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PLC I/O and Signals Cheat Sheet
PLC input and output essentials on one page: digital wiring logic, analog scaling and resolution, response timing, encoders and number codes.
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A one page A4 sheet to print, pin up in the workshop or keep on your phone. Everything on it is also shown below.
Download PDF A4 · 84 KBDigital I/O
| Sensor | Output switches | PLC input |
|---|---|---|
| PNP (sourcing) | +24 V to load | Sinking |
| NPN (sinking) | Load to 0 V | Sourcing |
| Dry contact | Either | Either |
Test: PNP output reads about +24 V to 0 V when active.
NAMUR sensor (EN 60947-5-6)
| Supply | about 8.2 V via 1 kΩ |
| Off | ≤ 1.2 mA |
| On | ≥ 2.1 mA |
Analog scaling
| Value | LRV + (raw − rawmin) ÷ (rawmax − rawmin) × span |
| Siemens S7 | 0 to 27648 = 0 to 100 % |
| Current | I = 4 + 16 × % |
| Square root | value = LRV + span × √% |
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ADC resolution (0 to 20 mA)
| Bits | Steps | Step size |
|---|---|---|
| 12 | 4096 | 4.9 µA |
| 14 | 16,384 | 1.2 µA |
| 16 | 65,536 | 0.31 µA |
Timing
| Worst case response | tin + 2 × tscan + tout |
| Digital filter | y = yprev + (x − yprev) × Ts ÷ (τ + Ts) |
| 63 % response | after one time constant τ |
Encoders
| Speed | RPM = f × 60 ÷ PPR |
| Quadrature | up to 4 counts per pulse |
| Example | 1024 PPR at 25.6 kHz = 1500 RPM |
Number codes
| Gray from binary | G = B XOR (B ≫ 1) |
| BCD | 4 bits per digit: 0001 0010 0101 = 125 |
| Hex | 0x1234 = 4660 decimal |
24 V DC power
- Size supplies for 22 mA per analog loop.
- Add 20 to 30 % margin.
- Separate supplies for field and system loads where possible.
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Frequently Asked Questions
Can I connect a PNP sensor to a sourcing input?
No, both would supply current; use a sinking input or an interface relay.
How many bits do I need?
Enough that one step is well below the accuracy needed; 12 bits is often enough, 16 bits is common.
Why 27648?
Siemens leaves headroom above 100 % for overrange detection.
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Why 24 volts is used in PLC systems
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