TI-99/4A
TMS9918A video, TMS9919 sound, TMS9901 I/O.
Open the TI-99/4A in Retro Bench- Also
- 99/4A
- Year
- 1981
- CPU
- TMS9900 @ 3 MHz
- RAM
- 256 B scratchpad (2 x MCM6810) + 16 KB VDP RAM (8 x 4116)
- ROM
- 8 KB console ROM (2 x TMS4732) + 18 KB GROM (3 x TMC0430)
- Video
- TMS9918A (NTSC) or TMS9929A (PAL): 32 x 24 text, 16 colors
- Sound
- TMS9919 (SN94624) or SN76489: 3 tones + noise
- OS
- TI BASIC (GPL)
Chips
| Chip | Where | Role |
|---|---|---|
| TMS9900 16-bit CPU | U600 | 16-bit CPU at 3 MHz. Needs -5 V, +5 V and +12 V, plus a 4-phase clock. |
| TIM9904 (SN74LS362) four-phase clock generator | U601 | Makes the four-phase 12 V clock for the 9900 (3 MHz) and the power-on reset. |
| TMS9918A / TMS9928A / TMS9929A video display processor | U100 | Video display processor (VDP) with its own 16 KB DRAM bus. Composite out on 9918A; Y, R-Y, B-Y on 9928A/9929A. |
| 4116 DRAM 16K x 1 | U102-U109 (VRAM) | Dynamic RAM, 16K x 1 (lower 16K on the Spectrum) |
| TMS9901 programmable systems interface | U300 | Keyboard scan, joysticks, cassette, interrupts and interval timer, over the CRU serial bus. |
| SN76489 / SN94624 (TMS9919) sound generator | U511 | Three tone voices and one noise voice, written over an 8-bit data bus. |
| TMC0430 GROM (graphics ROM) | U500-U502 (GROM 0-2) | 6 KB serial ROM for TI BASIC, the title screen and cartridge code. |
| TMS4732 console ROM (4K x 8) | U610 (even byte, D0-D7), U611 (odd byte, D8-D15) | Console ROM at $0000-$1FFF: two 4 KB chips, one per byte of the 16-bit word. |
| MCM6810 128 x 8 static RAM | U608 (even byte), U609 (odd byte) | The 256-byte CPU scratchpad at $8300: two chips, one per byte of the word. |
Ports and connectors
- Power in (from the transformer), 4-pin power receptacle UNVERIFIED
- Video / audio out (monitor or RF modulator), DIN 5-pin 180°
- Joystick port (both wired joysticks), DE-9
- Cassette port (CS1 and CS2), DE-9
- I/O port (right side, speech and Peripheral Expansion), 44-contact card edge
- Cartridge (GROM) port, 36-contact card-edge socket UNVERIFIED
- Regulator board output (to the logic board), 4-pin Molex (internal)
Power rails
| Rail | Expect | Measure at |
|---|---|---|
| -5 V | about -5.15 V (Sams reading) | TMS9900 pin 1 |
| +5 V | about +5.14 V (Sams reading) | TMS9900 pin 2 |
| +12 V | about +11.84 V (Sams reading) | TMS9900 pin 27 |
| Unregulated B+ | about 21.5 V DC | L6 on the regulator board |
Symptoms
- Power LED off, nothing works
- No title screen: check TMS9900, TIM9904, MCM6810, TMS4732
- Dead console, no title screen: check TMS9900, TIM9904
- No picture, or garbled picture: check TMS9918A, 4116
- Bad color: check TMS9918A
- Dead keys or a dead key row: check TMS9901
- No cassette, or stuck interrupts: check TMS9901
- No sound, or distorted sound: check SN76489
- Won't boot, or garbled graphics: check GROM, TMS4732