DSUB 9 Pin Male Connector at the computer
DSUB 9 Pin Female Connector at the cassette cable
The TI-99/4A Home Computer (Texas Instruments, 1981) features a 9-pin cassette port that is unique among home computers of its era in supporting two independent cassette decks simultaneously — Cassette 1 (CS1) for reading only, and Cassette 2 (CS2) for both reading and writing. Data is encoded using FSK (Frequency Shift Keying) at 1200 baud, with approximately 689 Hz representing a space (0) and 1379 Hz representing a mark (1) at a 3 MHz system clock. All cassette signals are buffered through opto-isolators for electrical isolation, and motor control for each deck is managed independently via CRU (Communications Register Unit) bits addressed through the TMS 9901 programmable system interface.
| Pin | Name | Dir | Comment |
|---|---|---|---|
| 1 | Cass 1 motor control | OUT | CRU bit 22 |
| 2 | Dito | OUT | neg |
| 3 | Output to tape 2 | OUT | CRU bit 25 |
| 4 | Audio gate | OUT | CRU bit 24 |
| 5 | Output to tape 2 | OUT | neg |
| 6 | Cass 2 motor control | OUT | CRU bit 23 |
| 7 | Dito | OUT | neg |
| 8 | Input from tape 1 or 2 | IN | CRU bit 27 |
| 9 | Dito | IN | neg |
Notes
- Direction is computer relative to cassette.
- Cassette 1 (CS1) is read-only; Cassette 2 (CS2) supports both read and write.
- All signals are optically isolated, providing electrical separation between the computer and the cassette decks.
- Motor control for each deck is independent: CRU bit 22 drives CS1 motor, CRU bit 23 drives CS2 motor.
- The audio gate (CRU bit 24) must be enabled for audio output to reach the tape.
- Jitter tolerance for reliable operation is less than 60 ms peak-to-peak.
- Timing is controlled by the TMS 9901 programmable system interface.
- The connector is a 9-pin arrangement; the physical shell resembles a D-sub but carries analogue audio and relay-control signals rather than RS-232 levels.
Signals
- Cass 1 motor control (pins 1, 2) — Opto-isolated relay output controlling the motor of Cassette Deck 1; driven by CRU bit 22 (pin 1, positive) and its complementary negative return (pin 2).
- Output to tape 2 (pins 3, 5) — FSK audio data output to Cassette Deck 2 for recording; driven by CRU bit 25 (pin 3, positive) and negative return (pin 5). Encoding: approximately 689 Hz = space (0), 1379 Hz = mark (1) at 1200 baud.
- Audio gate (pin 4) — Enables the audio output path to the tape; controlled by CRU bit 24. Must be asserted during write operations.
- Cass 2 motor control (pins 6, 7) — Opto-isolated relay output controlling the motor of Cassette Deck 2; driven by CRU bit 23 (pin 6, positive) and its complementary negative return (pin 7).
- Input from tape 1 or 2 (pins 8, 9) — FSK audio data input shared between both cassette decks; read by CRU bit 27 (pin 8, positive) and negative return (pin 9). The active source (CS1 or CS2) is selected by software.
References
- TI-99/4A Technical Servicing Manuals and Programmer Guides — Texas Instruments, October 1983