Robust io

Robust IO

A rugged industrial I/O board designed in KiCad 10.

Status

• Rev A: fabricated and assembled by JLCPCB (5 bare boards, 2 assembled), ordered 2026-10-05. Board 1 is under bring-up: power rails, MCU, UART console and MCP2515 CAN controller are verified; the MC33978 input interface does not yet answer on SPI (see Bring-up log). Board 2 is partly disassembled from the reverse-polarity incident and needs C44/C45, U5, U1, U14 and one BTS7008 refitted.
• Rev B: change list collected below, not started.
• Schematic: complete across four hierarchical sheets. ERC clean.
• PCB layout: fully routed. DRC reports 0 errors and 4 library-mismatch warnings (J20, U1, U14, U7), which are intentional local footprint edits.

Board

Architecture

Connectors

• J1: 2x3 2.54 mm SMD header, AVR ISP (SPI is also on test points).
• J20: 1x6 2.54 mm SMD header, 3.3 V TTL UART in FTDI cable pinout. RX and TX have series resistors and a TVS array.
• J14, J19, J18, J16, J9, J10 (top edge): 3.5 mm pluggable terminal headers for the switch inputs, each with a fused supply pin and ground.
• J15 (bottom edge): 24 V supply input and ground.
• J3, J5 (bottom edge): high-side outputs OUT_0 to OUT_7, with +24V and ground.
• J6 (bottom edge): two motor driver outputs, with +24V and ground.
• J4 (left edge): CAN bus, +5V and ground. SW1 is next to it.

Test points

41 test points (35 top, 6 bottom) cover the supply rails, SPI, UART, CAN interrupt, driver control and sense lines, and ground. Positions and nets are listed in fab/robust-io-testpoints.csv.

Known limits of rev A

• Supply polarity is not marked on the board. J15 pins 1 and 2 (the square pad end, nearest F1) are +24V; pins 3 and 4 are GND. There is no reverse-polarity protection, so a reversed supply forward-biases D6 and the substrate diodes of every part on the rail. Both rev A boards were powered reversed during bring-up; see the Bring-up log.

• MC33978 input LEDs read "input at ground". Each input LED sits between +24V (2.2k) and the IC pin, so it lights when the input is pulled low. That is correct for switch-to-ground sensing, which is all rev A supports; switch-to-battery on an SP channel needs that channel's LED removed, because the 2.2k sources more current than the IC can sink.

• BTS7008 GND pins have no series resistor. The datasheet calls for 47 ohm in the GND pin (RGND) and limits GND pin current to 50 mA.

• CS_CAN has no pull-up. With the MCU in reset (during ISP) the MCP2515 can drive MISO; programming is unreliable unless the board is power-cycled first. Fit 10k from TP8 to TP1 if it keeps failing.

• UART activity LEDs load the lines. C69/C70 (10 nF) with the 1k series resistors give a 10 us edge, too slow for 115200 baud. The console runs at 38400 on rev A.

• C57 to C60 use a polarized symbol in the schematic; the fitted parts are 50 V MLCCs and have no polarity.

• The BTS7008-2EPA is rated for a 28 V maximum supply. Use a regulated 24 V supply; do not connect directly to a charging battery system.

• Reverse polarity protection is the fuse and TVS only. There is no series reverse-blocking stage.

• The input polyfuses are rated 30 V, which is a small margin over 24 V.

• Y1 load capacitors (22 pF) have not been matched to the crystal's specified load capacitance.

• Regulator output tracks are 0.2 mm wide.

Bring-up log (rev A)

• Both boards were first powered with the supply reversed. The symptom was a rail that read like a diode (0.5 V both ways, about 2 Ohm at 1 A), with whichever part had the lowest forward drop getting warm: D6, then U5, then the BTS7008 switches and DRV8876 drivers as parts were removed. With the supply in current limit at about 1 V nothing was damaged except C44 and C45 on board 2, which vented after conducting in reverse for a long time. D6 was correctly placed on both boards; the earlier CPL change for it has been reverted.
• Board 1 after correct power: 5 V and 3.3 V present, fuses written (JTAG off), firmware flashed from a Raspberry Pi over J1, console at 38400 8N1 on J20, MCP2515 answers. MC33978 does not answer the SPI check: continuity from the MCU to U7 pins 2, 3, 4, 16, 31 and 32 is good, VBATP and VDDQ are present, all four SPI modes were tried. The MISO pull-up test in firmware 0.3 (MC33978 check frames line in the banner) is the next step: 0xFFFFFFFF means the chip never drives MISO, 0x00000000 means it is selected but not shifting.
• With JTAG enabled (fresh chip) outputs 2 to 5 switch on by themselves. They go off once the fuses are written.
• On a Pi 4, avrdude leaves GPIO25 (reset) driven low after it exits. Run pinctrl set 25 ip pu (or raspi-gpio set 25 ip pu) after programming, or unplug J1 pin 5, before expecting the board to run.
• The Pi's mini UART (/dev/ttyS0) is unreliable for the console even at 38400. Use an FTDI cable on J20 for the console and the Pi only for programming, or switch the Pi to the PL011 (dtoverlay=disable-bt).

Rev B change list

• Mark + and - on the silkscreen at J15; add a reverse-polarity stage at the input (P-FET or ideal-diode controller) so a reversed supply does nothing.
• Input indicator LEDs: two-lead bicolour LED (red/green) in series with each input, replacing the 2.2k pull-ups R62 to R83, so the LED shows current in either direction and switch-to-battery works on the SP channels. Check the MC33978 comparator thresholds against the LED drop before committing.
• 47 ohm in series with each BTS7008 GND pin (U8 to U11).
• 10k pull-up on CS_CAN.
• C69/C70 to 220 pF, or drive the UART LEDs from the MCU.
• C57 to C60: non-polarized capacitor symbol.
• Test points: Keystone 5015 SMT loops on the rails, SPI, UART, interrupt, fault and analog sense lines; the four corner mounting holes are GND and take a ring lug, which covers the ground clips.
• Rev A fab outputs move to fab/revA/ and docs/revA/ before any rev B output is generated.

Files

• robust-io.kicad_pro - project file
• robust-io.kicad_sch - root sheet
• power.kicad_sch, processor.kicad_sch, inputs.kicad_sch, outputs2.kicad_sch - schematic sheets
• robust-io.kicad_pcb - PCB layout
• robust-io.kicad_sym, sym-lib-table - project symbol library
• robust-io.kicad_dru - custom design rules
• DRC.rpt, ERC.rpt - latest check reports

Documentation (docs/)

Regenerate with bash docs/make_docs.sh after saving the schematic and board.

Firmware (firmware/)

ATmega1284P firmware 0.3: UART service console (38400 8N1), CAN protocol with host timeout, output current trip, motor control, EEPROM settings and a self-test. Runs on rev A hardware; the MC33978 driver is unverified until the chip answers (see Bring-up log). Build with make in firmware/ (avr-gcc); make fuses and make flash program the board from a Raspberry Pi wired to J1. See firmware/DESIGN.md for the pin map, console commands, CAN frame layouts and settings.

Host tools (tools/)

Everything for talking to the board over CAN from a Pi, laptop or robot: a DBC file, a Python package with the robustio command-line tool, a terminal dashboard, a browser dashboard, a bring-up and end-of-line test runner that writes a report per board, and a ROS 2 driver node. All of them also run against the firmware simulator, so they can be used before hardware exists. See tools/README.md.

Fabrication outputs (fab/)

To regenerate the Gerbers after a board change, save the board in KiCad and run:

bash fab/make_gerbers.sh

The placement file is not produced by the script. JLCPCB's rotation convention differs from KiCad's for some packages, so the rotation for each part in robust-io-CPL-JLC.csv is the KiCad rotation plus the offset in cpl_rotation_offsets.json. Regenerate it whenever parts move, and check the result in JLCPCB's placement preview before ordering.

Bring-up order

1. Fit F1 and J15. Apply 24 V from a current-limited supply with nothing else connected.
2. Check +5V (TP2) and +3.3V (TP1) against ground.
3. Wire the Raspberry Pi to J1 (firmware/DESIGN.md section 4) and run make fuses, then make flash, in firmware/. Do this before connecting any load to J3 or J5: until the fuses disable JTAG, outputs 2 to 5 can switch on by themselves. Release the reset line afterward (pinctrl set 25 ip pu).
4. On the J20 console (FTDI cable, 38400 8N1): ver, then selftest.
5. Connect CAN and run robustio bringup --console /dev/serial0 --serial <n> from tools/, which writes a report for the board.
6. Connect loads and enable the outputs and motor drivers one channel at a time (robustio bringup --loads once all are connected).

License

See LICENSE. Free for personal, educational, and open-source use. For commercial licensing, contact ryan.lush@gmail.com.