Board Architecture
Component roles on the original I/O board. Established by measurement and confirmed against the IKA Victor schematic recreation; reference designators follow the schematic.
| Block | Parts | Role |
|---|---|---|
| Address buffer | 74LS245 (U1) |
Carries address bus A0–A7 from the DB25, host to board |
| Data transceiver | 74LS245 (U2) |
Bidirectional D0–D7. Direction from IOGATEDIR; enable IOGATEEN from on-board decode |
| MCU port gates | 74HC245 ×3 (U3, U4, U8) |
Gate the AT89C51's port outputs onto the data bus under P0CS/P2CS/P3CS — the MCU-served read ports |
| Key input buffers | 74HC245 ×3 (U9–U11) |
Button ports, enabled by IN0CS/IN1CS/IN2CS, corresponding to reads of 0x101, 0x102 and 0x106 |
| Address decode | 74LS138 ×2 (U6, U7) |
One decodes the read address range, one the write range |
| Output latches | 74HC273 ×4 (U12, U14, U16, U18) |
OUTLATCH0–3, write ports 0x100–0x103. Clocked from the write decoder |
| Lamp drivers | ULN2803A ×4 |
Open-collector sinks. Lamp commons are +12 V. One channel drives the mechanical coin counter (COIN_CNTR) |
| Turntable and coin engine | AT89C51 (8051), 11.0592 MHz crystal, KIA7042 voltage-detector reset |
Decodes the turntable encoders' quadrature (P1_TT_S0/S1, P2_TT_S0/S1 on port 1) and serves absolute positions and the coin counter on its chip-selects |
| Coin input | 1N4148 series diode with RC into the MCU |
Diode-isolated and MCU-processed, not a directly buffered switch |
| Glue | 74LS14 hex inverter, 10 kΩ pull-up networks on all switch lines, 330 Ω series resistors on lamp and IDC lines |
Decoder count
The schematic shows two 74LS138 decoders. One measurement session during
this work suggested a third. A board revision difference is possible but
miscounting is the likelier explanation — the two parts are identical and
indistinguishable by marking, and were confused with each other repeatedly
during measurement. Recount on your own board if it matters to you.
This has no effect on the bus protocol or on any firmware.
Notes for replacement designs
Everything in the table above except the lamp drivers, the input pull-ups and
the connectors exists to interface an ISA card. The MCU, both decoders, all
bus transceivers and all output latches are unnecessary in a design that owns
the USB endpoint directly. See replacement-board.md.
The input pull-up and RC pattern and the ULN2803A lamp sinks are worth
retaining. They remain appropriate for the loads involved.
This page is synced from docs/board-architecture.md in the ez2-io repo.