Mark I Hardware
The Monolog Mark I is a single-line call logger developed by British Telecom for use in disputed accounts cases. Small enough to ship in a Jiffy bag, the unit is entirely line-powered with no on-site configuration required. All call details, dialled digits and timestamps are stored in internal RAM, with a dial-up control line enabling full remote interrogation over the PSTN.
The Mark I predates BT's April 1991 rebrand. Units carry original British Telecom livery. The control line also provides trickle charging to the internal battery pack.
Specification
| Parameter | Value | Notes |
|---|---|---|
| RAM options | 32K / 64K / 160K | 2,000 / 4,000 / 10,000 call capacity |
| Standby current | 180 µA | Increases to ~2 mA during a call |
| Timing resolution | 125 mS | Answered outgoing calls; 1 s for others |
| Loop dial pulse | 6 – 50 pps | Make/break ratio 10/90 – 90/10 |
| Meter pulse threshold | > 18 V | Positive or negative; SPM isolated or on line |
| Clock accuracy | ≤ 1 sec/day | Perpetual calendar valid to year 2099 |
| Dial-up settings | V21 · 300 bd · 7E1 | XON/XOFF handshaking; no config needed |
| Line impedance | 10 MΩ | A–B leg, A/B–ground, M wire–ground |
Line Connection
The standard connecting cord allows Monolog to be connected to the customer's line, metering wire and control line either through a Long Line Extender (LLE) or a Line Jack Unit (LJU). Where line resistance makes voltage sensing alone inadequate, the Long Line Extender introduces current sensing.
Connection Diagram
Wiring diagram to be added — see Appendix D of the handbook
Batteries
The internal battery pack allows data retention for several weeks without external power. The control line provides trickle charging during normal operation. Reversed battery protection is fitted as standard.
Battery Detail
Battery type, capacity and replacement procedure to be added
Memory Options
| RAM | Typical Capacity | Notes |
|---|---|---|
| 32K | 2,000 calls | Standard model |
| 64K | 4,000 calls | Extra 32K fitted |
| 160K | 10,000 calls | Extra 128K fitted |
A circular buffer allows continuous logging — once full, the earliest records are overwritten. Call storage varies with content; the figures above are typical.
EPROM
The Mark I firmware is held in EPROM. A patched version of the EPROM image is available in the Downloads section that extends the customer number (NUM) field from 10 to 11 digits, accommodating current UK number formats.