Signals
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Most names come from the official schematics.
Some help to decipher the names:
- / means the signal is active low.
- OE means Output Enable.
- U is used for Upper (even) byte.
- L is used for Lower (odd) byte.
- B as suffix often means the signal is inverted.
Clocks
| Name | Description | Comes from | Fault consequence |
| 24M | 24MHz master clock | NEO-D0 | Nothing works, no video at all |
| 12M | 12MHz clock (24/2) non-inverted | NEO-D0 | Bad graphics |
| 8M | 8MHz clock (24/3) non-inverted | LSPC | No sound |
| 6MB | 6MHz clock (24/4) inverted | NEO-D0 | Black screen ? |
| 4M | 4MHz clock (24/6) non-inverted | LSPC | No sound |
| 4MB | 4MHz clock (24/6) inverted | LSPC + inverter | ? |
| 1MB | 3MHz (!) clock (24/8) inverted | NEO-D0 | ? |
| 68KCLK | 12MHz clock (24/2) non-inverted | NEO-D0 | Click of death |
| 68KCLKB | 12MHz clock (24/2) inverted | NEO-D0 | Bad audio in games using PCM |
68k CPU
| Name | Description | Type | Comes from | Fault consequence |
| A1~A23 | 68k address bus | Address | 68k | Click of death |
| A22I,A23I | Altered address lines for 68k vector table swap | Address | NEO-E0 | Probably click of death |
| /AS | 68k Address Strobe | Control | 68k | Probably click of death |
| D0~D15 | 68k data bus | Data | Multiple | Click of death |
| /DTACK | 68k Data Transmit ACKnowledge (see wait cycle) | Control | NEO-C1 | Click of death |
| /PORTADRS | $200000-$2FFFFF (P2 ROM/Security chip) any access | Control | NEO-C1 | Game crashes, bad graphics |
| /PORTOEL | $200000-$2FFFFF (P2 ROM/Security chip) odd byte read | Control | NEO-C1 | Game crashes, bad graphics |
| /PORTOEU | $200000-$2FFFFF (P2 ROM/Security chip) even byte read | Control | NEO-C1 | Game crashes, bad graphics |
| /PORTWEL | $200000-$2FFFFF (P2 ROM/Security chip) odd byte write | Control | NEO-C1 | Game crashes, multiplayer comm. error |
| /PORTWEU | $200000-$2FFFFF (P2 ROM/Security chip) even byte write | Control | NEO-C1 | Game crashes, bad graphics |
| /PWAIT0, /PWAIT1 | Delay configuration for P2 ROM reads, see wait cycle | Control | NEO-C1 | Game crashes (very likely) |
| R/W | 68k Read/Write | Control | 68k | Click of death |
| /ROMOE | $000000-$0FFFFF (P1 ROM) read | Control | NEO-C1 + AND | Game doesn't start |
| /ROMOEL | $000000-$0FFFFF (P1 ROM) odd byte read | Control | NEO-C1 | Game doesn't start |
| /ROMOEU | $000000-$0FFFFF (P1 ROM) even byte read | Control | NEO-C1 | Game doesn't start |
| /ROMWAIT | Add 1-cycle delay for P1 ROM reads | Control | NEO-C1 | Game freezes (unlikely) |
| /SROMOE | $C00000-$CFFFFF (System ROM) read | Control | NEO-E0 | Click of death |
| /SROMOEL | $C00000-$CFFFFF (System ROM) odd byte read | Control | NEO-C1 | Click of death |
| /SROMOEU | $C00000-$CFFFFF (System ROM) even byte read | Control | NEO-C1 | Click of death |
Graphics
| Name | Description | Type | Comes from | Fault consequence |
| 2H1 | S ROM A3 | Address | LSPC | Bad fix graphics |
| CA4 | C ROMs A4 | Address | LSPC | Bad sprite graphics |
| C* or CR0~CR31 | C ROMs data bus (2*16bits), data for one 8-pixels line | Data | C ROM | Bad sprite graphics |
| FIXD0~FIXD7 | Fix layer data bus, data for 2 pixels | Data | S ROM | Static vertical lines, solid color screen |
| P0~P23 | Multiplexed P bus | Muxed | Multiple | Garbled or absent graphics |
| PA0~PA11 | Palette RAM address bus | Address | NEO-B1 | Bad colors, snow, black screen |
| PAL | Palette RAM decode for 68k access | Control | NEO-C1 | Bad colors, snow, black screen |
| PCK1, PCK2 | Graphics rendering control | Control | LSPC2-A2 | Bad graphics, solid colors, black screen |
| PCK1B, PCK2B | Graphics rendering control | Control | AS04 inverters | Bad graphics |
- PCK1B: Low 55ns, high 610 (1.5MHz) Clock to latch C ROM address from P0~P23 (mapping) on rising edge.
- PCK2B: Low 55ns, high 610 (1.5MHz) Clock to latch S ROM address from P0~P15 (mapping) on rising edge.
Misc.
| Name | Description | Type | Comes from | Fault consequence |
| /DIPRD0 | $300000~$31FFFF odd byte reads | Control | NEO-C1 | Can't read hardware DIPs, test button state and system type. See REG_DIPSW and REG_SYSTYPE. |
| /DIPRD1 | $320000~$33FFFF odd byte reads | Control | NEO-C1 | Coins aren't detected, RTC access problems. See REG_STATUS_A. |
| /BITW0 | $380000~$39FFFF odd byte writes | Control | NEO-C1 | No joypad output control, memory card access problems, MVS slot selection problems, erratic credits displays and marquee control, RTC access problems, no coin counters and coin lockouts control. See Memory mapped registers. |
| /BITW1 | $3A0000~$3BFFFF odd byte writes | Control | NEO-C1 | Spurious or impossible writes to system registers. |
| /BITWD0 | $3A0000~$3BFFFF odd byte writes | Control | NEO-F0 | No joypad output control, memory card access problems. See REG_POUTPUT and REG_CRDBANK. Depends on /BITW0. |
| BNK0~2 | Memory card bank selection | Control | NEO-D0 | Access problems with big memory cards. |
- SYSTEM: low when onboard ROMs selected
- SYSTEMB: inverted SYSTEM
Z80 CPU
- SDA0~SDA15 : Z80 address bus
- SDD0~SDD7: Z80 data bus
Sound
- SDRAD0~SDRAD7: ADPCM-A ROM multiplexed bus (data/address)
- SDRA8,SDRA9,SDRA20~SDRA23: ADPCM-A ROM address bus
- SDPAD0~SDPAD7: ADPCM-B ROM multiplexed bus (data/address)
- SDPA8,SDPA9,SDPA10,SDPA11: ADPCM-B ROM address bus