Keybed Scanning

The KN5000’s 61-key velocity-sensitive keyboard connects directly to the tone generator IC303 (TC183C230002), which performs hardware key scanning internally. The Sub CPU reads completed note events from IC303’s keyboard output interface — the CPU does not scan the keyboard matrix itself.

Status: Note encoding and voice slot management fully reverse-engineered from SubCPU firmware. HLE keybed device implemented in MAME driver.

Architecture

The note flow is bidirectional — keybed events travel through the Sub CPU to the Main CPU (for display and MIDI output), then note-on commands travel back from the Main CPU through the Sub CPU to IC303 (for sound generation):

┌──────────────────────────────────────────────────────────────────┐
│                        NOTE EVENT FLOW                           │
│                                                                  │
│  ┌─────────┐    ┌──────────┐    ┌──────────┐    ┌──────────┐     │
│  │ KEYBED  │───>│  IC303   │───>│ SUB CPU  │───>│ MAIN CPU │     │
│  │ (keys)  │    │ ToneGen  │    │ firmware │    │ firmware │     │
│  └─────────┘    └──────────┘    └──────────┘    └──────────┘     │
│                   0x110000        DMA latch       Display,       │
│                  (HW scan)       @ 0x120000       MIDI out       │
│                                                                  │
│                 ┌──────────┐    ┌──────────┐    ┌──────────┐     │
│                 │  IC303   │<───│ SUB CPU  │<───│ MAIN CPU │     │
│                 │ ToneGen  │    │ firmware │    │ firmware │     │
│                 └──────────┘    └──────────┘    └──────────┘     │
│                   0x100000        DMA latch     Sound select,    │
│                  (voice cfg)     @ 0x120000     note routing     │
└──────────────────────────────────────────────────────────────────┘

Forward path (keybed to display):

  1. Physical key press detected by IC303 hardware scanner
  2. IC303 presents note/velocity at 0x110000, sets status bit at 0x110002
  3. ToneGen_Read_Voice_Data reads the event and allocates a voice slot
  4. InterCPU_DMA_Send transmits [0x90, note, velocity] to Main CPU via latch
  5. Main CPU updates display (key indicators, voice allocation) and emits MIDI

Return path (sound generation):

  1. Main CPU sends note-on command via latch to Sub CPU
  2. Sub CPU Voice_NoteOn configures IC303 voice parameters at 0x100000
  3. IC303 begins waveform playback from ROMs IC304-IC307

Hardware Interface

Address Width Direction Purpose
0x110002 16-bit Read Status register
0x110000 16-bit Read Voice data (note + velocity)

Status Register (0x110002)

Bit Name Description
0 DATA_READY 1 = note event available for reading
1 MODE 0 = note-on context (normal operation)
15:2 — Reserved / unused

Data Register (0x110000)

Bits Name Description
7:0 RAW_NOTE Note number (see encoding below). Bit 7 = “has velocity” flag
15:8 VELOCITY Velocity value. 0xFF = note-off

Note Encoding

Raw Note to MIDI Note

ToneGen_Calc_Pitch (at 0x03D11F) adds a fixed offset of 0x24 (36) to convert the raw note number to a MIDI note:

MIDI_note = raw_note + 0x24
Raw Note MIDI Note Name Octave
0 36 C2 2
12 48 C3 3
24 60 C4 (Middle C) 4
36 72 C5 5
48 84 C6 6
60 96 C7 7

The KN5000 has 61 keys: C2 (raw 0) through C7 (raw 60).

Note-On Event Format

When a key is pressed with measurable velocity:

data_word = (velocity << 8) | (raw_note | 0x80)
status: bit 0 = 1 (data ready), bit 1 = 0 (note-on context)
  • Bit 7 of the low byte is SET (0x80 OR’d in), indicating “has velocity data”
  • Velocity range: 0x01-0xFE (1-254), where higher = harder press

Note-Off Event Format

When a key is released:

data_word = (0xFF << 8) | raw_note
status: bit 0 = 1 (data ready)
  • Bit 7 of the low byte is CLEAR (raw note value only)
  • Velocity byte = 0xFF triggers the note-off code path in ToneGen_Read_Voice_Data

Voice Slot Table

The Sub CPU maintains a 16-entry voice slot table at RAM address 0x4A4C:

Address Size Description
0x4A4A 2 bytes DMA enable flag (non-zero = DMA relay active)
0x4A4C 16 bytes Voice slot states (one byte per slot)

Each slot byte:

  • 0xFF = note active (slot in use)
  • 0x00 = slot available

When a note-on event arrives, ToneGen_Read_Voice_Data scans the 16 slots for an available one. When a note-off arrives, the corresponding slot is freed.

DMA Packet Format

After processing a note event, the Sub CPU relays it to the Main CPU via the inter-CPU latch using InterCPU_DMA_Send:

Note-On Packet

Byte 0: 0x90 (MIDI Note On status)
Byte 1: MIDI note number (raw_note + 0x24)
Byte 2: velocity (0x01-0xFE)

Note-Off Packet

Byte 0: 0x90 (MIDI Note On status — velocity 0 = note off per MIDI convention)
Byte 1: MIDI note number (raw_note + 0x24)
Byte 2: 0x00 (zero velocity = note off)

Key Firmware Routines

Routine Address Description
ToneGen_Init 0x03D016 Set tone generator mode to 6, begin polling
ToneGen_Process_Notes 0x03D01E Main loop: read and process all pending events
ToneGen_Read_Voice_Data 0x03D0C5 Read one event from 0x110000, manage voice slots
ToneGen_Calc_Pitch 0x03D11F Convert raw note to pitch value (adds 0x24)
ToneGen_Poll_Init 0x03D227 Initial polling: read 16 slots with delay loops
ToneGen_Config_Init 0x02DFCF Configure all 64 IC303 voices and global registers
InterCPU_DMA_Send (varies) Send 3-byte note packet to Main CPU via latch

MAME HLE Implementation

Since IC303 is a custom ASIC with no public documentation, the MAME driver uses an HLE (High-Level Emulation) device to inject keybed events:

  • 6 input ports (KEY0-KEY5) map PC keyboard keys to 61 piano notes
  • A 1ms scan timer compares current key states against previous states
  • Key press/release transitions generate events in the IC303 output format
  • Events are queued and served when the SubCPU reads 0x110000/0x110002

The HLE produces no sound (IC303 voice parameter writes at 0x100000 are discarded), but the full note event pipeline works end-to-end.

PC Keyboard Mapping

The keybed notes are available as MAME input entries (visible in the Tab menu under “Input (This Machine)”) but have no default PC keyboard assignments because all candidate keys conflict with control panel button mappings.

To play notes, use MAME’s input configuration UI (Tab menu) to assign keys. A suggested piano layout:

Lower octave (Z row — assign to KEY2 / C4-B4):

Suggested Key Note MAME Input Name
Z C4 C4
S C#4 C#4
X D4 D4
D D#4 D#4
C E4 E4
V F4 F4
G F#4 F#4
B G4 G4
H G#4 G#4
N A4 A4
J A#4 A#4
M B4 B4

Upper octave (Q row — assign to KEY3 / C5-B5):

Suggested Key Note MAME Input Name
Q C5 C5
2 C#5 C#5
W D5 D5
3 D#5 D#5
E E5 E5
R F5 F5
5 F#5 F#5
T G5 G5
6 G#5 G#5
Y A5 A5
7 A#5 A#5
U B5 B5

Note: Assigning these keys will also trigger control panel buttons that share the same keys. This is a known limitation — the control panel and keybed share the same physical keyboard namespace.

Fixed velocity of 100 for all key presses (PC keyboards have no velocity sensitivity).