• Installed inside a dry indoor electrical/control enclosure.
• Powered continuously from the panel 24 VDC control supply.
• Connected to 24 VDC PLC digital-input circuits through isolated relay contacts.
• Receives EU868 LoRa packets through an enclosure-mounted external antenna.
• Intended initially for prototype and site validation, with a production-intent component and PCB architecture.

• Receive and authenticate packets from two paired transmitter nodes.
• Track four reels, with independent low and depleted states for each reel.
• Provide eight normally-open reel alarm contacts.
• Provide two hardware fail-safe communications alarm contacts.
• Assert a communications alarm after three missed nominal 10-second reports, subject to final protocol timing.
• Assert communications alarms during receiver boot, reset, MCU lockup, loss of 3.3 V logic, missed external-watchdog service, or loss of 5 V relay-coil power.
• Recover automatically when valid communications return, with optional firmware hysteresis/debounce.
• Provide local indication for power, radio activity, system status, eight reel outputs, and two communications outputs.
• Support firmware programming, debugging, factory test, and field diagnostics.

• Protected 18–30 VDC nominal industrial input; design target tolerates at least 36 V continuous at the regulator input after protection selection.
• 5 V switching rail sized for all ten relays plus logic conversion.
• 3.3 V logic rail for MCU, radio, watchdog logic, and indicators.
• Pre-certified EU868 LoRa module with an external 50-ohm antenna connection; no onboard antenna.
• Dedicated host MCU with internal independent watchdog.
• Two-channel external missing-pulse watchdog path for communications-fault relays.
• Ten Form-C relays with field-accessible COM/NO/NC terminals.
• Pluggable screw terminals and clear channel labeling.
• Reverse-polarity, overcurrent, surge/transient, ESD, and EMI protection.

Diagram

• Input connector: two-position pluggable terminal, 24V_IN and 0V.
• Nominal input: 24 VDC; working design range 18–30 VDC.
• Series protection order: replaceable fuse or resettable protection, reverse-polarity MOSFET, bidirectional TVS, common/differential-mode filtering, local bulk capacitance.
• Preferred reverse protection: P-channel MOSFET or ideal-diode controller rather than a series diode, to reduce dissipation and voltage loss.
• TVS selection must be coordinated so normal 30 V operation does not conduct and the downstream buck remains below its absolute maximum during expected panel transients.
• Add input power LED after protection and test points before/after protection.

• 24 V to 5 V synchronous or asynchronous buck regulator.
• Minimum regulator input rating: 36 V; 60 V preferred for industrial margin.
• Continuous output target: at least 1.0 A; select 1.5 A or greater for thermal and transient margin.
• Include datasheet-recommended inductor, catch diode if required, ceramic input/output capacitors with DC-bias derating, compensation, enable biasing, and optional soft start.
• Keep the switching hot loop compact and physically separated from the RF module and antenna feed.

• 5 V to 3.3 V low-noise LDO rated for at least 300 mA.
• Target load below 200 mA peak.
• Provide 10 uF-class input/output bulk capacitance plus local 100 nF decoupling at every IC supply pin.
• Add 3.3 V power-good/reset supervision if not adequately provided by the MCU brownout detector.

• Preferred: STM32G030C8T6 or a similar STM32G0 device in LQFP-48 to provide adequate GPIO, UART, timers, SWD, watchdogs, and expansion margin.
• Operates at 3.3 V using the internal oscillator unless radio timing or test results justify an external crystal.
• Required peripherals: UART for LoRa module, I2C or SPI for GPIO expansion if used, timer outputs for communications watchdog service, GPIO for relay/status control, SWD, reset, and optional factory UART.
• Enable the independent watchdog and brownout reset.
• All actuator outputs must have hardware pull-down/default states so reel relays remain de-energized during reset.

• Preferred module: RAKwireless RAK3172 EU868 variant; alternatives are Seeed LoRa-E5 or Ezurio/Laird 453-00139R subject to interface and certification review.
• Module must support 863–870 MHz operation and expose a 50-ohm RF port for an external antenna.
• Host interface: UART preferred for modular firmware separation; direct STM32WL application firmware is acceptable if the module integration model requires it.
• Fit a board U.FL/I-PEX connector and a short qualified coax pigtail to an enclosure bulkhead SMA connector, or use the module RF pad to a controlled-impedance board connector.
• No onboard PCB antenna.
• Keep antenna feed 50 ohms, short, via-fenced where appropriate, and away from relay terminals, buck switching nodes, and enclosure wiring.
• Firmware and test procedures must enforce EU868 duty-cycle and channel-plan requirements.

• Eight Form-C relays, driven as conventional alarm outputs using COM-NO.
• Relay coil off at boot and when no alarm is present.
• MCU/GPIO expansion output high activates a low-side N-MOSFET or transistor driver.
• Each coil receives a flyback diode or driver-array clamp.
• Per-channel LED indication should not load the PLC contact circuit; indicate coil command/state on the logic side.
• Default pull-down on every driver gate/base.

• Two Form-C relays wired externally using COM-NC.
• Healthy state energizes each relay, holding COM-NC open.
• Alarm, boot, reset, firmware lockup, 3.3 V loss, 5 V coil loss, or receiver power loss de-energizes the relay and closes COM-NC.
• External watchdog logic must be independent of normal application execution. A dual retriggerable monostable or two dedicated watchdog ICs may be used.
• Firmware emits a periodic service pulse for each transmitter only while that transmitter link is considered healthy. The hardware timeout can be short (for example 1–3 seconds) because firmware performs the 30-second radio-health decision before continuing or stopping service pulses.
• The watchdog output must directly gate each communications relay driver so firmware cannot force a healthy output without generating valid recurring service activity.

• Preferred relay family: Omron G5V-1 5 VDC Form-C, subject to final availability and footprint verification.
• Alternative: Omron G6S-2-5VDC DPDT, using one pole where Form-C G5V-1 sourcing is unsuitable.
• Contact target: at least 1 A at 24–30 VDC, comfortably above the S7-1500 digital-input wetting current.
• Contacts are dry and isolated; do not connect PLC field voltage to receiver logic ground.
• Use three-position 5.08 mm pitch pluggable terminals per output, labeled NC/COM/NO.
• Maintain contact-to-coil and field-to-logic clearances according to the relay rating and intended low-voltage installation category.

• Preferred reel-channel driver: ULN2803A or individual logic-level N-MOSFETs.
• If MCU GPIO count is insufficient, use MCP23017 I2C GPIO expansion for reel relays and LEDs only.
• Communications relay paths must not depend solely on the GPIO expander.
• Provide series resistors where useful for edge control and test points for every relay-drive signal.

• LEDs: 24 V present, 5 V present, 3.3 V present/system, radio RX/activity, system fault, and one per relay command/state.
• Buttons: reset and optional commissioning/pairing button.
• Optional rotary/DIP switch for receiver address or installation mode; configuration can otherwise be stored in flash.
• Avoid excessive LED current: target 1–2 mA per indicator.

• 10-pin Cortex SWD header or compact Tag-Connect footprint exposing SWDIO, SWCLK, NRST, 3.3 V reference, and ground.
• Separate 3-pin or 4-pin UART diagnostic header is recommended.
• Add accessible test points for 24 V protected, 5 V, 3.3 V, ground, radio UART, watchdog service/output, and each relay driver.
• Firmware update through SWD is mandatory; field bootloader update is optional.

• Maintain a unique identity/key for each paired transmitter.
• Validate packet integrity, expected transmitter ID, sequence number, and freshness.
• Treat three missed nominal 10-second heartbeat/report intervals as a communications failure unless the protocol specification defines a stricter rule.
• Service each external communications watchdog only while its associated link is healthy.
• Configure reel outputs off before peripheral initialization.
• Use the independent MCU watchdog and brownout reset.
• Store configuration with CRC/versioning and safe defaults.
• Record reset cause and communications-fault diagnostics where practical.
• Avoid automatically clearing alarms on a single questionable packet; require valid recovery criteria.

• Recommended four-layer PCB: top signal/components, solid ground plane, power/secondary signal plane, bottom signal/components.
• Estimated board class: approximately 160–220 mm wide by 100–140 mm high, driven mainly by thirty relay terminal positions, relay spacing, and enclosure choice.
• Prefer relays and field terminals along one or two board edges with unambiguous channel grouping.
• Place power input and converter together, separated from RF.
• Place the radio and U.FL connector at the opposite edge, with a grounded keepout/corridor for the RF feed.
• Keep relay coil loops compact and route coil returns away from radio/MCU ground references before joining the ground plane.
• Provide four or more mounting holes and compatibility with a DIN-rail carrier or enclosure mounting plate.
• Indoor enclosure target: IP20 minimum inside a protected cabinet; use a metal or flame-retardant plastic DIN-rail enclosure as installation requires.
• External antenna bulkhead must be mechanically supported by the enclosure, not by the PCB U.FL connector.

• Four-layer, 1.6 mm FR-4, 1 oz copper is the preliminary baseline.
• Mixed SMD and through-hole assembly; relays and pluggable terminals are through-hole for field robustness.
• Use active-lifecycle parts with at least two credible distribution channels where possible.
• Avoid unnecessary fine-pitch packages; LQFP/TSSOP/SOIC and 0603/0805 passives are preferred for prototype rework.
• Provide polarity, pin-1, connector-function, and relay-contact markings on silkscreen.
• Include test points suitable for bed-of-nails or manual probing.
• Perform AOI for SMD assembly and functional testing of every relay/contact path.

1. Protected input operates from 18–30 VDC without overheating.
2. No component exceeds its thermal limits with all ten relays energized continuously at maximum ambient.
3. 5 V and 3.3 V rails remain within tolerance during LoRa transmit and simultaneous relay switching.
4. Each reel alarm closes only its assigned COM-NO contact.
5. Each communications contact is open in the healthy powered state and closes COM-NC on boot, reset, missed watchdog service, MCU halt, 3.3 V failure, 5 V relay failure, and total receiver power loss.
6. Relay contacts remain galvanically isolated from receiver ground and from one another.
7. Radio range and packet-loss performance meet the site requirement with the selected external antenna and enclosure.
8. Conducted/radiated emissions and immunity are suitable for the industrial installation and final RED/EMC testing.
9. Firmware programming, reset, diagnostics, and factory test points are accessible after assembly.
10. Labels and documentation state that outputs are not safety-rated.

• Use a pre-certified EU868 module and external enclosure antenna rather than an onboard antenna.
• Use electromechanical Form-C relays for true dry-contact behavior and PLC wiring flexibility.
• Use COM-NC, energized-healthy communications relays for hardware fail-safe fault indication.
• Use COM-NO reel alarms because they are operational indications, not safety functions.
• Use a 5 V relay rail to keep coil current manageable while retaining common relay availability.
• Use a four-layer PCB for robust grounding, RF control, and EMC performance.

• PLC input module is a standard Siemens S7-1500 24 VDC Type 3 input; exact module MPN and field wiring must be confirmed.
• Exact transmitter packet format, encryption/authentication method, RF spreading factor, transmit power, and channel plan remain to be frozen.
• Required point-to-point range, antenna cable length, enclosure material, ambient temperature, and EMC environment require site confirmation.
• Final radio-module certification and approved antenna conditions must be verified for the exact module SKU and antenna.
• Final relay choice depends on availability, footprint confirmation, and contact-life expectations.
• Final surge class and protection network depend on the panel supply and cable environment.

• Replaced the earlier non-fail-safe communications output description with energized-healthy COM-NC behavior.
• Expanded the architecture, interfaces, power budget, physical requirements, and verification criteria.
• Added a production-intent preliminary BOM as a separate project file.