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Monolog

[mon-uh-lawg, -log]

A Monolog is a Single Telephone Line Call Logging Device manufactured by British Telecom in the UK. The reason for connecting Monolog to a telephone line is to collect independent call and charging data to help resolve customer queries or complaints.

Monolog is usually connected to a customer's line at the telephone exchange although it is possible to monitor the line at the customer's premises.

Monolog is based around the Mitsubishi M50734SP-10 8-bit processor that uses an enhanced 6502 instruction set. The unit comprises two boards: a digital board that contains EPROM and RAM for storage of call records and an analogue board that provides the necessary interface components to the monitored telephone line.

Monolog is powered via four AA rechargeable batteries which are trickle charged at approximately 2 mA from a control line. This control line is also used for remote connection to the unit for the purposes of data retrieval.

Specifications

Monitored Line Input Impedance

A leg -> B leg = 10 MΏ
A/B leg -> Ground = 10 MΏ
Meter -> Ground = 10 MΏ
Gnd ref -> Ground = 10 MΏ

These impedances exceed the open loop requirements, therefore there is no loading on the telephone line.

Call Answer and Meter Input

Meter Pulses (+ve or -ve battery pulses above 18 volts)
Detection of pulses occurring on the 4th or P-wire
SPM isolated from customers line
SPM on customers line
Line reversal on answer

Call Storage

Call storage varies with content, typically Monolog will store:

32K RAM 2,000 calls (standard model)
64K RAM 4,000 calls (with extra 32K memory)
160K RAM 10,000 calls (with extra 128K memory)

A circular memory buffer allows calls to be stored continuously. Once full, the earliest calls are overwritten.

Analogue Board

The analogue board provides the interface circuitry between the monitored and control telephone lines and the microprocessor. The board contains the following devices ;

U1 = OP490 Quad Op-Amp

U2 = Mitel MT8870CE 300baud modem

U3 = Motorola MC145442 DTMF decoder

U4 = LM358N

U5 = Phillips 74HC157P

The board also contain a 25-way female D-type connector that provides the electrical interface to the monitored/control lines. Alternatively, it provides an RS-232 interface thereby enabling the direct connection of a PC running the Dialog software.

Battery Charging circuit

Monolog's batteries can be charged via 2 spare pins on the 25-way D-type connector; pin 4 (RTS) and pin 20 (DTR). If a 12volt power source is applied to either pin, the batteries are charged at 22mA. If both pins are used, the batteries are charged at 44mA. The 12volt feed is via two 330Ω current limiting resistors and the wired in fuse. It should be noted this circuitry does not provide any protection against over-charging.

Digital Board

The digital board contains 7 Integrated Circuits as follows ;

U1 = M50734-SP10 Mitsubishi Microprocessor

U2 = 74HC373P address latch

U3 = CD4060BCN 14-bit asynchronous binary counter

U4 = 74HC139N Address decoder

U5 = Static RAM (130 kB)

U6 = 27C256BQ EPROM

U7 = Static RAM (32 kB)

In addition to these components are the reverse battery protection diode associated timing circuitry for the Processor and CMOS divider IC.

Real Time Clock

To enable the call records to be date stamped, the CMOS divider IC produces an interrupt pulse every 125 ms which activates the processor. The interrupt service routing updates the system clock and checks for any activity on the line. If there is none, the processor goes back to the SLEEP mode. In this mode, the CPU consumes very little power thereby enabling the unit to be battery powered. In its quiescent state, Monolog consumes around 180 µA while the monitored telephone line is inactive, rising to a peak of about 6 mA during a call while the DTMF decoder is on during the first 20 to 30 seconds of a call.