I/O Map
The following table lists the I/O definitions of the BT CT Payphone (CT24/25/34).
| Q | Serial transmit (1200 baud, 7O1) |
| EF1 | Serial receive (active-low, inverted) |
| EF2 | SPM (Supervisory Pulse Metering) - call answer/metering detect |
| OUT 1 | Display & Tone Generator Bit 0: Tone A Bit 1: Tone B Bit 2: Tone C Bits [7:3]: LCD text indicators (999 CALLS ONLY, INSERT MONEY, CREDIT, MINIMUM FEE, REFUND) |
| OUT 2 | Voice & Validator Control Bit 0: RX MUTE Bit 1: TX MUTE Bit 2: COIN ACK Bit 3: LP DIAL Bit 4: SPCH EN Bit 5: SPARE |
| OUT 3 | MF4 Tone Generator (active low) Bits [3:0]: Column C1–C4 Bits [7:4]: Row R1–R4 (DTMF) |
| OUT 4 | Coin Gates & Sensor LEDs Bit 0: Gate 1 (accept/reject) Bit 1: Gate 2 (escrow routing) Bit 2: Gate 3 (lower escrow → return) Bit 3: Gate 4 (upper escrow → return) Bit 4: Gate 5 (lower escrow → cash box) Bit 5: Sensor LEDs 3 & 4 + Gate 6 (upper escrow → cash box) Bit 6: Sensor LEDs 1, 2, & 5 |
| OUT 5 | Keypad Drive & Phone Control Bits [3:0]: Keypad column drive Bit 4: BF RLY Bit 5: CFG WE (config RAM write-enable) Bit 6: SEIZE/EN BELL (line seize / bell enable) Bit 7: PHONE ON (power hold) |
| OUT 6 | LCD pounds digit (BCD) |
| OUT 7 | LCD pence digit (BCD) |
| INP 1 | Sensors (active low, multiplexed via OUT 4 LED bits) Bit 5: Sensor 1/3 (depending on LED drive) Bit 6: Sensor 2/4 (depending on LED drive) Bit 7: Sensor 5 |
| INP 3 | Coin Validator Bit 0: Coin A Bit 1: Coin B Bit 2: Coin C Bit 3: Coin D Bit 4: Coin E Bit 5: Coin F Bit 6: DAV (data available - coin validated) Bit 7: Test mode switch |
| INP 4 | FOC, Tester & Keypad Rows Bits [3:0]: Keypad row read Bit 4: FOC (Follow On Call) Bit 5: TSTR SN (Tester Sense) Bit 6: TSTR SP Bit 7: LD/MF |
| INP 5 | Power & Status Bit 0: LP OPEN Bit 1: RINGING Bit 2: ¬LG CBX Bit 3: DR OPEN (door open) Bit 4: SPARE Bit 5: LINE PWR Bit 6: OFF HOOK Bit 7: ¬CBX IN |