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.

NOTE

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.

Dual Interface

RS-232 at 9600 baud 8N1 for direct bench use; V21 300 baud dial-up retained for remote PSTN access.

Exchange Compatible

Tested against System X, AXE10, TXE4, Strowger and UXD5 across the full BT network.

Park Detection

Forced-release detection on TXE4, System X and AXE10 via line reversal or voltage conditions.

Specification

Parameter	Value	Notes
Dimensions (H × W × D)	45 mm × 92 mm × 154 mm	Plastic enclosure
Weight (without batteries)	0.32 kg
Weight (with batteries)	0.436 kg	4 × AA NiCad/NiMH
Power supply	4.8 V DC	4 × AA rechargeable batteries
Interface connector	25-way D-type (female)
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

The Mark I and Mark II are hardware identical — the same PCBs, the same components, the same physical enclosure dimensions. The fundamental difference is entirely in the firmware.

Feature	Mark I	Mark II
Hardware	Identical
Direct RS-232	Yes — 9600 bd 8N1	Yes — 9600 bd 8N1
Dial-up	300 bd V21	300 bd V21
Protocol	Plain text (human readable)	Binary byte commands
Firmware	EPROM — original image	EPROM — entirely rewritten
BT livery	British Telecom (pre-1991)	BT Piper (post-1991)

Firmware Architecture

The Mark I firmware stored all terminal commands and human-readable responses directly in the EPROM. Anyone with a modem could dial into a Monolog Mark I and — provided they got past the password prompt — interact with the unit directly using plain text commands such as STATUS, TIME and DL, receiving readable responses in return.

The Mark II firmware was entirely rewritten around a binary protocol, requiring the DIALOG software running on the PC to act as an interpreter. Commands are sent as:

      [count] [command] [bytes…] [checksum]

      Command frame sent to Monolog

      [count] [type] [bytes…] [checksum]

      Response frame returned by Monolog

Without DIALOG (or equivalent software) to decode these frames, a raw connection to a Mark II produces only binary data — providing a degree of security by obscurity that the plain-text Mark I lacked. The rewritten firmware also likely introduced flexibility for ROM images to be updated in the field.

Known Fault — Enclosure Cracking

A common structural weakness affects both plastic enclosure variants. The spring clips that retain the four AA batteries exert a persistent outward pressure on the moulded plastic case, eventually causing a crack to develop along the seam line. The fault is progressive — once started, normal handling accelerates it. It is likely that this design weakness was a contributing factor in BT's decision to move to a metal enclosure for later Mark II production runs.

Cracked plastic enclosure on a Mark II Monolog — Mark II (grey plastic) — crack along the case seam caused by outward battery clip pressure. Click to enlarge.

Cracked plastic enclosure on a Mark I Monolog — Mark I (blue plastic) — the same fault affects both variants. Click to enlarge.

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Mark II Photographs

Unit photographs to be added