60-pin card-edge connector.
The TI-99/4A Peripheral Expansion Box (PE Box), introduced alongside the TI-99/4A home computer in the early 1980s, uses a 60-pin card-edge bus to connect expansion cards. The connector carries an 8-bit data bus (D0–D7), a 16-bit address bus (A0–A15), and the TI-99/4A’s unique Communications Register Unit (CRU) serial I/O lines. Power is supplied as unregulated voltages — approximately +8 V, −16 V, and +16 V — that each expansion card must regulate locally to ±5 V or ±12 V as required. The bus is defined in Texas Instruments’ 1983 document Console And Peripheral Expansion System Technical Data, which remains the primary hardware reference for card developers.
| Pin | Name | Dir | Description |
|---|---|---|---|
| 1 | +5V 3-T regulator voltage supply (about +8V) | ||
| 2 | - | Dito | |
| 3 | GND | - | Ground |
| 4 | READYA | - | System ready (10K pull-up to +5V) |
| 5 | GND | - | Ground |
| 6 | RESET* | OUT | System reset (active low) |
| 7 | GND | - | Ground |
| 8 | SCLK | nc | System clock (not connected) |
| 9 | LCP* | nc | CPU indicator 1=TI99 0=2nd generation (not connected) |
| 10 | AUDIO | IN | Input audio (=AUDIOIN) |
| 11 | RDBENA* | IN | Active low: enable flex cable data bus drivers (1K pull-up) |
| 12 | PCBEN | H | PCB enable for burn-in (always High) |
| 13 | HOLD* | H | Active low CPU hold request (always High) |
| 14 | IAQHA | nc | IAQ [or] HOLDA (logical or) |
| 15 | SENILA* | H | Interrupt level A sense enable (always High) |
| 16 | SENILB* | H | Interrupt level B sense enable (always High) |
| 17 | INTA* | IN | Active low interrupt level A (=EXTINT*) |
| 18 | LOAD* | nc | Unmaskable interrupt (not connected) |
| 19 | D7 | IN/OUT | Data bus, bit 7 (least significant) |
| 20 | GND | - | Ground |
| 21 | D5 | IN/OUT | |
| 22 | D6 | IN/OUT | |
| 23 | D3 | IN/OUT | |
| 24 | D4 | IN/OUT | |
| 25 | D1 | IN/OUT | |
| 26 | D2 | IN/OUT | |
| 27 | GND | - | Ground |
| 28 | D0 | IN/OUT | Data bus, bit 0 (most significant) |
| 29 | A14 | OUT | |
| 30 | A15 | OUT | Address bus, lsb. Also CRU output bit. |
| 31 | A12 | OUT | |
| 32 | A13 | OUT | |
| 33 | A10 | OUT | |
| 34 | A11 | OUT | |
| 35 | A8 | OUT | |
| 36 | A9 | OUT | |
| 37 | A6 | OUT | |
| 38 | A7 | OUT | |
| 39 | A4 | OUT | |
| 40 | A5 | OUT | |
| 41 | A2 | OUT | |
| 42 | A3 | OUT | |
| 43 | A0 | OUT | Address bus, bit 0 (most significant) |
| 44 | A1 | OUT | |
| 45 | AMB | H | Extra address bit. Always High. |
| 46 | AMA | H | Extra address bit. Always High. |
| 47 | GND | - | Ground |
| 48 | AMC | H | Extra address bit. Always High. |
| 49 | GND | - | Ground |
| 50 | CLKOUT* | OUT | Inversion of phase 3 clock (=PHI3*) |
| 51 | CRUCLK* | OUT | Inversion of TMS9900 CRUCLOCK pin |
| 52 | DBIN | OUT | Active high = read memory |
| 53 | GND | - | Ground |
| 54 | WE* | OUT | Write Enable (derived from TMS9900 WE* pin) |
| 55 | CRUIN | IN | CRU input bit to TMS9900 |
| 56 | MEMEN* | OUT | Memory access enable (active low) |
| 57 | -12 Volts 3-T regulator supply voltage (about -16V) | ||
| 58 | Dito | ||
| 59 | +12 Volts 3-T regulator supply voltage (about +16V) | ||
| 60 | Dito |
Signals
- D0–D7 — 8-bit bidirectional data bus. Note that by TI convention D7 is the least significant bit and D0 is the most significant bit, the reverse of most other architectures.
- A0–A15 — 16-bit address bus, output from the CPU. A0 is the most significant bit; A15 is the least significant and doubles as the CRU output bit for serial I/O addressing.
- AMA, AMB, AMC — Extra address bits always driven High; used to extend the address space for future expansion.
- DBIN — Data Bus IN; active high indicates a memory read cycle.
- WE* — Write Enable; active low, derived from the TMS9900 WE* pin.
- MEMEN* — Memory access enable; active low, qualifies address bus for memory (as opposed to CRU) cycles.
- CRUCLK* — Inverted TMS9900 CRUCLOCK signal; clocks CRU serial output shifts.
- CRUIN — CRU serial input bit returning data to the TMS9900.
- READYA — System ready signal with 10 kΩ pull-up to +5 V; expansion cards may hold this low to insert wait states.
- RESET* — Active-low system reset, output from the PE Box backplane.
- RDBENA* — Active-low signal that enables the flex-cable data bus drivers (74LS245); 1 kΩ pull-up.
- INTA* — Active-low interrupt level A, equivalent to EXTINT*.
- SENILA*, SENILB* — Interrupt level sense enables; always driven High in the base system.
- PCBEN — PCB burn-in enable; always High in normal operation.
- HOLD* — Active-low CPU hold request; always High (not used).
- IAQHA — Logical OR of IAQ (Instruction Acquisition) and HOLDA; not connected in the standard PE Box.
- LOAD* — Unmaskable interrupt input; not connected in the standard PE Box.
- SCLK — System clock; not connected in the standard PE Box.
- LCP* — CPU indicator (1 = TI-99/4A, 0 = second-generation CPU); not connected in the standard PE Box.
- CLKOUT* — Inverted phase-3 clock output (PHI3*).
- AUDIO — Analogue audio input to the PE Box sound mixing circuit.
- Pins 1–2 — Unregulated +8 V supply (nominal; for +5 V regulation on card).
- Pins 57–58 — Unregulated −16 V supply (nominal; for −12 V regulation on card).
- Pins 59–60 — Unregulated +16 V supply (nominal; for +12 V regulation on card).
Notes
- The connector is a 60-pin card-edge type; pin numbering runs from rear (odd side, pins 1–59) to front (even side, pins 2–60) of the PE Box chassis.
- Address and control signals are buffered by 74LS244 drivers in the flex-cable connection module: A0–A15, DBIN, MEMEN*, WE*, RESET*, CLKOUT*.
- The data bus is buffered by two 74LS245 transceivers (one at each end of the flex cable), with 47 Ω series resistors; direction is controlled by DBIN, gated by RDBENA*.
- Unbuffered signals passed directly: CRUIN, INTA*, AUDIO, READYA.
- All signals must be re-buffered on each expansion card; the bus does not drive cards directly.
- Always-High lines (AMA, AMB, AMC, SENILA*, SENILB*, PCBEN, HOLD*) are pulled up to +5 V via 47 Ω resistors on the backplane.
- Power rails are unregulated; each card is responsible for on-board regulation to the required ±5 V or ±12 V levels.
References
- TI 99/4A Console And Peripheral Expansion System Technical Data (1983) — Texas Instruments primary hardware document
- The Peripheral Expansion Box — TI-99/4A Tech Pages — Thierry Nouspikel, University of Geneva
- TI-99/4A Pinouts — TI-99/4A Tech Pages — Thierry Nouspikel, University of Geneva