The TI-99/4A (Texas Instruments, 1981) keyboard connects to the console mainboard via an internal 15-pin ribbon connector; pin 1 is marked on the PCB and pin 15 is identified by the red wire. The keyboard forms an 8-row × 7-column passive scan matrix with no isolation diodes; it is scanned entirely by the TMS9901 Programmable Systems Interface IC through the CRU (Communications Register Unit) bus shared with the TMS9900 CPU. Rows are read on CRU bits 3–10 (R12 address >0006) and the active column is selected by writing a 3-bit value to CRU bits 18–20 (R12 address >0024); a pressed key returns 0 (active-low) and an open key returns 1. The governing reference is Texas Instruments document number 1049717-2, published 1983.
| Pin | 12 | 13 | 14 | 15 | 9 | 8 | 6 |
|---|---|---|---|---|---|---|---|
| 5 | = | . | , | M | N | / | |
| 4 | space | L | K | J | H | ; | |
| 1 | enter | O | I | U | Y | P | |
| 2 | 9 | 8 | 7 | 6 | 0 | ||
| 7 | fctn | 2 | 3 | 4 | 5 | 1 | lock |
| 3 | shift | S | D | F | G | A | |
| 10 | ctrl | W | E | R | T | Q | |
| 11 | X | C | V | B | Z |
Notes
- Pin 1 is marked on the PCB; pin 15 is identified by the red wire.
- The table rows are keyboard matrix row lines (connected to TMS9901 input pins INT3*–INT10*); the table columns are the column select lines driven by a 74LS138 decoder controlled by the TMS9901 output pins P2–P4.
- Pressing a key closes the contact between the corresponding row and column line.
- No isolation diodes are fitted. Pressing three keys simultaneously can generate a phantom fourth keypress at the fourth corner of the rectangle formed by those keys (e.g. pressing keys at rows 8, 7 and 3 with a shared column may assert a fourth row combination through the uninhibited current path).