ZX81
Four chips and a ULA.
Open the ZX81 in Retro Bench- Also
- ZX81 (UK), Timex Sinclair 1000 (US, see ts1000)
- Year
- 1981
- CPU
- Z80A @ 3.25 MHz
- RAM
- 1 KB base (2 x 2114 or 1 x 4118); 16 KB via RAM pack
- ROM
- 8 KB BASIC (floating point)
- Video
- Monochrome, 32 x 24 text, 64 x 44 block graphics
- Sound
- None
- OS
- Sinclair BASIC 8K
Chips
| Chip | Where | Role |
|---|---|---|
| ZX81 ULA 2C184E / 2C210E | IC1 | Video sync, character access, keyboard, tape, NMI, ROM/RAM select |
| Z80A CPU | IC3 | Main CPU (8-bit, 40-DIP) |
| 2114 SRAM 1K x 4 | IC4a, IC4b (some boards fit one 1K x 8 4118 at IC4 instead) | Static RAM (ZX80 and ZX81 1K) |
| MK4118 SRAM 1K x 8 | IC4, the 24-pin option in place of the 2114 pair | Static RAM: the single-chip 1 KB option of the ZX81 |
| ZX81 8K BASIC ROM (2364 type) | IC2 | BASIC, editor, character set and floating point at $0000-$1FFF |
Ports and connectors
- Power in (9V DC), 3.5 mm mono jack
- EAR (tape in), 3.5 mm mono jack
- MIC (tape out), 3.5 mm mono jack
- TV (UHF RF out), Phono (Cinch) socket, RF
- Rear edge connector (2 x 23), Card edge, 23 + 23 contacts, keyway at 3
Power rails
| Rail | Expect | Measure at |
|---|---|---|
| +9 V DC in | +7 to +11 V DC on load | Power jack; C3 (22 uF) at the regulator input; zx81-edge pin B2 |
| +5 V | +5 V (7805) | regulator output; Z80A pin 11; ULA pin 40; 2114 pin 18; ROM pin 24 |
| GND | 0 V | Z80A pin 29; ULA pin 34; 2114 pin 9; ROM pin 12 |
Symptoms
- No picture, no boot: check ULA, Z80A
- Black bar across the screen or garbage: check 2114, ROM
- Wobbly RAM pack (crashes when touched): check 2114
- Dead key or a dead row: check ULA
- Keyboard membrane: dead keys: check ULA
- Hot ULA, no display: check ULA
- Stripes through the picture: check ULA
- Random resets: check Z80A