Replacement Board Design

Design notes for a single board replacing both the original I/O board and the ISA card. The cabinet harness plugs in unmodified, using the same connector types and pinouts; the host connects over USB as a composite HID device.

This is a draft. No schematic, layout or firmware exists. It is recorded here so the work can be picked up. The Arduino bridge remains the working path and can stay in service throughout development, so the cabinet does not need to come down.

Core: a Raspberry Pi Pico soldered flat via its castellated pads. This gives a USB connector, a 3.3 V regulator and hand-solderable assembly for free. A bare RP2040 can replace it later for a slimmer board.

Signal inventory

Switch inputs are B1–B10, both Starts, four Effectors, both Pedals, Test, Service and Coin. Lamp outputs are the same set plus both turntable lamps, the neon channel, the mechanical coin counter and four generic cabinet lamps.

Power

• J12, 4-pin, same type as the original: +5V, GND, GND, +12V from the cabinet PSU.
• +12 V feeds the ULN2803 COM pins and the lamp common pin on every harness connector, through a 2 A polyfuse. Bulk capacitor 100 µF 25 V.
• +5 V from the cabinet supplies the turntable encoders only. It is not used for logic.
• The Pico is powered from USB. The logic rail is its 3V3 output; three 74HC165 and four 74HC595 draw a few milliamps, well inside the regulator budget.
• Grounds — cabinet ground and USB ground joined at a single star point near the Pico.
• 100 nF decoupling at every logic IC.

Input chain

Three 74HC165 on SPI0 RX. Each input line is pulled up to 3V3 through 10 kΩ (bussed resistor networks) and RC filtered — 100 Ω series with 10 nF to ground. The capacitors can be left unpopulated and debouncing done in firmware. Switches short the line to ground, so inputs are active-low.

Chain U1.QH to U2.SER, U2.QH to U3.SER, U3.QH to the Pico's MISO. Clock is shared with SCK, SH//LD is driven by /PL, and CLK INH is tied to ground.

Bit assignment mirrors the original read ports so the firmware mapping can be copied directly:

Output chain

Four 74HC595 into four ULN2803A. Chain the Pico's MOSI to U4.SER, then QH' of each device to SER of the next. SRCLK is shared with SCK, RCLK is on GP6, and /SRCLR is tied to 3V3.

/OE is on GP7 with a 10 kΩ pull-up to 3V3, so the lamps stay dark until firmware drives it low. Hold it high until USB is configured to avoid a lamp flash at boot.

Each 595 output drives one ULN2803 input 1:1, and each ULN2803 output sinks one lamp cathode on the harness. ULN2803 COM, pin 10, must go to +12 V — it clamps the flyback from the coin counter solenoid and the neon relay coil.

Four devices rather than three, so that the bytes map exactly onto the original write ports 0x100–0x103:

Encoders

TT1A, TT1B, TT2A, TT2B on GP8–GP11. Keeping them consecutive allows one PIO state machine per player.

Encoder power comes from the cabinet +5 V, with ground to the star point.

RP2040 GPIO is not 5 V tolerant. Each signal line needs a 10 kΩ pull-up to 3V3, which is sufficient for open-collector encoders, plus a divider footprint — 2.2 kΩ top, 3.3 kΩ bottom — in case the encoders turn out to be push-pull 5 V. Scope them and populate one option or the other.

Firmware counts edges in PIO and wraps to an unsigned 8-bit absolute position, matching what the game and 2EZConfig expect, and also exposes it as an HID axis.

Pico pin map

Scan loop: assert /PL, run one duplex SPI transfer reading the 165 chain while writing the 595 chain, then pulse RCLK. At 1 MHz SPI this is roughly 30 µs for a full scan, leaving ample margin for a 1 kHz poll rate.

Connectors

Clone the original footprints. Board-side mates for Molex 5051 housings are Molex 5045 vertical or 5046 right-angle wafers at 2.5 mm pitch; choose orientation to suit your cabinet's cable dressing.

Per-pin assignments are in connector-pinout.md. Transcribing them into the KiCad project is the first design task.

WS2812 data expects 5 V logic. A 74AHCT1G125 handles the level shift, or a spare ULN2803 channel with a pull-up.

Bill of materials

Excluding the PCB.

Firmware outline

1. TinyUSB composite HID: gamepad input report with 24 buttons and 2 absolute axes; output report carrying the lamp bitmap.
2. Two PIO quadrature state machines producing 8-bit absolute positions.
3. 1 kHz scan loop: duplex SPI reading switches and writing lamps, firmware debounce by 2-of-3 sample majority, then HID report update.
4. Hold /OE high until USB is configured.
5. Optional: a CDC or vendor channel answering reads and writes against the original 0x100–0x106 semantics, so software written for the original port map runs unchanged.

Much of this ports directly from the Arduino bridge. The USB layer, report descriptor, coin handling and test routines carry over; only the bus read and write functions are replaced by the shift-register scan and PIO counter reads.

This page is synced from docs/replacement-board.md in the ez2-io repo.