Compact Safety PLC Reference Design
An illustrative compact safety PLC (safety controller) design for safety circuits on machine tools, robot cells, packaging and conveyor lines: DC 24V input, dual-MCU redundant architecture (cross-checking + external watchdog), dual-channel safety inputs (E-stop, safety door, light curtain, etc., with test pulses) and dual-channel safety outputs (OSSD), dual-port industrial Ethernet carrying a safety protocol (e.g. FSoE / PROFIsafe), and a USB Type-C configuration port. This page covers the function, selection notes and available VOOHU parts for 27 component positions.
Illustrative PCB ⇄ Schematic
The illustrative layout shows placement and BOM positions; click any position to see selection notes, recommended parts and package. The board is an illustration arranged by functional zone (not the actual layout); Ethernet ports, the config port, safety input / output terminals and the power terminal sit on the bottom edge, while field safety devices and the upstream network are external nodes.
Ports P1 / P2 are identical; only one port and one differential pair are drawn (the other 10/100 pair is the same). Each port goes through a discrete VOOHU 10/100 LAN transformer (T1 / T2, WHS16037TG, 1500 Vrms isolation), is clamped on the PHY side by ESD arrays D1 / D2, and reaches a 10/100 PHY (U1 / U2, JL1111BI-NI); the PHYs connect via MII / RMII to the dual-port safety bus communication controller U8, which passes data to MCU A over its host interface. The safety protocol (e.g. FSoE / PROFIsafe) is handled in the communication controller and the two MCUs; the physical layer is the same as ordinary industrial Ethernet. Center-tap wiring, termination and crystal load capacitance per the PHY datasheet.
Board bill of materials
Click any row to jump to that position and view its selection notes
| No. | Zone | Position | Ref. | VOOHU category | Recommended part | Availability |
|---|---|---|---|---|---|---|
| 01 | A Safety bus · Config port | RJ45 ports ×2 (with LEDs) | J1 J2 | RJ45 Connectors | SYT561188AB1A1DB4 | VOOHU |
| 02 | A Safety bus · Config port | 10/100 LAN transformer ×2 | T1 T2 | 10/100 LAN Transformers | WHS16037TG | VOOHU |
| 03 | A Safety bus · Config port | Ethernet ESD array ×2 | D1 D2 | TVS / ESD | WHALD03G4U | VOOHU |
| 04 | A Safety bus · Config port | 10/100 PHY ×2 | U1 U2 | Ethernet PHY | JL1111BI-NI | VOOHU |
| 05 | A Safety bus · Config port | PHY 25MHz crystal | Y1 | — | — | Non-VOOHU |
| 06 | A Safety bus · Config port | USB Type-C configuration port (USB 2.0 only) | J3 | USB / Type-C | WHTCAX01167303 | VOOHU |
| 07 | A Safety bus · Config port | USB ESD array | D3 | TVS / ESD | WHTA5V01P2C | VOOHU |
| 08 | B Safety inputs | Safety input terminal (dual-channel + test pulses) | J4 | — | — | Non-VOOHU |
| 09 | B Safety inputs | Safety input ESD ×2 | D4 D5 | TVS / ESD | WHALD30E2B | VOOHU |
| 10 | B Safety inputs | Input conditioning + digital isolator ×2 | U3 U4 | — | — | Non-VOOHU |
| 11 | B Safety inputs | Test pulse output driver | U5 | — | — | Non-VOOHU |
| 12 | C Safety outputs | Safety output terminal (OSSD) | J5 | — | — | Non-VOOHU |
| 13 | C Safety outputs | Safety output ESD ×2 | D6 D7 | TVS / ESD | WHALD30E2B | VOOHU |
| 14 | C Safety outputs | Dual-channel high-side switch ×2 | U6 U7 | — | — | Non-VOOHU |
| 15 | D Dual-MCU core | Safety bus communication controller | U8 | — | — | Non-VOOHU |
| 16 | D Dual-MCU core | Safety MCU A | U9 | — | — | Non-VOOHU |
| 17 | D Dual-MCU core | Safety MCU B | U10 | — | — | Non-VOOHU |
| 18 | D Dual-MCU core | MCU crystals ×2 | Y2 Y3 | — | — | Non-VOOHU |
| 19 | D Dual-MCU core | External watchdog / voltage supervisor | U11 | — | — | Non-VOOHU |
| 20 | D Dual-MCU core | Core supply filter molded inductor ×2 | L1 L2 | Molded Inductors | WHYTA0420 Series | VOOHU |
| 21 | E Power | DC 24V input terminal | J6 | — | — | Non-VOOHU |
| 22 | E Power | Input fuse + TVS | F1 D8 | TVS / ESD | WHTB058VA | VOOHU |
| 23 | E Power | Power line common-mode choke (EMI) | L3 | Power Line CMC | WHAL-7060A-102T0 | VOOHU |
| 24 | E Power | Synchronous buck ×2 (24V→5V→3.3V) | U12 U13 | — | — | Non-VOOHU |
| 25 | E Power | DC-DC molded inductor ×2 | L4 L5 | Molded Inductors | WHYT0630 Series | VOOHU |
| 26 | E Power | Push-pull isolated power transformer | T3 | Push-Pull Transformers | WHST06K14A0 | VOOHU |
| 27 | E Power | Push-pull driver + isolated-side LDO | U14 U15 | — | — | Non-VOOHU |
Resources & downloads
Board bill of materials
Reference designators, zones, VOOHU categories, recommended parts and availability for all 27 positions. Opens directly in Excel.
Datasheet collection
Datasheet PDFs for the VOOHU parts recommended on this board.
Design files & footprint library
Schematic, PCB, and VOOHU part footprints and 3D models.
Coming soonRequest the full Safety PLC reference design packagefull package
Includes the BOM, datasheets for the recommended VOOHU parts and position-by-position selection notes. If you need advice for your safety I/O count, safety bus protocol or isolation requirements, describe them in the request. We reply within 24 hours on working days.
