The TI-99/4A Cartridge Port is a 36-pin edge connector located on the top of the Texas Instruments TI-99/4A home computer, introduced in 1981. It accepts the proprietary Solid State Software Command Module cartridges and exposes address lines A3–A15, the 8-bit bidirectional data bus D0–D7, bus control signals, and four GROM-specific lines. The connector carries two predecoded select outputs: ROMG* (active low for addresses >6000–7FFF, selecting cartridge ROM or RAM) and GS* (active low for addresses >9800–9FFF, selecting on-cartridge GROMs). GROMs (Graphic Read-Only Memories, e.g. TMC0430) use a multiplexed serial interface with an internal auto-incrementing address counter; up to eight GROMs sharing one bus are distinguished by a 3-bit base address, giving 64 KB of addressable GROM space per cartridge. The governing technical reference is the Texas Instruments document “TI-99/4A Console and Peripheral Expansion System Technical Data” (part no. 1049717-2, 1983), whose schematic fold-outs define the full pin assignment.
| Pin | Name | Dir | Description |
|---|---|---|---|
| 1 | RESET | IN | Resets the system (active high) |
| 2 | GND | - | Signal groud |
| 3 | D7 | IN/OUT | Data bus, bit 7 (least significant) |
| 4 | CRUCLK* | OUT | Inversion of TMS9900 CRUCLOCK pin |
| 5 | D6 | IN/OUT | |
| 6 | CRUIN | IN | CRU input to TMS9900 |
| 7 | D5 | IN/OUT | |
| 8 | A15 | OUT | Address bus, bit 15 / also CRU output bit |
| 9 | D4 | IN/OUT | |
| 10 | A13 | OUT | |
| 11 | D3 | IN/OUT | |
| 12 | A12 | OUT | |
| 13 | D2 | IN/OUT | |
| 14 | A11 | OUT | |
| 15 | D1 | IN/OUT | |
| 16 | A10 | OUT | |
| 17 | D0 | IN/OUT | Data bus, bit 0 (most significant) |
| 18 | A9 | OUT | |
| 19 | VCC | - | +5 Volts power supply |
| 20 | A8 | OUT | |
| 21 | GS* | OUT | Grom select. Active low is addr in >9800-9FFF |
| 22 | A7 | OUT | |
| 23 | A14 | OUT | Address bus, bit 14. Select mode: low=data / high=addr |
| 24 | A3 | OUT | |
| 25 | DBIN | OUT | Active high = read memory |
| 26 | A6 | OUT | |
| 27 | GRC | OUT | GROM clock: color burst of VDP 9918A |
| 28 | A5 | OUT | |
| 29 | VDD | - | -5 Volts power supply |
| 30 | A4 | OUT | |
| 31 | GR | IN | Active high = GROM ready |
| 32 | WE* | OUT | Active low = write enable (derived from TMS9900 WE*) |
| 33 | VSS | - | |
| 34 | ROMG* | OUT | Active low if addr in >6000-7FFF |
| 35 | GND | - | Ground |
| 36 | GND | - | Ground |
Notes
The cartridge port is an edge connector on the top surface of the console. Cartridges insert from above with the PCB edge making contact with the connector fingers. Address lines A0–A2 are not present on the cartridge port; the lowest address line available is A3. The connector supplies +5 V (VCC, pin 19), −5 V (VDD, pin 29), VSS (pin 33), and two ground returns (pins 35–36). Signal lines are driven by 74LS-series TTL logic in the console; the maximum capacitive load per signal line is 90 pF for address and data lines. Cartridge ROM or RAM occupies the predecoded address range >6000–7FFF (8 KB); on-cartridge GROMs are accessed via >9800–9FFF.
Signals
- RESET — System reset, active high; resets the TMS9900 CPU and peripherals.
- D0–D7 — 8-bit bidirectional data bus; D0 is the most significant bit (TMS9900 big-endian convention), D7 is the least significant.
- A3–A15 — Address bus outputs from the TMS9900 CPU; A0–A2 are not routed to this connector.
- A15 — Also serves as the CRU serial output bit (CRUOUT) during CRU output instructions.
- A14 — Selects GROM operating mode: low = data access, high = address access.
- CRUCLK* — Inverted TMS9900 CRUCLOCK signal; clocks the CRU serial interface.
- CRUIN — CRU serial data input to the TMS9900.
- DBIN — Active high; indicates a memory read cycle.
- WE* — Active low write enable, derived from the TMS9900 WE* pin.
- GS* — GROM select, active low; asserted by the console when the address is in the range >9800–9FFF.
- GRC — GROM clock output, 447 kHz, derived from the VDP TMS9918A colour burst oscillator.
- GR — GROM ready, active high; a GROM asserts this line when it has completed an access cycle.
- ROMG* — ROM/RAM select, active low; asserted when the address is in the range >6000–7FFF, enabling cartridge ROM or RAM.
- VCC — +5 V supply (pin 19).
- VDD — −5 V supply (pin 29), required by GROM chips.
- VSS — Ground reference for GROM chips (pin 33).
- GND — Ground returns (pins 2, 35, 36).
References
- TI-99/4A Console and Peripheral Expansion System Technical Data (part no. 1049717-2) — Texas Instruments, 1983
- TI-99/4A Console and Peripheral Expansion System Technical Data (PDF mirror) — Texas Instruments, 1983
- TI-99/4A Pinouts — Thierry Nouspikel’s TI-99/4A Tech Pages
- GROMs — Thierry Nouspikel’s TI-99/4A Tech Pages