Pinout diagram
Summary specifications
Full pin reference
| Pin | Name | Group | Direction | Levels | Description |
|---|---|---|---|---|---|
| 1 | CAP1A | Analog | Bidirectional | Analogue filter cap | Filter capacitor C1, terminal A. Connects across pins 1 and 2. |
| 2 | CAP1B | Analog | Bidirectional | Analogue filter cap | Filter capacitor C1, terminal B. C1 sits between pins 1 and 2; 470 pF to 2200 pF typical. |
| 3 | CAP2A | Analog | Bidirectional | Analogue filter cap | Filter capacitor C2, terminal A. Connects across pins 3 and 4. |
| 4 | CAP2B | Analog | Bidirectional | Analogue filter cap | Filter capacitor C2, terminal B. Match C1 and C2 for a tracked filter. |
| 5 | /RES | Control | Input | TTL, active low | Active low. Reset. Held low for at least 10 Phi 2 cycles to clear all registers and silence the output. |
| 6 | PHI2 | Clock / Timing | Input | TTL clock | Phi 2 system clock. All bus access is timed to this clock; data is valid only when Phi 2 is high. |
| 7 | R/W | Control | Input | TTL | Read / Write. High reads a register, low writes one. |
| 8 | /CS | Control | Input | TTL, active low | Active low. Chip select. Low enables the data bus; high tri-states D0 to D7. |
| 9 | A0 | Address Bus | Input | TTL | Register select bit 0. |
| 10 | A1 | Address Bus | Input | TTL | Register select bit 1. |
| 11 | A2 | Address Bus | Input | TTL | Register select bit 2. |
| 12 | A3 | Address Bus | Input | TTL | Register select bit 3. |
| 13 | A4 | Address Bus | Input | TTL | Register select bit 4. A0 to A4 select one of 32 register addresses. |
| 14 | GND | Ground | Power | 0 V | Ground. Run separately to the supply to keep digital noise off the analogue stages. |
| 15 | D0 | Data Bus | Bidirectional | TTL, 0 V / 5 V | Data bus bit 0. |
| 16 | D1 | Data Bus | Bidirectional | TTL, 0 V / 5 V | Data bus bit 1. |
| 17 | D2 | Data Bus | Bidirectional | TTL, 0 V / 5 V | Data bus bit 2. |
| 18 | D3 | Data Bus | Bidirectional | TTL, 0 V / 5 V | Data bus bit 3. |
| 19 | D4 | Data Bus | Bidirectional | TTL, 0 V / 5 V | Data bus bit 4. |
| 20 | D5 | Data Bus | Bidirectional | TTL, 0 V / 5 V | Data bus bit 5. |
| 21 | D6 | Data Bus | Bidirectional | TTL, 0 V / 5 V | Data bus bit 6. |
| 22 | D7 | Data Bus | Bidirectional | TTL, 0 V / 5 V | Data bus bit 7. |
| 23 | POTY | Analog | Input | Analogue paddle | Paddle Y input. An RC charge time on this pin is digitised to read a potentiometer. |
| 24 | POTX | Analog | Input | Analogue paddle | Paddle X input. Same RC ADC as POTY for the second axis. |
| 25 | VCC | Power | Power | +5 V | Logic supply. Decouple with 100 nF close to the pin. |
| 26 | EXT IN | Analog | Input | Analogue, max 0.6 V | External audio input. Mixed with the internal voices before the filter; keep below 0.6 V. |
| 27 | AUDIO OUT | Analog | Output | Analogue, rides at 6 V DC | Audio output. AC-couple with 1 to 10 uF; the pin sits at about 6 V DC. |
| 28 | VDD | Power | Power | +12 V | Analogue supply. Decouple separately from VCC; the 8580 uses 9 V here, not 12 V. |
Common uses and repair diagnostics
The MOS 6581 is the Sound Interface Device at the heart of the Commodore 64: three programmable oscillators, a four-channel envelope generator, a multimode filter and two paddle ADCs, all addressed through 29 registers. It runs from a 5 V logic supply and a separate 12 V analogue supply, and its filter behaviour is so tied to the 6581 die revision that the 8580 — same pinout, different sound — is not a drop-in substitute.
The 6581 is the voice of the Commodore 64. The CPU writes frequency, waveform, envelope and filter settings into its 29 registers over the 5 V bus, and the chip produces a single audio output at pin 27 that the board AC-couples to the RF modulator and the audio jack. The two paddle ADCs at pins 23 and 24 are the same inputs the C64 uses to read its analogue joysticks and mice.
On the repair bench the SID is a common casualty: it runs warm, it sits between two supplies, and a failed 12 V regulator or a reversed power feed takes it out. The classic symptom is a C64 that boots but is silent, or one whose sound is badly distorted — usually a dead filter or a dried-out C1/C2 capacitor. The 8580 has the same pinout and will plug in, but it sounds different and expects 9 V on VDD, so it is not a true replacement for a 6581 board.
- Power up and check pin 25 (VCC) at 5 V and pin 28 (VDD) at 12 V. A missing 12 V rail is the most common cause of a silent SID.
- Scope pin 27 (AUDIO OUT) while playing a tone; it should ride at about 6 V DC with the audio riding on top. A flat 0 V or 12 V means the output stage has failed.
- Check pins 1 to 4 for the filter capacitors; an open or missing C1/C2 gives a thin, buzzy sound with no filter action.
- Probe pin 6 (PHI2) for a 1 MHz clock and pin 8 (CS) for activity during a register write; no CS means the PLA or address decode is at fault, not the SID.
- Feel the chip after a few minutes. A 6581 runs warm normally, but one that is too hot to touch has usually suffered internal damage and must be replaced.