor every input for anomalies

TYPE DigitalInputDiag :
  STRUCT
    raw_value       : BOOL;
    prev_value      : BOOL;
    transition_count: DWORD;   // Total state changes
    last_change_ms  : DWORD;   // Timestamp of last change (ms)
    state_duration  : DWORD;   // How long in current state (ms)
    expected_hz     : REAL;    // Expected change rate (for cyclic inputs)
    actual_hz       : REAL;    // Measured change rate
    stuck_timeout   : TIME := T#30s; // Alarm if no change in this time
    alarm_stuck     : BOOL;
    alarm_fast      : BOOL;    // Changing much faster than expected
    alarm_slow      : BOOL;    // Changing much slower than expected
  END_STRUCT
END_TYPE

// Update function (call every PLC scan)
PROCEDURE UpdateDiag
VAR_IN_OUT d : DigitalInputDiag; END_VAR
VAR
  now_ms    : DWORD;  // System time in ms
  dt_ms     : DWORD;
END_VAR

now_ms := GetSystemTime_ms();

IF d.raw_value <> d.prev_value THEN
  d.transition_count := d.transition_count + 1;
  dt_ms := now_ms - d.last_change_ms;
  d.actual_hz := 1000.0 / DWORD_TO_REAL(dt_ms + 1);
  d.last_change_ms := now_ms;
  d.alarm_stuck := FALSE;  // Activity: clear stuck alarm
END_IF;

d.state_duration := now_ms - d.last_change_ms;

// Stuck-at detection: no change for too long
d.alarm_stuck := (d.state_duration > TIME_TO_DWORD(d.stuck_timeout))
                 AND (d.expected_hz > 0.0);  // Only if cyclic expected

// Rate alarms (for cyclic inputs like cam-driven sensors)
IF d.expected_hz > 0.0 THEN
  d.alarm_fast := d.actual_hz > d.expected_hz * 2.0;  // >2× expected
  d.alarm_slow := d.actual_hz < d.expected_hz * 0.5;  // <50% expected
END_IF;

d.prev_value := d.raw_value;
END_PROCEDURE