Mark II Hardware
The Mark II Monolog was introduced following BT's April 1991 rebrand. The principal addition over the Mark I is a direct RS-232 port operating at 9600 baud 8N1, enabling fast bench retrieval without the overhead of modem negotiation. The dial-up interface is retained for remote field use.
RS-232 Direct Connection
Do not toggle DTR/RTS lines when connecting. Toggling may generate a BRK signal which triggers a full hardware reset, erasing all logged call records from RAM.
01
Dual Interface
RS-232 at 9600 baud 8N1 for direct bench use; V21 300 baud dial-up retained for remote PSTN access.
02
Exchange Compatible
Tested against System X, AXE10, TXE4, Strowger and UXD5 across the full BT network.
03
Park Detection
Forced-release detection on TXE4, System X and AXE10 via line reversal or voltage conditions.
Specification
| Parameter | Value | Notes |
|---|---|---|
| RS-232 direct | 9600 bd · 8N1 | Do not toggle DTR/RTS |
| Dial-up modem | 300 bd · 7E1 | V21 originate, XON/XOFF |
| RAM options | 32K / 64K / 160K | 2,000 / 4,000 / 10,000 calls |
| Standby current | 180 µA | ~2 mA during active call |
| Timing resolution | 125 mS | Answered outgoing; 1 s for others |
| Clock accuracy | ≤ 1 sec/day | Calendar valid to year 2099 |
| A2D channels | 2 | Battery voltage + line voltage |
| Battery voltage | raw × 5.0 / 255 | e.g. 248 raw → 4.86 V |
Differences from Mark I
| Feature | Mark I | Mark II |
|---|---|---|
| Direct RS-232 | No | Yes — 9600 bd 8N1 |
| Dial-up | 300 bd V21 | 300 bd V21 (retained) |
| A2D channels | 4 | 2 (battery + line) |
| Firmware | EPROM | EPROM (different image) |
| BT livery | British Telecom (pre-1991) | BT Piper (post-1991) |
Mark II Photographs
Unit photographs to be added