1. CPU Unit: The Brain of the PLC (4 Selection Factors)
The CPU is the core of the PLC. Selection depends on4 factors:
| Feature | What it should show | Example |
|---|---|---|
| I/O count | Total DI + DO + AI + AO points + 20% margin | 64 DI / 32 DO points — choose CPU 1214C |
| Program capacity | User program + data storage | 50 KB for mid-range, 4 MB+ for large |
| Communication interfaces | Ethernet / serial / bus | PROFINET×1 + RS485×1 |
| Processing speed | Bit instruction execution time | <0.1 μs per instruction (S7-1500) |
2. I/O Modules: Signal Input and Output (DI/DO/AI/AO Categories)
I/O falls into4 categories, select by field signal type:
- DI (digital input): connects pushbuttons, limit switches, proximity switches. 0.75 mm² wire per point; common terminal wiring depends on NPN/PNP. Typical module: Siemens SM1221 DI16×24VDC.
- DO (digital output): connects indicator lights, interposing relays, solenoid valves. Relay output 2 A/point, transistor output 0.5 A/point. Typical module: Siemens SM1222 DO16×Relay.
- AI (analog input): connects temperature sensors (PT100/thermocouple) and pressure transmitters (4–20 mA/0–10 V). 12–16 bit resolution. Typical module: Siemens SM1231 AI4×13bit.
- AO (analog output): connects VFD frequency setpoints and control valves. 4–20 mA or 0–10 V. Typical module: Siemens SM1232 AO2×14bit.
3. Power supply modules: from AC 220 V to DC 24 V (switching power supply selection)
The PLC power supply module converts AC 220 V/380 V to DC 24 V to power the CPU, I/O, and sensors. Selection formula:
P_total = P_CPU + P_IO + P_sensors + P_HMI + 20% margin
| configuration | Power consumption estimate | Recommended switching power supply |
|---|---|---|
| S7-1200 CPU1214C + 4 modules | ~30W | SITOP 24V/5A (120W) |
| S7-1200 CPU1214C + 8 modules | ~55W | SITOP 24V/10A (240W) |
| S7-1500 CPU1515 + 12 modules | ~100W+ | SITOP 24V/20A (480W) |
4. Interposing relays: isolation and amplification (DO contact protection layer)
PLC DO points do not drive high-power loads directly; an interposing relay layer provides isolation and amplification. Selection depends oncoil voltage, contact count, contact rating:
| Model | Coil | Contacts | Contact rating | Application |
|---|---|---|---|---|
| Omron MY2NJ | DC24V | 2 Form C | 5A/AC250V | Drives contactor coils |
| Omron LY2NJ | DC24V | 2 Form C | 5A/AC250V | Same as above |
| Schneider RXM2LB2BD | DC24V | 2 Form C | 5A/AC250V | Drives solenoid valves/indicator lights |
| Omron G2R-1-S | DC24V | 1 Form C | 5A/AC250V | Single-contact control |
Afreewheeling diodemust be connected across the coil terminals (e.g., 1N4007) or a varistor to absorb the reverse EMF at coil de-energization and protect the PLC output point. Relay sockets are supplied withQuick-connect terminals, for easy maintenance and replacement.
5. Terminal blocks: the wiring hub (numbering rules and color code)
Terminal blocks are thewiring hubinside the cabinet. The numbering scheme directly affects maintenance efficiency:
| Wire number range | Application | Color code |
|---|---|---|
| U/V/W | Main circuit, three-phase | Yellow / green / red |
| L/N/PE | Control circuit power | Brown / blue / yellow-green |
| +24V/0V | DC power | Red / blue |
| I0.0~I7.7 | PLC input signals | White / gray |
| Q0.0~Q7.7 | PLC output signals | Black |
Zhejiang Golden Unicorn produces drawings to GB/T 6988.1, and every cabinet ships witha terminal block diagram + component list. Terminal wire markers useheat-shrink printing, oil-resistant, water-resistant and heat-resistant.
FAQ FAQ
How much PLC I/O spare capacity should be reserved?
Recommended20% spare. For example, if 48 DI points are actually needed, specify a 64-point DI module. The spare capacity covers later process improvements and replacement of failed points; without it, future expansion means changing the CPU or adding expansion modules.
Do interposing relays always need a flyback diode?
It is mandatory.. When a DC coil opens, the reverse EMF can reach several hundred volts; without a flyback diode it will break down the PLC output transistor. It is also recommended for relay-output PLCs to protect the relay contacts and extend service life.
How should heat be managed in a PLC control cabinet?
The CPU and power supply are the main heat sources. Keep the internal cabinet temperature at or below 55°C; above that, addcooling fans(for enclosures rated below IP54) orcabinet air conditioners(for enclosures rated IP54 or higher). Leave at least 100 mm of clearance above the switching power supply and around the CPU for airflow.
How should terminal wire markers be printed?
Recommended:heat-shrink tube printing, which withstands higher temperatures than PVC (125°C) and resists oil and water. Use a character height of at least 2 mm, even spacing, and a consistent top-to-bottom, left-to-right orientation.
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