Pinout diagram
Summary specifications
Full pin reference
| Pin | Name | Group | Direction | Levels | Description |
|---|---|---|---|---|---|
| 1 | FE | Control | Input | TTL | Field Enable / programming pin. Leave it open or grounded in normal operation; it is only used during programming. |
| 2 | I7 | I/O | Input | TTL | Input 7. In the C64: connected to A13 of the address bus. Inputs I7 down to I0 are on pins 2 to 9. |
| 3 | I6 | I/O | Input | TTL | Input 6. In the C64: connected to A14. |
| 4 | I5 | I/O | Input | TTL | Input 5. In the C64: connected to A15. |
| 5 | I4 | I/O | Input | TTL | Input 4. In the C64: connected to /VA14 on the VIC-II. |
| 6 | I3 | I/O | Input | TTL | Input 3. In the C64: connected to /CHAREN on the 6510 I/O port. |
| 7 | I2 | I/O | Input | TTL | Input 2. In the C64: connected to /HIRAM on the 6510 I/O port. |
| 8 | I1 | I/O | Input | TTL | Input 1. In the C64: connected to /LORAM on the 6510 I/O port. |
| 9 | I0 | I/O | Input | TTL | Input 0. In the C64: connected to /CAS on the VIC-II. |
| 10 | F7 | I/O | Tri-state output | TTL, three-state | Output 7. In the C64: /ROMH. The outputs are split either side of the ground pin and run high-to-low: F7 to F4 on pins 10-13, then F3 to F0 on pins 15-18. |
| 11 | F6 | I/O | Tri-state output | TTL, three-state | Output 6. In the C64: /ROML. |
| 12 | F5 | I/O | Tri-state output | TTL, three-state | Output 5. In the C64: /I/O. |
| 13 | F4 | I/O | Tri-state output | TTL, three-state | Output 4. In the C64: GR/W, connected to /WE on the colour RAM. |
| 14 | GND | Ground | Power | 0 V | Ground. The outputs are split either side of this pin: F7 to F4 above, F3 to F0 below. |
| 15 | F3 | I/O | Tri-state output | TTL, three-state | Output 3. In the C64: connected to /CS1 on the CHAROM. |
| 16 | F2 | I/O | Tri-state output | TTL, three-state | Output 2. In the C64: connected to /CS on the KERNAL ROM. |
| 17 | F1 | I/O | Tri-state output | TTL, three-state | Output 1. In the C64: connected to /CS on the BASIC ROM. |
| 18 | F0 | I/O | Tri-state output | TTL, three-state | Output 0. In the C64: /CASRAM, connected to the /CAS pin on the DRAM. |
| 19 | /CE | Control | Input | TTL, active low | Active low. Chip Enable. Low enables the outputs; high floats all eight three-state outputs. |
| 20 | I15 | I/O | Input | TTL | Input 15. In the C64: connected to VA12 on the VIC-II. Inputs I15 down to I8 are on pins 20 to 27. |
| 21 | I14 | I/O | Input | TTL | Input 14. In the C64: connected to VA13 on the VIC-II. |
| 22 | I13 | I/O | Input | TTL | Input 13. In the C64: connected to /GAME on the cartridge port. |
| 23 | I12 | I/O | Input | TTL | Input 12. In the C64: connected to /EXROM on the cartridge port. |
| 24 | I11 | I/O | Input | TTL | Input 11. In the C64: connected to R/W of the bus. |
| 25 | I10 | I/O | Input | TTL | Input 10. In the C64: connected to /AEC, the inverted address enable from the VIC-II. |
| 26 | I9 | I/O | Input | TTL | Input 9. In the C64: connected to /CAS on the VIC-II. |
| 27 | I8 | I/O | Input | TTL | Input 8. In the C64: connected to A12 of the address bus. |
| 28 | VCC | Power | Power | +5 V | Positive supply. Decouple with 100 nF close to the pin. |
Common uses and repair diagnostics
The Signetics 82S100 is a bipolar fuse-link field-programmable logic array: 16 inputs, 8 three-state outputs and 48 product terms. In the Commodore 64 it is the PLA, Commodore part 906114-01, and it does all the memory decoding — which ROM, RAM or I/O block answers a given address. It is one of the most failure-prone parts in a C64: it runs hot, and a failing PLA typically gives a black screen, a garbled display, or a machine that works cold and fails after ten minutes.
The 82S100 is the C64 PLA, Commodore part 906114-01. It does all the memory decoding — which ROM, RAM or I/O block answers a given address — and it is one of the most failure-prone parts in a C64. It runs hot, and a failing PLA typically gives a black screen, a garbled or flickering display, or a machine that works cold and fails after ten minutes. Warming it with a hairdryer or chilling it with freeze spray to change the symptom is the classic field test.
Modern drop-in replacements exist — GAL-based PLAnkton, PLA20V8 and similar — which run far cooler and are the practical repair. The 82S100 is a generic part, so a blank one can be programmed from the published C64 fuse map, which is why PLA repair is DIY-able. Note the output order: F7 to F4 on pins 10-13, then GND on 14, then F3 to F0 on pins 15-18 — the outputs are split either side of the ground pin and run high-to-low throughout.
- Feel the PLA after a few minutes. It runs hot normally, but one that is too hot to touch has usually failed.
- Warm it with a hairdryer or chill it with freeze spray. If the symptom changes, the PLA is the prime suspect.
- Black screen on power-up with no other obvious fault: probe the PLA outputs (pins 10-13, 15-18) for activity. A dead PLA gives a black screen.
- A machine that works cold and fails after ten minutes is a classic PLA failure — the fuse links drift with temperature.
- If replacing, a GAL-based PLAnkton or PLA20V8 runs far cooler and is the recommended modern substitute.