STM32F446 CNC Control Board

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A custom CNC control interface board designed with KiCad 9.0, intended for use with LinuxCNC and the Remora STM32F446 / W5500 firmware real-time firmware. The WeAct Studio STM32F446 64-Pin Core Board plugs onto this control board, and a WIZnet W5500 Ethernet module connects via the on-board SPI socket for Remora Ethernet operation.

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WARNING — Read before use

This design is provided as-is, without warranty of any kind, express or implied. CNC machines involve high voltages, high-speed mechanical components, and industrial motor drivers. Improper construction, wiring, or use of this hardware design can cause serious injury, death, fire, or property damage.

The author(s) of this design accept no liability for any damage, injury, or loss resulting from the use, misuse, reproduction, or inability to use this hardware design or any derivative thereof. You are solely responsible for verifying the suitability, safety, and correctness of any implementation before powering or operating it.

This design is intended for personal and educational use by individuals with appropriate knowledge of electronics and machine safety. It is not a finished or certified product.

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1. Board Overview

MCU Module	WeAct Studio STM32F446 64-Pin Core Board
Ethernet	WIZnet W5500 (SPI)
Step/Dir Outputs	5 axes — RS-422 differential (X, Y, Z, A, B)
Signal Isolation	CBMUD1200LAS8 high-speed digital isolators
Differential Driver	AM26LS31CDRG4 quad RS-422 line driver
Digital Inputs	8 × optocoupler-isolated (EL357N)
Digital Outputs	6 × optocoupler-isolated (configurable pull-up / pull-down)
Motor Enable Outputs	2 × optocoupler-isolated (STEP_EN, STEP_EN_Z)
Halt / E-Stop	Dedicated halt circuit — selectable NO / NC polarity
Alarm Input	Servo driver alarm → hardware halt trigger
Probe Input	Parallel dual-signal touch probe input
Spindle Control	Analog 0–10 V output via LM358 op-amp
Encoder Input	3-channel differential RS-422 (A/B/Z) with isolation
PCB Tool	KiCad 9.0

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2. PCB Layout

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3. Hardware Modules

3.1 MCU: WeAct Studio STM32F446 64-Pin Core Board

The WeAct STM32F446 development module plugs onto this control board via two 30-pin sockets. The sockets expose all GPIO ports (PA0–PA15, PB0–PB15, PC0–PC15, PD2) as well as 3.3 V and 5 V power rails.

Full MCU pin assignment:

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3.2 Ethernet: WIZnet W5500 Module

Ethernet connectivity is provided by a WIZnet W5500 module connected to the on-board SPI socket. This enables Remora Ethernet operation, where LinuxCNC communicates with the STM32F446 over UDP in real time.

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4. I/O Features

4.1 5-Axis Step / Direction Outputs (RS-422 Differential)

Each of the five axes (X, Y, Z, A, B) has a fully isolated, differential step and direction output channel. The signal chain consists of:

1. CBMUD1200LAS8 — Dual-channel high-speed digital isolator (SOIC-8, LCSC C476470). Provides galvanic isolation between the MCU 3.3 V domain and the driver 5 V domain.
2. AM26LS31CDRG4 — Quad RS-422 line driver (SOIC-16). Converts single-ended signals from the isolator output to balanced differential pairs (STEP+/STEP−, DIR+/DIR−).
3. ZMM5V6 Zener diode — Transient protection on the 5 V driver supply.

Each axis provides a 4-pin screw terminal: STEP+, STEP−, DIR+, DIR−.

MCU GPIO  →  CBMUD1200  →  AM26LS31C  →  STEP± / DIR±
(3.3 V)      (isolator)    (RS-422 driver)   (to motor driver)

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4.2 Isolated Digital Inputs (8 Channels)

Eight optocoupler-isolated digital inputs (EL357N, 1 kΩ series / 4.7 kΩ pull-up / 100 nF filter) connect via 2-pin screw terminals. Each channel is labelled by its intended function:

Channel	Net Label	Function
OPT_0	OPT_0_DOOR	Safety door switch
OPT_1	OPT_1_B_LIM	B-axis limit switch
OPT_2	OPT_2_X_LIM	X-axis limit switch
OPT_3	OPT_3_Y_LIM	Y-axis limit switch
OPT_4	OPT_4_Z_LIM	Z-axis limit switch
OPT_5	OPT_5_FEED_HOLD	Feed hold button
OPT_6	OPT_6_CYCLE_START	Cycle start button
OPT_7	OPT_7_A_LIM	A-axis limit switch

Note — HALT triggering: OPT_0 (DOOR) and OPT_1 (B_LIM) are wired to also trigger the hardware halt circuit in addition to being read as standard digital inputs by the firmware.

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4.3 Isolated Digital Outputs (6 Channels)

Six optocoupler-isolated digital outputs are available for switching relays, solenoids, and other loads. Output polarity (pull-up or pull-down) is configured per channel via on-board jumpers.

Channel	Net Label	Default Use
AUX_0	AUX_0_COOLANT	Flood coolant pump
AUX_1	AUX_1_MIST	Mist coolant
AUX_2	AUX_2	General purpose
AUX_3	AUX_3	General purpose
AUX_4	AUX_4	General purpose
AUX_5	AUX_5	General purpose

Pull-Down Configuration

Pull-Up Configuration

Each output connector exposes three pins: Vcc, SIG, GND.

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4.4 Motor Enable Outputs

Two dedicated optocoupler-isolated outputs (EL357N) are provided for controlling the enable (ENA) pins of motor drivers:

Signal	Function
STEP_EN	Enable output for all axes
STEP_EN_Z	Independent enable output for the Z axis

Unlike the AUX outputs (section 4.3), the motor enable outputs have a fixed output configuration — there is no pull-up / pull-down jumper. They connect directly to motor driver ENA inputs via screw terminals.

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4.5 Halt / E-Stop Input

A dedicated halt input is provided with optocoupler isolation (EL357N) and AO3400A N-channel MOSFET switching. The trigger polarity is selected by an on-board jumper:

Jumper Position	Trigger Condition
Position A	Normally-open (NO) contact — triggers on circuit open
Position B	Normally-closed (NC) contact — triggers on V+ applied

Multiple sources can trigger the HALT signal in hardware:

• External E-stop switch — connected to the dedicated HALT screw terminal.
• Door input (OPT_0_DOOR) — opening the safety door triggers a halt.
• B-axis limit (OPT_1_B_LIM) — B-axis over-travel triggers a halt.
• Servo ALARM input — described separately below.

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4.6 Servo Alarm Input

A separate ALARM input is dedicated to receiving the alarm signal from servo motor drivers. When a servo driver asserts its alarm output, this input hardware-triggers the HALT signal, bringing the machine to an immediate stop independently of the firmware state.

The alarm input circuit uses the same optocoupler isolation and transistor topology as the halt channels, wired in parallel with the main halt circuit.

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4.7 Probe / Touch Probe Input

The probe input (PROBE) uses a parallel dual-signal circuit implemented in parallel_switch.kicad_sch. Two independent probe signals (SIG_1, SIG_2) are combined through BAV23 dual-diode packages in a wired-OR configuration before entering the optocoupler (EL357N) and reaching the MCU.

Signal 1 ──┐
           ├── BAV23 (OR) ── EL357N ── MCU PROBE
Signal 2 ──┘

This allows two different probe switch contacts (e.g. a tool-length sensor and a 3D touch probe) to share the same MCU input. Either signal independently triggers the probe detection.

The probe screw terminal (J21, labeled PROBE) provides three connections: Vcc, SIG 1, SIG 2.

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4.8 Spindle Speed Control

An analog spindle speed output is generated using an LM358 dual operational amplifier. The MCU generates a PWM signal (SPINDLE_PWM) which is filtered and scaled by the LM358 stage to produce a 0–10 V analog reference voltage suitable for VFD spindle speed control.

Additional spindle signals: SPINDLE_DIR (direction) and SPINDLE_EN (enable).

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4.9 Encoder Input

The encoder input accepts a 3-channel differential quadrature encoder with index (A, B, Z). The signal chain:

1. AM26LS32ACDRG4 — Quad RS-422 differential line receiver (SOIC-16). Converts the differential pairs (A+/A−, B+/B−, Z+/Z−) to single-ended logic signals.
2. CBMUD1200LAS8 (×2) — High-speed dual-channel digital isolators. Provide galvanic isolation between the encoder cable side (5 V) and the MCU side (3.3 V).
3. 8-pin screw terminal — Encoder connector pinout: A+, A−, B+, B−, Z+, Z−, 5 V, GND.
4. 100 Ω series resistors + ZMM5V6 Zener — Line termination and transient protection.

Encoder  →  8-pin terminal  →  AM26LS32A  →  CBMUD1200  →  MCU (ENC_A / ENC_B / ENC_Z)
(RS-422)                       (receiver)     (isolator)    (3.3 V logic)

The 8-pin terminal is wired 1:1 to a custom encoder board via a straight-through Ethernet (Cat5/Cat6) cable, using the four twisted pairs for the A+/A−, B+/B−, Z+/Z−, and power pairs.

Note: Most industrial encoders provide single-ended (open-collector or push-pull) outputs. To connect such an encoder, an external line driver module must be used to convert the single-ended ABZ signals to RS-422 differential pairs before connecting to the RJ45: axvelo/PCB-Encoder-Line-Driver

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5. Software Compatibility

5.1 LinuxCNC

LinuxCNC runs on a real-time Linux PC and acts as the CNC machine controller. It sends motion commands to the Remora firmware over Ethernet.

5.2 Remora Firmware

The STM32F446 runs a Remora port specifically built for this board's hardware combination (STM32F446 + WIZnet W5500). It receives step/direction commands from LinuxCNC via UDP over the W5500 Ethernet interface and generates real-time pulse trains.

Firmware repository: axvelo/Firmware-F446-W5500

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6. Schematics

Full schematics are available as a PDF:

• documents/schematics.pdf

The KiCad 9.0 source files (.kicad_sch, .kicad_pcb, .kicad_pro) are included in this repository.

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7. Key Components

Component	Part Number	Package	Function
MCU Module	WeAct STM32F446 64-Pin	Module	Main controller
Ethernet Module	WIZnet W5500	Module	SPI Ethernet
Digital Isolator	CBMUD1200LAS8	SOIC-8	High-speed 2-ch isolation
RS-422 Line Driver	AM26LS31CDRG4	SOIC-16	Step/dir differential output
RS-422 Line Receiver	AM26LS32ACDRG4	SOIC-16	Encoder differential input
Optocoupler	EL357N(B)(TA)-G	DIP-4	I/O galvanic isolation
N-ch MOSFET	AO3400A	SOT-23	Halt circuit switching
NPN Transistor	MMBT5551	SOT-23	Probe circuit signal switching
Dual Diode	BAV23A / BAV23C	SOT-23	Probe wired-OR logic
Op-Amp	LM358	SOIC-8	Spindle 0–10V analog output
Zener Diode	ZMM5V6	LL-34	5.6 V transient protection

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8. License

This project is licensed under the CERN Open Hardware Licence Version 2 — Strongly Reciprocal (CERN-OHL-S-2.0).

You may redistribute and modify this design under the terms of the CERN-OHL-S v2. Any product or design that incorporates or derives from this work must be released under the same licence, with all source files (schematics, PCB layout, BOMs) made publicly available.

See the LICENSE file for the full licence text.